Magnetic resonance of a single molecular spin
Identifieur interne : 000D09 ( Main/Corpus ); précédent : 000D08; suivant : 000D10Magnetic resonance of a single molecular spin
Auteurs : Jürgen KöhlerSource :
- Physics Reports [ 0370-1573 ] ; 1998.
Abstract
Magnetic-resonance spectroscopy on single molecules represents the ultimate limit in sensitivity of electron spin resonance: the detection of a single molecular spin. This is achieved by combining single molecule spectroscopy and optically detected magnetic resonance. Experimental results on pentacene in p-terphenyl both in zero-field and in the presence of a weak magnetic field demonstrate that magnetic-resonance spectroscopy on single molecules adds on to the specificity of single-molecule spectroscopy. It proved possible to identify single molecules which contain 13C nuclei in natural abundance and to observe the splitting of the electron spin resonance line resulting from the hyperfine intercation of a single molecular spin with a single 13C nuclear spin. From the Zeeman effect the orientation of the symmetry axes of individual molecules with respect to the direction of the external magnetic field is obtained. This allows to compare the orientation of individual molecules with their substitutional-site specific transition frequencies depending on the quality of the host crystal.
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DOI: 10.1016/S0370-1573(98)00057-X
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<record><TEI wicri:istexFullTextTei="biblStruct"><teiHeader><fileDesc><titleStmt><title>Magnetic resonance of a single molecular spin</title><author><name sortKey="Kohler, Jurgen" sort="Kohler, Jurgen" uniqKey="Kohler J" first="Jürgen" last="Köhler">Jürgen Köhler</name><affiliation><mods:affiliation>Centre for the Study of Excited States of Molecules, Huygens Laboratory, University of Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands</mods:affiliation></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">ISTEX</idno><idno type="RBID">ISTEX:CE5235C68496649D77502A841749C0A07CBBF079</idno><date when="1999" year="1999">1999</date><idno type="doi">10.1016/S0370-1573(98)00057-X</idno><idno type="url">https://api.istex.fr/document/CE5235C68496649D77502A841749C0A07CBBF079/fulltext/pdf</idno><idno type="wicri:Area/Main/Corpus">000D09</idno></publicationStmt><sourceDesc><biblStruct><analytic><title level="a">Magnetic resonance of a single molecular spin</title><author><name sortKey="Kohler, Jurgen" sort="Kohler, Jurgen" uniqKey="Kohler J" first="Jürgen" last="Köhler">Jürgen Köhler</name><affiliation><mods:affiliation>Centre for the Study of Excited States of Molecules, Huygens Laboratory, University of Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands</mods:affiliation></affiliation></author></analytic><monogr></monogr><series><title level="j">Physics Reports</title><title level="j" type="abbrev">PLREP</title><idno type="ISSN">0370-1573</idno><imprint><publisher>ELSEVIER</publisher><date type="published" when="1998">1998</date><biblScope unit="volume">310</biblScope><biblScope unit="issue">5–6</biblScope><biblScope unit="page" from="261">261</biblScope><biblScope unit="page" to="339">339</biblScope></imprint><idno type="ISSN">0370-1573</idno></series><idno type="istex">CE5235C68496649D77502A841749C0A07CBBF079</idno><idno type="DOI">10.1016/S0370-1573(98)00057-X</idno><idno type="PII">S0370-1573(98)00057-X</idno></biblStruct></sourceDesc><seriesStmt><idno type="ISSN">0370-1573</idno></seriesStmt></fileDesc><profileDesc><textClass></textClass><langUsage><language ident="en">en</language></langUsage></profileDesc></teiHeader><front><div type="abstract" xml:lang="en">Magnetic-resonance spectroscopy on single molecules represents the ultimate limit in sensitivity of electron spin resonance: the detection of a single molecular spin. This is achieved by combining single molecule spectroscopy and optically detected magnetic resonance. Experimental results on pentacene in p-terphenyl both in zero-field and in the presence of a weak magnetic field demonstrate that magnetic-resonance spectroscopy on single molecules adds on to the specificity of single-molecule spectroscopy. It proved possible to identify single molecules which contain 13C nuclei in natural abundance and to observe the splitting of the electron spin resonance line resulting from the hyperfine intercation of a single molecular spin with a single 13C nuclear spin. From the Zeeman effect the orientation of the symmetry axes of individual molecules with respect to the direction of the external magnetic field is obtained. This allows to compare the orientation of individual molecules with their substitutional-site specific transition frequencies depending on the quality of the host crystal.</div></front></TEI><istex><corpusName>elsevier</corpusName><author><json:item><name>Jürgen Köhler</name><affiliations><json:string>Centre for the Study of Excited States of Molecules, Huygens Laboratory, University of Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands</json:string></affiliations></json:item></author><subject><json:item><lang><json:string>eng</json:string></lang><value>76.70.Hb</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>42.62.Fi</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>33.50.Bq</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>Single-molecule magnetic resonance</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>Single-molecule spectroscopy</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>Pentacene in p-terphenyl</value></json:item></subject><language><json:string>eng</json:string></language><abstract>Magnetic-resonance spectroscopy on single molecules represents the ultimate limit in sensitivity of electron spin resonance: the detection of a single molecular spin. This is achieved by combining single molecule spectroscopy and optically detected magnetic resonance. Experimental results on pentacene in p-terphenyl both in zero-field and in the presence of a weak magnetic field demonstrate that magnetic-resonance spectroscopy on single molecules adds on to the specificity of single-molecule spectroscopy. It proved possible to identify single molecules which contain 13C nuclei in natural abundance and to observe the splitting of the electron spin resonance line resulting from the hyperfine intercation of a single molecular spin with a single 13C nuclear spin. From the Zeeman effect the orientation of the symmetry axes of individual molecules with respect to the direction of the external magnetic field is obtained. This allows to compare the orientation of individual molecules with their substitutional-site specific transition frequencies depending on the quality of the host crystal.</abstract><qualityIndicators><score>6.884</score><pdfVersion>1.2</pdfVersion><pdfPageSize>504 x 674 pts</pdfPageSize><refBibsNative>true</refBibsNative><keywordCount>6</keywordCount><abstractCharCount>1099</abstractCharCount><pdfWordCount>29344</pdfWordCount><pdfCharCount>165641</pdfCharCount><pdfPageCount>79</pdfPageCount><abstractWordCount>157</abstractWordCount></qualityIndicators><title>Magnetic resonance of a single molecular spin</title><pii><json:string>S0370-1573(98)00057-X</json:string></pii><genre><json:string>research-article</json:string></genre><host><volume>310</volume><pii><json:string>S0370-1573(00)X0221-9</json:string></pii><pages><last>339</last><first>261</first></pages><issn><json:string>0370-1573</json:string></issn><issue>5–6</issue><genre></genre><language><json:string>unknown</json:string></language><title>Physics Reports</title><publicationDate>1999</publicationDate></host><categories><wos><json:string>PHYSICS</json:string><json:string>PHYSICS, MULTIDISCIPLINARY</json:string></wos></categories><publicationDate>1998</publicationDate><copyrightDate>1999</copyrightDate><doi><json:string>10.1016/S0370-1573(98)00057-X</json:string></doi><id>CE5235C68496649D77502A841749C0A07CBBF079</id><fulltext><json:item><original>true</original><mimetype>application/pdf</mimetype><extension>pdf</extension><uri>https://api.istex.fr/document/CE5235C68496649D77502A841749C0A07CBBF079/fulltext/pdf</uri></json:item><json:item><original>false</original><mimetype>application/zip</mimetype><extension>zip</extension><uri>https://api.istex.fr/document/CE5235C68496649D77502A841749C0A07CBBF079/fulltext/zip</uri></json:item><istex:fulltextTEI uri="https://api.istex.fr/document/CE5235C68496649D77502A841749C0A07CBBF079/fulltext/tei"><teiHeader><fileDesc><titleStmt><title level="a">Magnetic resonance of a single molecular spin</title></titleStmt><publicationStmt><authority>ISTEX</authority><publisher>ELSEVIER</publisher><availability><p>ELSEVIER</p></availability><date>1999</date></publicationStmt><notesStmt><note>editor: S.D. Peyerimhoff Contents 1.Introduction 264 2. The principles of optically detected magnetic-resonance 267 2.1.Photophysics of pentacene 267 2.2.The triplet state and the spin Hamiltonian 269 2.3.Fluorescence-detected magnetic-resonance 272 3.Optical single-molecule spectroscopy 277 3.1.The principle of single-molecule spectroscopy in solids 277 3.2.Optical saturation 280 4.Experimental 282 4.1.Sample preparation 283 4.2.The experimental set-up 283 4.3.The cryogenic insert 285 4.4.Illustrations 288 5.Single-spin spectroscopy 290 5.1.Basic considerations 290 5.2.Magnetic-resonance experiments on single pentacene molecules 293 5.3.Microwave transition frequency versus spectral site O1 or O2 298 5.4.The hyperfine interaction of a single-triplet spin with a single-nuclear spin 303 6.``Spin-off” 320 6.1.Molecules with poor photophysical properties 320 6.2.Spectroscopy of isotopomers without chemical synthetization 326 7.Conclusions and outlook 330 Acknowledgements 332 Appendix A. Spin–spin interaction 333 A.1. Like spins 333 A.2. Unlike spins 334 References 335</note><note type="content">Fig. 2.1: (a) Molecular structure and axis system of pentacene. (b) Energy level diagram of the lowest excited states of pentacene. The full arrows indicate the optical excitation-emission cycle (transition rates k0 and k1) and the dotted arrows represent the intersystem crossings (rates kISC1 and kISC2).</note><note type="content">Fig. 2.2: The crystal structure of p-terphenyl in the triclinic phase (below 193K). P1 to P4 denote the four inequivalent crystal sites.</note><note type="content">Fig. 2.3: Schematic representation of the pumping cycle for pentacene. The relative steady-state populations of the three triplet zero-field states are indicated by the circles. The arrows sketch the relative magnitude of the decay rates for pentacene in p-terphenyl (see Table 1).</note><note type="content">Fig. 2.4: The three optically detected zero-field transitions of the lowest excited triplet state of pentacene for an ensemble of molecules. From top to bottom the respective transitions are between Tx–Tz, Ty–Tz, an Tx–Ty as indicated. Each spectrum corresponds to a decrease of the fluorescence intensity with respect to the steady-state fluorescence intensity without microwaves. The vertical scale is different for each spectrum. The Tx–Ty spectrum has been obtained from a different sample and using different microwave equipment with respect to the other two spectra.</note><note type="content">Fig. 2.5: Labelling scheme of the six inequivalent carbon positions of pentacene. The magnitude of the triplet spin density at the respective carbon positions is indicated by the diameter of the circles, see Table 2.</note><note type="content">Fig. 2.6: Comparison of the magnetic-resonance lineshape of the Tx–Tz transition for pentacene-h14 in p-terphenyl-h14 (top) and pentacene-d14 in p-terphenyl-d14 (bottom). The signals correspond to a decrease of the fluorescence. The full width at half maximum (FWHM) for each line is given in the figure.</note><note type="content">Fig. 3.1: Schematic representation of the phenomenon of inhomogeneous line broadening in solids. In the upper right three different impurity molecules are sketched being subjected to different local environments due to random strain. For the ensemble of molecules this results in a broad distribution of transition frequencies. Using a narrow band laser a subensemble of molecules can be selected (line centre) or tuning the laser into the wing a single molecule is isolated. The figure has been taken from [43].</note><note type="content">Fig. 3.2: (a) The inhomogeneously broadened fluorescence excitation line of an ensemble of pentacene molecules in p-terphenyl. The “noisy” structure is denoted as statistical fine structure and is highly repeatable. (b) Enlarged view of the circled region of part (a) of the figure resolving several fluorescence excitation lines of individual molecules. The spectra have been taken from [46].</note><note type="content">Fig. 4.1: Schematic representation of the experimental set-up. Details are given in the text.</note><note type="content">Fig. 4.2: (a) Front view of the sample mounting assembly along the x-axis. The large inner circle indicates the suprasil cover plate. The three, small, open circles refer to the dots of vacuum grease which glue the cover plate to the LiF substrate. The black dot represents the aluminium mirror used for the autocollimation and the horizontal strip indicates the area accessible by the laser focus. (b) A simplified section of the insert in the x–z plane. L is the focusing lens (f=10mm), P the parabolic mirror (f=5mm), S the sample, C the suprasil cover plate, I the microwave coil, and B is the beam block. M indicates the superconducting magnet which is mounted in a Helmholtz configuration with the magnetic field along the z-axis.</note><note type="content">Fig. 4.3: (a) A section of the insert in the y–z plane showing the principle of the horizontal adjustment of the sample. (b) A section of the insert in the x–z plane showing the principle of the adjustment of the focusing lens. For further explanation see the text.</note><note type="content">Fig. 4.4: The lower trace shows the fluorescence-excitation spectrum of a single pentacene molecule which is embedded in a p-terphenyl host crystal. The spectrum has been recorded with an incident laser power of 5nW corresponding to 25mW/cm2 in focus. The line exhibits a Lorentzian profile with a width of 11.0MHz (FWHM). The upper trace is the output of the 750mm long Fabry-Pérot Interferometer recorded simultaneously to provide the calibration of the laser frequency axis in relative units. The spacing between the two modes is 100MHz. The vertical scale is valid for the lower trace.</note><note type="content">Fig. 4.5: Fluorescence-excitation spectra of a single molecule of pentacene in p-terphenyl by scanning the laser over a range of 500MHz in the red wing of the inhomogeneously broadened 1S1←1S0 transition for the O1 spectral site. The laser frequency is given in relative units. The spectra have been obtained for different settings of the dial controlling the position of the focusing lens L. The laser power incident to the cryostat was 5nW corresponding to 25mW/cm2 when in focus. The vertical scale is valid for the lowest trace while all other traces are offset for clarity. For further explanation see text.</note><note type="content">Fig. 5.1: A schematic representation of a magnetic-resonance experiment of a single molecule. The upper panel sketches the situation without microwaves and the lower panel gives the situation in presence of a microwave field which is in resonance with the Tx–Tz transition. Under influence of the incident laser radiation the pentacene molecule carries out excitation-emission cycles (situations 1 and 3). This process is interrupted when the molecule crosses over to the triplet state. With highest probability it ends up in the Tx sublevel where it resides on the average for 50μs, a period during which no fluorescence is observable (situation 2). In the presence of the microwave field the triplet mean residence time of the molecule is prolonged to about 70μs (situation 4). Below the energy-level diagrams the temporal distribution of the emitted photons is represented by the stripes.</note><note type="content">Fig. 5.2: Dependence of the emission rate R for a single pentacene molecule on the incident laser intensity for two different values of the mean triplet residence time. The upper curve refers to a situation without microwaves and the lower curve is valid for a situation where a microwave field saturates the Tx–Tz transition. The calculation is based on the values reported in [92] but has been modified according to the numbers given in [83], see also Table 1.</note><note type="content">Fig. 5.3: Two fluorescence-excitation spectra of the same pentacene molecule. For the lower trace the incident laser intensity is 5nW corresponding to 25mW/cm2 in focus and for the upper spectrum the intensity is increased to 100nW equivalent to 500mW/cm2. While the background increases by a factor of about 20 from 105 counts per second (cps) to 2200cps the single molecule signal grows only from 1000 to 2700cps. The vertical scale is valid for both spectra.</note><note type="content">Fig. 5.4: (a) FMDR spectrum of the Tx–Tz transition of a single pentacene molecule in a p-terphenyl host crystal. The molecule is excited at the fixed laser frequency ν1 in the red wing of the inhomogeneously broadened O1 ensemble line. For each dot the signal is accumulated for 10s while the lines serve to guide the eye. The average photon-count rate during acquisition is 2×104, and the laser intensity is 200mW/cm2. The signal corresponds to the decrease of the fluorescence relative to the situation without microwaves. (b) No FDMR signal is visible when the laser frequency is held at ν2. In the inset two consecutively registered fluorescence-excitation spectra of the molecule are shown. The spectra are recorded with a lower laser intensity than that used for the FDMR signals.</note><note type="content">Fig. 5.5: Comparison of the lineshapes of the Ty–Tz magnetic-resonance transition for a single molecule (top) and an ensemble of about 104 molecules (bottom). Both signals correspond to a decrease of the fluorescence intensity. The ensemble spectrum is recorded with the laser tuned to the top of the inhomogeneously broadened O1 absorption line whereas the molecule is selected in the red wing of this line.</note><note type="content">Fig. 5.6: Hyperfine splitting of the pentacene triplet sublevels in zero-field. The eigenvalues are obtained through a full diagonalization of the Hamiltonian given in Eq. (5.6)with terms for all fourteen proton spins. The lowest box displays the result on a continuous scale while the upper three boxes show expanded views for the three electronic substates.</note><note type="content">Fig. 5.7: Tx–Tz transition of a single molecule for different incident microwave power which is given in dBm (0dBm corresponds to 10mW) and decreases from top to bottom. For better comparison of the lineshapes the spectra are normalised. The peak FDMR effect at the maximum, corrected for the background, is 10.2% (5dBm), 10.2% (2.5dBm), 7.7% (0dBm), and 6.3% (−3dBm), respectively.</note><note type="content">Fig. 5.8: Tx–Tz transition of pentacene in p-terphenyl for various experimental conditions. (a) and (b) Conventional FDMR spectra for large ensembles of molecules from a Bridgman grown crystal (10−5mol pentacene per mol p-terphenyl) with optical excitation in the O1 and O2 spectral sites, respectively. The vertical scale is in arbitrary units. (c)–(e) Single-molecule FDMR spectra for a thin sublimed crystal with excitation wavelengths of the laser to the red of the O1 ensemble line. (f) and (g) Single-molecule FDMR spectra of the same crystal flake for a molecule selected to the blue of the O2 ensemble line. For spectra (c)–(f) the spacing of the dots is 500kHz, for spectrum (g) the spacing is 250kHz. For the single molecule spectra the vertical scale represents the decrease of the fluorescence counts per ten seconds.</note><note type="content">Fig. 5.9: Comparison of the onsets of the Tx–Tz transition for ten different single molecules of sample 2. The vertical bars labelled O1 and O2 indicate the onset of the transition for the respective ensembles of the same sample volume where the single molecules have been isolated.</note><note type="content">Fig. 5.10: Comparison of the shift of the Tx–Tz transition in a magnetic field for two different pentacene molecules. Molecule V is selected close to the O1 ensemble line whereas molecule VI is selected in the vicinity of the O2 line. For better comparison all spectra are normalised. The background corrected maximum FDMR effect amounts for molecule V to 10.5% (zero-field), 7% (3.8mT), and 5.2% (7.5mT) and for molecule VI to 7.7% (zero-field) and 6% (5.6mT), respectively.</note><note type="content">Fig. 5.11: The fluorescence-excitation spectrum for the O1 (top) and O2 (bottom) spectral sites of pentacene in p-terphenyl. The intensity scale in each spectrum is valid for the lowest traces, each recorded with an incident laser power of 1nW. For the other traces the following parameters are valid: O1 b) offset 50, scale factor 2.5, laser power 5nW, c) offset 150, scale factor 5, laser power 10nW, d) offset 150, scale factor 5, laser power 100nW; O2 b) offset 250, scale factor 10, laser power 1nW, c) offset 750, scale factor 2.5, laser power 10nW, d) offset 450, scale factor 10, laser power 100nW. The O2 spectrum shows a larger fluorescence count-rate with respect to the O1 spectrum because the incident excitation light is stronger focused onto the sample in order to reduce the probe volume. This leads to an increase of the statistical fine structure.</note><note type="content">Fig. 5.12: Labelling scheme of the six inequivalent carbon positions of pentacene.</note><note type="content">Fig. 5.13: The upper box displays the fluorescence-excitation spectrum of the O1 spectral region. The location of the optical absorption frequencies of three different molecules are indicated by the arrows. The lower three boxes show the respective FDMR spectra for the Ty–Tz transition of these molecules. The additional broadening of the FDMR line observed for molecules II and III is interpreted in terms of the hyperfine interaction of the triplet spin with a single 13C nucleus. The position of the 13C nucleus within the molecule is indicated and is obtained from the analysis of the hyperfine interaction which considers the triplet spin density distribution across the molecule.</note><note type="content">Fig. 5.14: Simulations of the hyperfine broadened Ty–Tz transition for pentacene with either no or one 13C in the ε or γ position. The transition probabilities for microwaves polarised along the molecular x-axis are summed into histograms of 20kHz wide bins. The hyperfine eigenstates are obtained from a diagonalization of the zero-field Hamiltonian with terms for all but the protons bound to carbons in positions β. The histograms are compared with the corresponding experimental FDMR ensemble spectra.</note><note type="content">Fig. 5.15: Comparison of the Ty–Tz magnetic-resonance spectra observed for single molecules with ensemble spectra for a 13C substitution in position ε (left) and γ (right). The ensemble spectra were obtained by tuning the laser into resonance with satellite 4 or satellite 5 of the O1 fluorescence-excitation spectrum, respectively. The single molecules were located by tuning the laser into the wing of the respective satellite line.</note><note type="content">Fig. 5.16: FDMR spectra for the Ty–Tz transition of two doubly 13C substituted pentacene molecules. The molecules are selected optically in-between the combination bands labelled C and D in Fig. 5.11. The positions of the 13C substitutions within the molecule correspond to ε–γ (or symmetry equivalent) for molecule IV, and γ–γ for molecule V, as indicated. The assignment is obtained from the analysis of the hyperfine interactions.</note><note type="content">Fig. 5.17: Comparison of the Tx–Tz magnetic-resonance transition for two pentacene molecules in zero-field and in a magnetic field of 9.4mT. Molecule VI is selected in the red wing of the O1 fluorescence-excitation line while molecule VII is selected close to satellite 5. From the splitting of the FDMR line of molecule VII for a magnetic field of 9.4mT it is concluded that it contains a 13C nucleus in the γ position of pentacene as indicated in the figure. For better comparison all spectra are normalised. The background corrected peak FDMR effect amounts for molecule VI to 4.8% (zero-field), and 4% (9.4mT) and for molecule VII to 5.1% (zero-field), and 3.7% (5.6mT), respectively.</note><note type="content">Fig. 5.18: Comparison of the Tx–Tz magnetic-resonance transition for a pentacene-h14 (top), and a pentacene-d14 (bottom) molecule. The respective linewidth of the spectra are 4.3MHz for the upper and 440kHz for the lower trace. Both spectra are recorded with medium microwave power incident onto the sample. The background corrected relative decrease of the fluorescence intensity is 10% for both spectra.</note><note type="content">Fig. 5.19: Fluorescence-excitation spectrum of the O1 spectral site of pentacene-d14 in p-terphenyl-d14. The arrows indicate the spectral positions of molecules I and II, respectively.</note><note type="content">Fig. 5.20: Comparison of the Tx–Tz magnetic-resonance transition for two different pentacene-d14 molecules. Molecule I has been selected to the red of the main line while molecule II has been selected to the blue of satellite 3, as sketched by the arrows in Fig. 5.19. The insets indicate the molecular constitution as obtained from the analysis of the hyperfine interaction. The vertical scale in each spectrum represents the background corrected relative decrease of the fluorescence intensity.</note><note type="content">Fig. 5.21: Comparison of the Tx–Tz magnetic-resonance transition for two different pentacene-d14 molecules. The splitting of the lines could be attributed to the hyperfine interaction of the triplet spin with two 13C nuclear spins. The location of the 13C substitutions within the molecules correspond to the indicated (or symmetry-equivalent) positions. For convenience both spectra are normalised. The background corrected FDMR effect amounts to 7.7% for molecule III and to 6.2% for molecule IV.</note><note type="content">Fig. 5.22: Fluorescence-detected Tx–Tz magnetic-resonance transition for three different pentacene-d14 molecules in weak magnetic fields. The vertical scale is different for each molecule. The background corrected FDMR effects are for molecule VII: 18.9% (zero-field), 13.2% (1.9mT), 10.9% (3.8mT), and 7.5% (7.5mT); for molecule VIII: 10% (zero-field), 8.1% (1.9mT), 6.6% (3.8mT), and 4.2% (7.5mT); and for molecule IX: 13.3% (zero-field), and 6% (3.8mT).</note><note type="content">Fig. 6.1: Dependence of the Tx–Tz and Ty–Tz transition frequencies on the strength of the external magnetic field for the spectral sites O1/O4 (top) and O2/O3 (bottom).</note><note type="content">Fig. 6.2: Fluorescence-excitation spectra of the spectral sites O1–O4. For better comparison the spectra are displayed on a relative scale and are shifted such that their strongest lines coincide at zero. Spectral features labelled 1–5 result from 13C substituted pentacene molecules. The absolute positions of the main lines and the respective shifts of the satellites are given in Table 14. The spectra have been recorded with an incident laser power of 125mW/cm2 (O1), 50mW/cm2 (O2), 250mW/cm2 (O3), and 500mW/cm2 (O4).</note><note type="content">Fig. 6.3: Fluorescence-excitation spectra of pentacene-h14 in p-terphenyl-h14 in the spectral region of the 1S1←1S0 transition of the O1 spectral site for different positions of the sample in the exciting beam. The sample position is given relative to the lowest spectrum for which the position was set arbitrarily to zero.</note><note type="content">Fig. 6.4: Fluorescence-excitation spectra of pentacene-d14 in p-terphenyl-d14 in the spectral region of the 1S1←1S0 transition of the O1 spectral site for different positions of the sample in the exciting beam. The sample position is given relative to the lowest spectrum for which the position was set arbitrarily to zero.</note><note type="content">Fig. 6.5: Fluorescence-excitation spectrum of the O1/O2 spectral region for pentacene-d14 in p-terphenyl-d14. The bars above the spectrum assign spectral features to the O1 or O2 spectral site. For explanation see text.</note><note type="content">Fig. 6.6: Comparison of the lineshape of the Tx–Tz magnetic-resonance transition for optically selected ensembles of pentacene molecules. For better comparison the spectra have been normalised. The ensembles have been selected by tuning the laser frequency to the top of the respective ensemble zero-phonon line. The left hand part of the figure features pentacene-h14 molecules which contain a single or two 13C nuclei the positions of which are indicated by the black dots. From top to bottom the laser has been tuned into resonance with O1, satellite 4 (O1), satellite 5 (O1), satellite D (O2), and satellite C (O2) (cf. Fig. 5.10). The right hand part of the figure shows deuterated pentacene molecules which contain exclusively 12C nuclei but for which the deuteration was incomplete. The positions of the 1H nuclei within the molecules are indicated by the black ellipses. From top to bottom the laser has been tuned into resonance with O1, satellite 3, satellite 7, and satellite 6 (cf. Fig. 6.5).</note><note type="content">Table 1: The relative steady-state populations nu∼pu/ku and decay rates ku(u=x,y,z) for a single pentacene molecule in p-terphenyl (O1 or O2 spectral site) [83] and naphthalene [67]. The relative populations have been normalized to obtain 1 for the largest value</note><note type="content">Table 2: Spin density distribution of the lowest excited triplet state of pentacene. The data have been taken from [88]</note><note type="content">Table 3: FDMR transition frequencies and deduced triplet sublevel energies (divided by Planck’s constant h) in MHz for large ensembles of pentacene molecules (1013 molecules) in the O1 or O2 spectral sites of p-terphenyl, respectively</note><note type="content">Table 4: Frequency of the Tx–Tz transition for molecules V and VI in weak external magnetic fields. The calculation of the transition frequency is based on the diagonalization of the spin Hamiltonian (Eq.(5.7)), details are given in the text</note><note type="content">Table 5: The positions and relative intensities of the satellites 1–5. The positions are given as shifts with respect to the main lines located at 16882.739cm−1 (O1) and 16886.452cm−1 (O2). The intensities are normalised so as to have the total intensity of satellites 2–5 add up to 18. For satellites that show structure the position of each maximum is given</note><note type="content">Table 6: Assignment of the satellites observed in the O1 and O2 fluorescence-excitation spectra. The ε and the γ assignment is confirmed experimentally. Other assignments are arrived at through comparative reasoning (see text). For satellites that show splittings the average shift is given. The last column gives the result of a QCFF/PI calculation for the isotope shift for a 13C in the respective position of pentacene [136]</note><note type="content">Table 7: Isotope shifts of the 13C13C pentacene satellite bands A–D with respect to the O1 or O2 zero-phonon line. For O2 the positions of the subsatellites are less accurate due to the statistical fine structure. In the assignment column the experimentally observed shifts are compared with combinations of shifts observed for satellites 2–5</note><note type="content">Table 8: Lineposition, linewidth and linesplitting (MHz) of the Tx–Tz transition for two single molecules in zero-field and in a field of 9.4mT. The calculated values are obtained from the diagonalization of the spin Hamiltonian including the zero-field, the Zeeman and the hyperfine interaction for a 13C substitution either in the ε or γ-position of pentacene</note><note type="content">Table 9: Positions and assignments of the satellites 1–3 for pentacene-d14 in p-terphenyl-d14. The positions are given as shifts with respect to the main line at 16915.529cm−1. The assignments are based on the similarity of the observed shifts with those for the protonated system, see Section 5.4.1</note><note type="content">Table 10: Principal values of the total hyperfine tensor (anisotropic plus isotropic part) for 13C, 2H, and for comparison 1H. All values are given in MHz</note><note type="content">Table 11: Observed and calculated splittings of the Tx–Tz magnetic-resonance transition for several pentacene-d14 molecules. The assignments of the positions of the respective 13C substitutions are based on the analysis of the hyperfine interaction in terms of second-order perturbation theory</note><note type="content">Table 12: Comparison of the calculated and observed hyperfine splittings for various molecules. The labels ε and γ refer to the position of the 13C nucleus within the pentacene-d14 molecule</note><note type="content">Table 13: Transition frequencies and sublevels energies X,Y,Z (divided by Planck’s constant h) for the lowest triplet state of pentacene-h14 in p-terphenyl-h14 and of pentacene-d14 in p-terphenyl-d14. All values are given in MHz</note><note type="content">Table 14: Positions of the zero-phonon lines O1–O4 in cm−1 and relative positions of the satellite lines</note><note type="content">Table 15: Intensity distribution of the satellite lines in the different spectral sites. The intensity of satellite 4 has been normalised to 4 for each spectral site. The numbers in parenthesis are deduced from the known total intensity of 22</note><note type="content">Table 16: Shifts and assignments of the additional zero-phonon lines observed in Fig. 6.5 with respect to the main lines. The numbering of the features corresponds to the numbers given above the spectrum in Fig. 6.5 for O1 and O2, respectively. In the assignment column the carbon position to which the proton is bound is referred to by the subscript, where ε1 and ε2 distinguish the two non-equivalent ε positions in pentacene. The calculated values have been obtained by summing the individual shifts of the respective substitutions. Assignments marked with an asterisk have been confirmed by FDMR, see Fig. 6.6</note></notesStmt><sourceDesc><biblStruct type="inbook"><analytic><title level="a">Magnetic resonance of a single molecular spin</title><author><persName><forename type="first">Jürgen</forename><surname>Köhler</surname></persName><note type="biography">Tel.: +31-71-5275858; fax: +31-71-5275819; e-mail: koehler@molphys.leidenuniv.nl.</note><affiliation>Tel.: +31-71-5275858; fax: +31-71-5275819; e-mail: koehler@molphys.leidenuniv.nl.</affiliation><affiliation>Centre for the Study of Excited States of Molecules, Huygens Laboratory, University of Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands</affiliation></author></analytic><monogr><title level="j">Physics Reports</title><title level="j" type="abbrev">PLREP</title><idno type="pISSN">0370-1573</idno><idno type="PII">S0370-1573(00)X0221-9</idno><imprint><publisher>ELSEVIER</publisher><date type="published" when="1998"></date><biblScope unit="volume">310</biblScope><biblScope unit="issue">5–6</biblScope><biblScope unit="page" from="261">261</biblScope><biblScope unit="page" to="339">339</biblScope></imprint></monogr><idno type="istex">CE5235C68496649D77502A841749C0A07CBBF079</idno><idno type="DOI">10.1016/S0370-1573(98)00057-X</idno><idno type="PII">S0370-1573(98)00057-X</idno></biblStruct></sourceDesc></fileDesc><profileDesc><creation><date>1999</date></creation><langUsage><language ident="en">en</language></langUsage><abstract xml:lang="en"><p>Magnetic-resonance spectroscopy on single molecules represents the ultimate limit in sensitivity of electron spin resonance: the detection of a single molecular spin. This is achieved by combining single molecule spectroscopy and optically detected magnetic resonance. Experimental results on pentacene in p-terphenyl both in zero-field and in the presence of a weak magnetic field demonstrate that magnetic-resonance spectroscopy on single molecules adds on to the specificity of single-molecule spectroscopy. It proved possible to identify single molecules which contain 13C nuclei in natural abundance and to observe the splitting of the electron spin resonance line resulting from the hyperfine intercation of a single molecular spin with a single 13C nuclear spin. From the Zeeman effect the orientation of the symmetry axes of individual molecules with respect to the direction of the external magnetic field is obtained. This allows to compare the orientation of individual molecules with their substitutional-site specific transition frequencies depending on the quality of the host crystal.</p></abstract><textClass><keywords scheme="keyword"><list><head>PACS</head><item><term>76.70.Hb</term></item><item><term>42.62.Fi</term></item><item><term>33.50.Bq</term></item></list></keywords></textClass><textClass><keywords scheme="keyword"><list><head>Keywords</head><item><term>Single-molecule magnetic resonance</term></item><item><term>Single-molecule spectroscopy</term></item><item><term>Pentacene in p-terphenyl</term></item></list></keywords></textClass></profileDesc><revisionDesc><change when="1998-03-01">Received</change><change when="1998">Published</change></revisionDesc></teiHeader></istex:fulltextTEI><json:item><original>false</original><mimetype>text/plain</mimetype><extension>txt</extension><uri>https://api.istex.fr/document/CE5235C68496649D77502A841749C0A07CBBF079/fulltext/txt</uri></json:item></fulltext><metadata><istex:metadataXml wicri:clean="Elsevier, elements deleted: ce:floats; body; tail"><istex:xmlDeclaration>version="1.0" encoding="utf-8"</istex:xmlDeclaration><istex:docType PUBLIC="-//ES//DTD journal article DTD version 4.5.2//EN//XML" URI="art452.dtd" name="istex:docType"><istex:entity SYSTEM="gr2y1" NDATA="IMAGE" name="gr2y1"></istex:entity><istex:entity SYSTEM="gr2y2" NDATA="IMAGE" name="gr2y2"></istex:entity><istex:entity SYSTEM="gr2y3" NDATA="IMAGE" name="gr2y3"></istex:entity><istex:entity SYSTEM="gr2y4" NDATA="IMAGE" name="gr2y4"></istex:entity><istex:entity SYSTEM="gr2y5" NDATA="IMAGE" name="gr2y5"></istex:entity><istex:entity SYSTEM="gr2y6" NDATA="IMAGE" name="gr2y6"></istex:entity><istex:entity SYSTEM="gr3y1" NDATA="IMAGE" name="gr3y1"></istex:entity><istex:entity SYSTEM="gr3y2" NDATA="IMAGE" name="gr3y2"></istex:entity><istex:entity SYSTEM="gr4y1" NDATA="IMAGE" name="gr4y1"></istex:entity><istex:entity SYSTEM="gr4y2" NDATA="IMAGE" name="gr4y2"></istex:entity><istex:entity SYSTEM="gr4y3" NDATA="IMAGE" name="gr4y3"></istex:entity><istex:entity SYSTEM="gr4y4" NDATA="IMAGE" name="gr4y4"></istex:entity><istex:entity SYSTEM="gr4y5" NDATA="IMAGE" name="gr4y5"></istex:entity><istex:entity SYSTEM="gr5y1" NDATA="IMAGE" name="gr5y1"></istex:entity><istex:entity SYSTEM="gr5y2" NDATA="IMAGE" name="gr5y2"></istex:entity><istex:entity SYSTEM="gr5y3" NDATA="IMAGE" name="gr5y3"></istex:entity><istex:entity SYSTEM="gr5y4" NDATA="IMAGE" name="gr5y4"></istex:entity><istex:entity SYSTEM="gr5y5" NDATA="IMAGE" name="gr5y5"></istex:entity><istex:entity SYSTEM="gr5y6" NDATA="IMAGE" name="gr5y6"></istex:entity><istex:entity SYSTEM="gr5y7" NDATA="IMAGE" name="gr5y7"></istex:entity><istex:entity SYSTEM="gr5y8" NDATA="IMAGE" name="gr5y8"></istex:entity><istex:entity SYSTEM="gr5y9" NDATA="IMAGE" name="gr5y9"></istex:entity><istex:entity SYSTEM="gr5y10" NDATA="IMAGE" name="gr5y10"></istex:entity><istex:entity SYSTEM="gr5y11" NDATA="IMAGE" name="gr5y11"></istex:entity><istex:entity SYSTEM="gr5y12" NDATA="IMAGE" name="gr5y12"></istex:entity><istex:entity SYSTEM="gr5y13" NDATA="IMAGE" name="gr5y13"></istex:entity><istex:entity SYSTEM="gr5y14" NDATA="IMAGE" name="gr5y14"></istex:entity><istex:entity SYSTEM="gr5y15" NDATA="IMAGE" name="gr5y15"></istex:entity><istex:entity SYSTEM="gr5y16" NDATA="IMAGE" name="gr5y16"></istex:entity><istex:entity SYSTEM="gr5y17" NDATA="IMAGE" name="gr5y17"></istex:entity><istex:entity SYSTEM="gr5y18" NDATA="IMAGE" name="gr5y18"></istex:entity><istex:entity SYSTEM="gr5y19" NDATA="IMAGE" name="gr5y19"></istex:entity><istex:entity SYSTEM="gr5y20" NDATA="IMAGE" name="gr5y20"></istex:entity><istex:entity SYSTEM="gr5y21" NDATA="IMAGE" name="gr5y21"></istex:entity><istex:entity SYSTEM="gr5y22" NDATA="IMAGE" name="gr5y22"></istex:entity><istex:entity SYSTEM="gr6y1" NDATA="IMAGE" name="gr6y1"></istex:entity><istex:entity SYSTEM="gr6y2" NDATA="IMAGE" name="gr6y2"></istex:entity><istex:entity SYSTEM="gr6y3" NDATA="IMAGE" name="gr6y3"></istex:entity><istex:entity SYSTEM="gr6y4" NDATA="IMAGE" name="gr6y4"></istex:entity><istex:entity SYSTEM="gr6y5" NDATA="IMAGE" name="gr6y5"></istex:entity><istex:entity SYSTEM="gr6y6" NDATA="IMAGE" name="gr6y6"></istex:entity></istex:docType><istex:document><converted-article version="4.5.2" docsubtype="fla"><item-info><jid>PLREP</jid><aid>769</aid><ce:pii>S0370-1573(98)00057-X</ce:pii><ce:doi>10.1016/S0370-1573(98)00057-X</ce:doi><ce:copyright year="1999" type="full-transfer">Elsevier Science B.V.</ce:copyright></item-info><head><ce:title>Magnetic resonance of a single molecular spin</ce:title><ce:author-group><ce:author><ce:given-name>Jürgen</ce:given-name><ce:surname>Köhler</ce:surname><ce:cross-ref refid="CORR1">*</ce:cross-ref></ce:author><ce:affiliation><ce:textfn>Centre for the Study of Excited States of Molecules, Huygens Laboratory, University of Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands</ce:textfn></ce:affiliation><ce:correspondence id="CORR1"><ce:label>*</ce:label><ce:text>Tel.: +31-71-5275858; fax: +31-71-5275819; e-mail: koehler@molphys.leidenuniv.nl.</ce:text></ce:correspondence></ce:author-group><ce:date-received day="1" month="3" year="1998"></ce:date-received><ce:miscellaneous>editor: S.D. Peyerimhoff<display><ce:table colsep="0" rowsep="0" frame="none"><tgroup cols="3"><colspec colname="col1" colsep="0"></colspec><colspec colname="col2" colsep="0"></colspec><colspec colname="col3" colsep="0"></colspec><tbody><row><entry namest="col1" nameend="col3"><ce:bold>Contents</ce:bold></entry></row><row><entry></entry><entry></entry><entry></entry></row><row><entry namest="col1" nameend="col2">1.<ce:hsp sp="0.5"></ce:hsp>Introduction</entry><entry colname="col3">264</entry></row><row><entry namest="col1" nameend="col2">2. The principles of optically detected magnetic-resonance</entry><entry colname="col3">267</entry></row><row><entry></entry><entry>2.1.<ce:hsp sp="0.5"></ce:hsp>Photophysics of pentacene</entry><entry>267</entry></row><row><entry></entry><entry>2.2.<ce:hsp sp="0.5"></ce:hsp>The triplet state and the spin Hamiltonian</entry><entry>269</entry></row><row><entry></entry><entry>2.3.<ce:hsp sp="0.5"></ce:hsp>Fluorescence-detected magnetic-resonance</entry><entry>272</entry></row><row><entry namest="col1" nameend="col2">3.<ce:hsp sp="0.5"></ce:hsp>Optical single-molecule spectroscopy</entry><entry colname="col3">277</entry></row><row><entry></entry><entry>3.1.<ce:hsp sp="0.5"></ce:hsp>The principle of single-molecule spectroscopy in solids</entry><entry>277</entry></row><row><entry></entry><entry>3.2.<ce:hsp sp="0.5"></ce:hsp>Optical saturation</entry><entry>280</entry></row><row><entry namest="col1" nameend="col2">4.<ce:hsp sp="0.5"></ce:hsp>Experimental</entry><entry colname="col3">282</entry></row><row><entry></entry><entry>4.1.<ce:hsp sp="0.5"></ce:hsp>Sample preparation</entry><entry>283</entry></row><row><entry></entry><entry>4.2.<ce:hsp sp="0.5"></ce:hsp>The experimental set-up</entry><entry>283</entry></row><row><entry></entry><entry>4.3.<ce:hsp sp="0.5"></ce:hsp>The cryogenic insert</entry><entry>285</entry></row><row><entry></entry><entry>4.4.<ce:hsp sp="0.5"></ce:hsp>Illustrations</entry><entry>288</entry></row><row><entry namest="col1" nameend="col2">5.<ce:hsp sp="0.5"></ce:hsp>Single-spin spectroscopy</entry><entry colname="col3">290</entry></row><row><entry></entry><entry>5.1.<ce:hsp sp="0.5"></ce:hsp>Basic considerations</entry><entry>290</entry></row><row><entry></entry><entry>5.2.<ce:hsp sp="0.5"></ce:hsp>Magnetic-resonance experiments on single pentacene molecules</entry><entry>293</entry></row><row><entry></entry><entry>5.3.<ce:hsp sp="0.5"></ce:hsp>Microwave transition frequency versus spectral site O<math altimg="si14.gif"><inf>1</inf></math> or O<math altimg="si15.gif"><inf>2</inf></math></entry><entry>298</entry></row><row><entry></entry><entry>5.4.<ce:hsp sp="0.5"></ce:hsp>The hyperfine interaction of a single-triplet spin with a single-nuclear spin</entry><entry>303</entry></row><row><entry namest="col1" nameend="col2">6.<ce:hsp sp="0.5"></ce:hsp>``Spin-off”</entry><entry colname="col3">320</entry></row><row><entry></entry><entry>6.1.<ce:hsp sp="0.5"></ce:hsp>Molecules with poor photophysical properties</entry><entry>320</entry></row><row><entry></entry><entry>6.2.<ce:hsp sp="0.5"></ce:hsp>Spectroscopy of isotopomers without chemical synthetization</entry><entry>326</entry></row><row><entry namest="col1" nameend="col2">7.<ce:hsp sp="0.5"></ce:hsp>Conclusions and outlook</entry><entry colname="col3">330</entry></row><row><entry namest="col1" nameend="col2">Acknowledgements</entry><entry colname="col3">332</entry></row><row><entry namest="col1" nameend="col2">Appendix A. Spin–spin interaction</entry><entry colname="col3">333</entry></row><row><entry></entry><entry>A.1. Like spins</entry><entry>333</entry></row><row><entry></entry><entry>A.2. Unlike spins</entry><entry>334</entry></row><row><entry namest="col1" nameend="col2">References</entry><entry colname="col3">335</entry></row></tbody></tgroup></ce:table></display></ce:miscellaneous><ce:abstract><ce:section-title>Abstract</ce:section-title><ce:abstract-sec><ce:simple-para>Magnetic-resonance spectroscopy on single molecules represents the ultimate limit in sensitivity of electron spin resonance: the detection of a single molecular spin. This is achieved by combining single molecule spectroscopy and optically detected magnetic resonance. Experimental results on pentacene in <ce:italic>p</ce:italic>-terphenyl both in zero-field and in the presence of a weak magnetic field demonstrate that magnetic-resonance spectroscopy on single molecules adds on to the specificity of single-molecule spectroscopy. It proved possible to identify single molecules which contain <ce:sup>13</ce:sup>C nuclei in natural abundance and to observe the splitting of the electron spin resonance line resulting from the hyperfine intercation of a single molecular spin with a single <ce:sup>13</ce:sup>C nuclear spin. From the Zeeman effect the orientation of the symmetry axes of individual molecules with respect to the direction of the external magnetic field is obtained. This allows to compare the orientation of individual molecules with their substitutional-site specific transition frequencies depending on the quality of the host crystal.</ce:simple-para></ce:abstract-sec></ce:abstract><ce:keywords class="pacs"><ce:section-title>PACS</ce:section-title><ce:keyword><ce:text>76.70.Hb</ce:text></ce:keyword><ce:keyword><ce:text>42.62.Fi</ce:text></ce:keyword><ce:keyword><ce:text>33.50.Bq</ce:text></ce:keyword></ce:keywords><ce:keywords class="keyword"><ce:section-title>Keywords</ce:section-title><ce:keyword><ce:text>Single-molecule magnetic resonance</ce:text></ce:keyword><ce:keyword><ce:text>Single-molecule spectroscopy</ce:text></ce:keyword><ce:keyword><ce:text>Pentacene in <ce:italic>p</ce:italic>-terphenyl</ce:text></ce:keyword></ce:keywords></head></converted-article></istex:document></istex:metadataXml><mods version="3.6"><titleInfo><title>Magnetic resonance of a single molecular spin</title></titleInfo><titleInfo type="alternative" contentType="CDATA"><title>Magnetic resonance of a single molecular spin</title></titleInfo><name type="personal"><namePart type="given">Jürgen</namePart><namePart type="family">Köhler</namePart><affiliation>Centre for the Study of Excited States of Molecules, Huygens Laboratory, University of Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands</affiliation><description>Tel.: +31-71-5275858; fax: +31-71-5275819; e-mail: koehler@molphys.leidenuniv.nl.</description><role><roleTerm type="text">author</roleTerm></role></name><typeOfResource>text</typeOfResource><genre type="research-article" displayLabel="Full-length article"></genre><originInfo><publisher>ELSEVIER</publisher><dateIssued encoding="w3cdtf">1998</dateIssued><dateCaptured encoding="w3cdtf">1998-03-01</dateCaptured><copyrightDate encoding="w3cdtf">1999</copyrightDate></originInfo><language><languageTerm type="code" authority="iso639-2b">eng</languageTerm><languageTerm type="code" authority="rfc3066">en</languageTerm></language><physicalDescription><internetMediaType>text/html</internetMediaType></physicalDescription><abstract lang="en">Magnetic-resonance spectroscopy on single molecules represents the ultimate limit in sensitivity of electron spin resonance: the detection of a single molecular spin. This is achieved by combining single molecule spectroscopy and optically detected magnetic resonance. Experimental results on pentacene in p-terphenyl both in zero-field and in the presence of a weak magnetic field demonstrate that magnetic-resonance spectroscopy on single molecules adds on to the specificity of single-molecule spectroscopy. It proved possible to identify single molecules which contain 13C nuclei in natural abundance and to observe the splitting of the electron spin resonance line resulting from the hyperfine intercation of a single molecular spin with a single 13C nuclear spin. From the Zeeman effect the orientation of the symmetry axes of individual molecules with respect to the direction of the external magnetic field is obtained. This allows to compare the orientation of individual molecules with their substitutional-site specific transition frequencies depending on the quality of the host crystal.</abstract><note>editor: S.D. Peyerimhoff Contents 1.Introduction 264 2. The principles of optically detected magnetic-resonance 267 2.1.Photophysics of pentacene 267 2.2.The triplet state and the spin Hamiltonian 269 2.3.Fluorescence-detected magnetic-resonance 272 3.Optical single-molecule spectroscopy 277 3.1.The principle of single-molecule spectroscopy in solids 277 3.2.Optical saturation 280 4.Experimental 282 4.1.Sample preparation 283 4.2.The experimental set-up 283 4.3.The cryogenic insert 285 4.4.Illustrations 288 5.Single-spin spectroscopy 290 5.1.Basic considerations 290 5.2.Magnetic-resonance experiments on single pentacene molecules 293 5.3.Microwave transition frequency versus spectral site O1 or O2 298 5.4.The hyperfine interaction of a single-triplet spin with a single-nuclear spin 303 6.``Spin-off” 320 6.1.Molecules with poor photophysical properties 320 6.2.Spectroscopy of isotopomers without chemical synthetization 326 7.Conclusions and outlook 330 Acknowledgements 332 Appendix A. Spin–spin interaction 333 A.1. Like spins 333 A.2. Unlike spins 334 References 335</note><note type="content">Fig. 2.1: (a) Molecular structure and axis system of pentacene. (b) Energy level diagram of the lowest excited states of pentacene. The full arrows indicate the optical excitation-emission cycle (transition rates k0 and k1) and the dotted arrows represent the intersystem crossings (rates kISC1 and kISC2).</note><note type="content">Fig. 2.2: The crystal structure of p-terphenyl in the triclinic phase (below 193K). P1 to P4 denote the four inequivalent crystal sites.</note><note type="content">Fig. 2.3: Schematic representation of the pumping cycle for pentacene. The relative steady-state populations of the three triplet zero-field states are indicated by the circles. The arrows sketch the relative magnitude of the decay rates for pentacene in p-terphenyl (see Table 1).</note><note type="content">Fig. 2.4: The three optically detected zero-field transitions of the lowest excited triplet state of pentacene for an ensemble of molecules. From top to bottom the respective transitions are between Tx–Tz, Ty–Tz, an Tx–Ty as indicated. Each spectrum corresponds to a decrease of the fluorescence intensity with respect to the steady-state fluorescence intensity without microwaves. The vertical scale is different for each spectrum. The Tx–Ty spectrum has been obtained from a different sample and using different microwave equipment with respect to the other two spectra.</note><note type="content">Fig. 2.5: Labelling scheme of the six inequivalent carbon positions of pentacene. The magnitude of the triplet spin density at the respective carbon positions is indicated by the diameter of the circles, see Table 2.</note><note type="content">Fig. 2.6: Comparison of the magnetic-resonance lineshape of the Tx–Tz transition for pentacene-h14 in p-terphenyl-h14 (top) and pentacene-d14 in p-terphenyl-d14 (bottom). The signals correspond to a decrease of the fluorescence. The full width at half maximum (FWHM) for each line is given in the figure.</note><note type="content">Fig. 3.1: Schematic representation of the phenomenon of inhomogeneous line broadening in solids. In the upper right three different impurity molecules are sketched being subjected to different local environments due to random strain. For the ensemble of molecules this results in a broad distribution of transition frequencies. Using a narrow band laser a subensemble of molecules can be selected (line centre) or tuning the laser into the wing a single molecule is isolated. The figure has been taken from [43].</note><note type="content">Fig. 3.2: (a) The inhomogeneously broadened fluorescence excitation line of an ensemble of pentacene molecules in p-terphenyl. The “noisy” structure is denoted as statistical fine structure and is highly repeatable. (b) Enlarged view of the circled region of part (a) of the figure resolving several fluorescence excitation lines of individual molecules. The spectra have been taken from [46].</note><note type="content">Fig. 4.1: Schematic representation of the experimental set-up. Details are given in the text.</note><note type="content">Fig. 4.2: (a) Front view of the sample mounting assembly along the x-axis. The large inner circle indicates the suprasil cover plate. The three, small, open circles refer to the dots of vacuum grease which glue the cover plate to the LiF substrate. The black dot represents the aluminium mirror used for the autocollimation and the horizontal strip indicates the area accessible by the laser focus. (b) A simplified section of the insert in the x–z plane. L is the focusing lens (f=10mm), P the parabolic mirror (f=5mm), S the sample, C the suprasil cover plate, I the microwave coil, and B is the beam block. M indicates the superconducting magnet which is mounted in a Helmholtz configuration with the magnetic field along the z-axis.</note><note type="content">Fig. 4.3: (a) A section of the insert in the y–z plane showing the principle of the horizontal adjustment of the sample. (b) A section of the insert in the x–z plane showing the principle of the adjustment of the focusing lens. For further explanation see the text.</note><note type="content">Fig. 4.4: The lower trace shows the fluorescence-excitation spectrum of a single pentacene molecule which is embedded in a p-terphenyl host crystal. The spectrum has been recorded with an incident laser power of 5nW corresponding to 25mW/cm2 in focus. The line exhibits a Lorentzian profile with a width of 11.0MHz (FWHM). The upper trace is the output of the 750mm long Fabry-Pérot Interferometer recorded simultaneously to provide the calibration of the laser frequency axis in relative units. The spacing between the two modes is 100MHz. The vertical scale is valid for the lower trace.</note><note type="content">Fig. 4.5: Fluorescence-excitation spectra of a single molecule of pentacene in p-terphenyl by scanning the laser over a range of 500MHz in the red wing of the inhomogeneously broadened 1S1←1S0 transition for the O1 spectral site. The laser frequency is given in relative units. The spectra have been obtained for different settings of the dial controlling the position of the focusing lens L. The laser power incident to the cryostat was 5nW corresponding to 25mW/cm2 when in focus. The vertical scale is valid for the lowest trace while all other traces are offset for clarity. For further explanation see text.</note><note type="content">Fig. 5.1: A schematic representation of a magnetic-resonance experiment of a single molecule. The upper panel sketches the situation without microwaves and the lower panel gives the situation in presence of a microwave field which is in resonance with the Tx–Tz transition. Under influence of the incident laser radiation the pentacene molecule carries out excitation-emission cycles (situations 1 and 3). This process is interrupted when the molecule crosses over to the triplet state. With highest probability it ends up in the Tx sublevel where it resides on the average for 50μs, a period during which no fluorescence is observable (situation 2). In the presence of the microwave field the triplet mean residence time of the molecule is prolonged to about 70μs (situation 4). Below the energy-level diagrams the temporal distribution of the emitted photons is represented by the stripes.</note><note type="content">Fig. 5.2: Dependence of the emission rate R for a single pentacene molecule on the incident laser intensity for two different values of the mean triplet residence time. The upper curve refers to a situation without microwaves and the lower curve is valid for a situation where a microwave field saturates the Tx–Tz transition. The calculation is based on the values reported in [92] but has been modified according to the numbers given in [83], see also Table 1.</note><note type="content">Fig. 5.3: Two fluorescence-excitation spectra of the same pentacene molecule. For the lower trace the incident laser intensity is 5nW corresponding to 25mW/cm2 in focus and for the upper spectrum the intensity is increased to 100nW equivalent to 500mW/cm2. While the background increases by a factor of about 20 from 105 counts per second (cps) to 2200cps the single molecule signal grows only from 1000 to 2700cps. The vertical scale is valid for both spectra.</note><note type="content">Fig. 5.4: (a) FMDR spectrum of the Tx–Tz transition of a single pentacene molecule in a p-terphenyl host crystal. The molecule is excited at the fixed laser frequency ν1 in the red wing of the inhomogeneously broadened O1 ensemble line. For each dot the signal is accumulated for 10s while the lines serve to guide the eye. The average photon-count rate during acquisition is 2×104, and the laser intensity is 200mW/cm2. The signal corresponds to the decrease of the fluorescence relative to the situation without microwaves. (b) No FDMR signal is visible when the laser frequency is held at ν2. In the inset two consecutively registered fluorescence-excitation spectra of the molecule are shown. The spectra are recorded with a lower laser intensity than that used for the FDMR signals.</note><note type="content">Fig. 5.5: Comparison of the lineshapes of the Ty–Tz magnetic-resonance transition for a single molecule (top) and an ensemble of about 104 molecules (bottom). Both signals correspond to a decrease of the fluorescence intensity. The ensemble spectrum is recorded with the laser tuned to the top of the inhomogeneously broadened O1 absorption line whereas the molecule is selected in the red wing of this line.</note><note type="content">Fig. 5.6: Hyperfine splitting of the pentacene triplet sublevels in zero-field. The eigenvalues are obtained through a full diagonalization of the Hamiltonian given in Eq. (5.6)with terms for all fourteen proton spins. The lowest box displays the result on a continuous scale while the upper three boxes show expanded views for the three electronic substates.</note><note type="content">Fig. 5.7: Tx–Tz transition of a single molecule for different incident microwave power which is given in dBm (0dBm corresponds to 10mW) and decreases from top to bottom. For better comparison of the lineshapes the spectra are normalised. The peak FDMR effect at the maximum, corrected for the background, is 10.2% (5dBm), 10.2% (2.5dBm), 7.7% (0dBm), and 6.3% (−3dBm), respectively.</note><note type="content">Fig. 5.8: Tx–Tz transition of pentacene in p-terphenyl for various experimental conditions. (a) and (b) Conventional FDMR spectra for large ensembles of molecules from a Bridgman grown crystal (10−5mol pentacene per mol p-terphenyl) with optical excitation in the O1 and O2 spectral sites, respectively. The vertical scale is in arbitrary units. (c)–(e) Single-molecule FDMR spectra for a thin sublimed crystal with excitation wavelengths of the laser to the red of the O1 ensemble line. (f) and (g) Single-molecule FDMR spectra of the same crystal flake for a molecule selected to the blue of the O2 ensemble line. For spectra (c)–(f) the spacing of the dots is 500kHz, for spectrum (g) the spacing is 250kHz. For the single molecule spectra the vertical scale represents the decrease of the fluorescence counts per ten seconds.</note><note type="content">Fig. 5.9: Comparison of the onsets of the Tx–Tz transition for ten different single molecules of sample 2. The vertical bars labelled O1 and O2 indicate the onset of the transition for the respective ensembles of the same sample volume where the single molecules have been isolated.</note><note type="content">Fig. 5.10: Comparison of the shift of the Tx–Tz transition in a magnetic field for two different pentacene molecules. Molecule V is selected close to the O1 ensemble line whereas molecule VI is selected in the vicinity of the O2 line. For better comparison all spectra are normalised. The background corrected maximum FDMR effect amounts for molecule V to 10.5% (zero-field), 7% (3.8mT), and 5.2% (7.5mT) and for molecule VI to 7.7% (zero-field) and 6% (5.6mT), respectively.</note><note type="content">Fig. 5.11: The fluorescence-excitation spectrum for the O1 (top) and O2 (bottom) spectral sites of pentacene in p-terphenyl. The intensity scale in each spectrum is valid for the lowest traces, each recorded with an incident laser power of 1nW. For the other traces the following parameters are valid: O1 b) offset 50, scale factor 2.5, laser power 5nW, c) offset 150, scale factor 5, laser power 10nW, d) offset 150, scale factor 5, laser power 100nW; O2 b) offset 250, scale factor 10, laser power 1nW, c) offset 750, scale factor 2.5, laser power 10nW, d) offset 450, scale factor 10, laser power 100nW. The O2 spectrum shows a larger fluorescence count-rate with respect to the O1 spectrum because the incident excitation light is stronger focused onto the sample in order to reduce the probe volume. This leads to an increase of the statistical fine structure.</note><note type="content">Fig. 5.12: Labelling scheme of the six inequivalent carbon positions of pentacene.</note><note type="content">Fig. 5.13: The upper box displays the fluorescence-excitation spectrum of the O1 spectral region. The location of the optical absorption frequencies of three different molecules are indicated by the arrows. The lower three boxes show the respective FDMR spectra for the Ty–Tz transition of these molecules. The additional broadening of the FDMR line observed for molecules II and III is interpreted in terms of the hyperfine interaction of the triplet spin with a single 13C nucleus. The position of the 13C nucleus within the molecule is indicated and is obtained from the analysis of the hyperfine interaction which considers the triplet spin density distribution across the molecule.</note><note type="content">Fig. 5.14: Simulations of the hyperfine broadened Ty–Tz transition for pentacene with either no or one 13C in the ε or γ position. The transition probabilities for microwaves polarised along the molecular x-axis are summed into histograms of 20kHz wide bins. The hyperfine eigenstates are obtained from a diagonalization of the zero-field Hamiltonian with terms for all but the protons bound to carbons in positions β. The histograms are compared with the corresponding experimental FDMR ensemble spectra.</note><note type="content">Fig. 5.15: Comparison of the Ty–Tz magnetic-resonance spectra observed for single molecules with ensemble spectra for a 13C substitution in position ε (left) and γ (right). The ensemble spectra were obtained by tuning the laser into resonance with satellite 4 or satellite 5 of the O1 fluorescence-excitation spectrum, respectively. The single molecules were located by tuning the laser into the wing of the respective satellite line.</note><note type="content">Fig. 5.16: FDMR spectra for the Ty–Tz transition of two doubly 13C substituted pentacene molecules. The molecules are selected optically in-between the combination bands labelled C and D in Fig. 5.11. The positions of the 13C substitutions within the molecule correspond to ε–γ (or symmetry equivalent) for molecule IV, and γ–γ for molecule V, as indicated. The assignment is obtained from the analysis of the hyperfine interactions.</note><note type="content">Fig. 5.17: Comparison of the Tx–Tz magnetic-resonance transition for two pentacene molecules in zero-field and in a magnetic field of 9.4mT. Molecule VI is selected in the red wing of the O1 fluorescence-excitation line while molecule VII is selected close to satellite 5. From the splitting of the FDMR line of molecule VII for a magnetic field of 9.4mT it is concluded that it contains a 13C nucleus in the γ position of pentacene as indicated in the figure. For better comparison all spectra are normalised. The background corrected peak FDMR effect amounts for molecule VI to 4.8% (zero-field), and 4% (9.4mT) and for molecule VII to 5.1% (zero-field), and 3.7% (5.6mT), respectively.</note><note type="content">Fig. 5.18: Comparison of the Tx–Tz magnetic-resonance transition for a pentacene-h14 (top), and a pentacene-d14 (bottom) molecule. The respective linewidth of the spectra are 4.3MHz for the upper and 440kHz for the lower trace. Both spectra are recorded with medium microwave power incident onto the sample. The background corrected relative decrease of the fluorescence intensity is 10% for both spectra.</note><note type="content">Fig. 5.19: Fluorescence-excitation spectrum of the O1 spectral site of pentacene-d14 in p-terphenyl-d14. The arrows indicate the spectral positions of molecules I and II, respectively.</note><note type="content">Fig. 5.20: Comparison of the Tx–Tz magnetic-resonance transition for two different pentacene-d14 molecules. Molecule I has been selected to the red of the main line while molecule II has been selected to the blue of satellite 3, as sketched by the arrows in Fig. 5.19. The insets indicate the molecular constitution as obtained from the analysis of the hyperfine interaction. The vertical scale in each spectrum represents the background corrected relative decrease of the fluorescence intensity.</note><note type="content">Fig. 5.21: Comparison of the Tx–Tz magnetic-resonance transition for two different pentacene-d14 molecules. The splitting of the lines could be attributed to the hyperfine interaction of the triplet spin with two 13C nuclear spins. The location of the 13C substitutions within the molecules correspond to the indicated (or symmetry-equivalent) positions. For convenience both spectra are normalised. The background corrected FDMR effect amounts to 7.7% for molecule III and to 6.2% for molecule IV.</note><note type="content">Fig. 5.22: Fluorescence-detected Tx–Tz magnetic-resonance transition for three different pentacene-d14 molecules in weak magnetic fields. The vertical scale is different for each molecule. The background corrected FDMR effects are for molecule VII: 18.9% (zero-field), 13.2% (1.9mT), 10.9% (3.8mT), and 7.5% (7.5mT); for molecule VIII: 10% (zero-field), 8.1% (1.9mT), 6.6% (3.8mT), and 4.2% (7.5mT); and for molecule IX: 13.3% (zero-field), and 6% (3.8mT).</note><note type="content">Fig. 6.1: Dependence of the Tx–Tz and Ty–Tz transition frequencies on the strength of the external magnetic field for the spectral sites O1/O4 (top) and O2/O3 (bottom).</note><note type="content">Fig. 6.2: Fluorescence-excitation spectra of the spectral sites O1–O4. For better comparison the spectra are displayed on a relative scale and are shifted such that their strongest lines coincide at zero. Spectral features labelled 1–5 result from 13C substituted pentacene molecules. The absolute positions of the main lines and the respective shifts of the satellites are given in Table 14. The spectra have been recorded with an incident laser power of 125mW/cm2 (O1), 50mW/cm2 (O2), 250mW/cm2 (O3), and 500mW/cm2 (O4).</note><note type="content">Fig. 6.3: Fluorescence-excitation spectra of pentacene-h14 in p-terphenyl-h14 in the spectral region of the 1S1←1S0 transition of the O1 spectral site for different positions of the sample in the exciting beam. The sample position is given relative to the lowest spectrum for which the position was set arbitrarily to zero.</note><note type="content">Fig. 6.4: Fluorescence-excitation spectra of pentacene-d14 in p-terphenyl-d14 in the spectral region of the 1S1←1S0 transition of the O1 spectral site for different positions of the sample in the exciting beam. The sample position is given relative to the lowest spectrum for which the position was set arbitrarily to zero.</note><note type="content">Fig. 6.5: Fluorescence-excitation spectrum of the O1/O2 spectral region for pentacene-d14 in p-terphenyl-d14. The bars above the spectrum assign spectral features to the O1 or O2 spectral site. For explanation see text.</note><note type="content">Fig. 6.6: Comparison of the lineshape of the Tx–Tz magnetic-resonance transition for optically selected ensembles of pentacene molecules. For better comparison the spectra have been normalised. The ensembles have been selected by tuning the laser frequency to the top of the respective ensemble zero-phonon line. The left hand part of the figure features pentacene-h14 molecules which contain a single or two 13C nuclei the positions of which are indicated by the black dots. From top to bottom the laser has been tuned into resonance with O1, satellite 4 (O1), satellite 5 (O1), satellite D (O2), and satellite C (O2) (cf. Fig. 5.10). The right hand part of the figure shows deuterated pentacene molecules which contain exclusively 12C nuclei but for which the deuteration was incomplete. The positions of the 1H nuclei within the molecules are indicated by the black ellipses. From top to bottom the laser has been tuned into resonance with O1, satellite 3, satellite 7, and satellite 6 (cf. Fig. 6.5).</note><note type="content">Table 1: The relative steady-state populations nu∼pu/ku and decay rates ku(u=x,y,z) for a single pentacene molecule in p-terphenyl (O1 or O2 spectral site) [83] and naphthalene [67]. The relative populations have been normalized to obtain 1 for the largest value</note><note type="content">Table 2: Spin density distribution of the lowest excited triplet state of pentacene. The data have been taken from [88]</note><note type="content">Table 3: FDMR transition frequencies and deduced triplet sublevel energies (divided by Planck’s constant h) in MHz for large ensembles of pentacene molecules (1013 molecules) in the O1 or O2 spectral sites of p-terphenyl, respectively</note><note type="content">Table 4: Frequency of the Tx–Tz transition for molecules V and VI in weak external magnetic fields. The calculation of the transition frequency is based on the diagonalization of the spin Hamiltonian (Eq.(5.7)), details are given in the text</note><note type="content">Table 5: The positions and relative intensities of the satellites 1–5. The positions are given as shifts with respect to the main lines located at 16882.739cm−1 (O1) and 16886.452cm−1 (O2). The intensities are normalised so as to have the total intensity of satellites 2–5 add up to 18. For satellites that show structure the position of each maximum is given</note><note type="content">Table 6: Assignment of the satellites observed in the O1 and O2 fluorescence-excitation spectra. The ε and the γ assignment is confirmed experimentally. Other assignments are arrived at through comparative reasoning (see text). For satellites that show splittings the average shift is given. The last column gives the result of a QCFF/PI calculation for the isotope shift for a 13C in the respective position of pentacene [136]</note><note type="content">Table 7: Isotope shifts of the 13C13C pentacene satellite bands A–D with respect to the O1 or O2 zero-phonon line. For O2 the positions of the subsatellites are less accurate due to the statistical fine structure. In the assignment column the experimentally observed shifts are compared with combinations of shifts observed for satellites 2–5</note><note type="content">Table 8: Lineposition, linewidth and linesplitting (MHz) of the Tx–Tz transition for two single molecules in zero-field and in a field of 9.4mT. The calculated values are obtained from the diagonalization of the spin Hamiltonian including the zero-field, the Zeeman and the hyperfine interaction for a 13C substitution either in the ε or γ-position of pentacene</note><note type="content">Table 9: Positions and assignments of the satellites 1–3 for pentacene-d14 in p-terphenyl-d14. The positions are given as shifts with respect to the main line at 16915.529cm−1. The assignments are based on the similarity of the observed shifts with those for the protonated system, see Section 5.4.1</note><note type="content">Table 10: Principal values of the total hyperfine tensor (anisotropic plus isotropic part) for 13C, 2H, and for comparison 1H. All values are given in MHz</note><note type="content">Table 11: Observed and calculated splittings of the Tx–Tz magnetic-resonance transition for several pentacene-d14 molecules. The assignments of the positions of the respective 13C substitutions are based on the analysis of the hyperfine interaction in terms of second-order perturbation theory</note><note type="content">Table 12: Comparison of the calculated and observed hyperfine splittings for various molecules. The labels ε and γ refer to the position of the 13C nucleus within the pentacene-d14 molecule</note><note type="content">Table 13: Transition frequencies and sublevels energies X,Y,Z (divided by Planck’s constant h) for the lowest triplet state of pentacene-h14 in p-terphenyl-h14 and of pentacene-d14 in p-terphenyl-d14. All values are given in MHz</note><note type="content">Table 14: Positions of the zero-phonon lines O1–O4 in cm−1 and relative positions of the satellite lines</note><note type="content">Table 15: Intensity distribution of the satellite lines in the different spectral sites. The intensity of satellite 4 has been normalised to 4 for each spectral site. The numbers in parenthesis are deduced from the known total intensity of 22</note><note type="content">Table 16: Shifts and assignments of the additional zero-phonon lines observed in Fig. 6.5 with respect to the main lines. The numbering of the features corresponds to the numbers given above the spectrum in Fig. 6.5 for O1 and O2, respectively. In the assignment column the carbon position to which the proton is bound is referred to by the subscript, where ε1 and ε2 distinguish the two non-equivalent ε positions in pentacene. The calculated values have been obtained by summing the individual shifts of the respective substitutions. Assignments marked with an asterisk have been confirmed by FDMR, see Fig. 6.6</note><subject><genre>PACS</genre><topic>76.70.Hb</topic><topic>42.62.Fi</topic><topic>33.50.Bq</topic></subject><subject><genre>Keywords</genre><topic>Single-molecule magnetic resonance</topic><topic>Single-molecule spectroscopy</topic><topic>Pentacene in p-terphenyl</topic></subject><relatedItem type="host"><titleInfo><title>Physics Reports</title></titleInfo><titleInfo type="abbreviated"><title>PLREP</title></titleInfo><genre type="Journal">journal</genre><originInfo><dateIssued encoding="w3cdtf">199903</dateIssued></originInfo><identifier type="ISSN">0370-1573</identifier><identifier type="PII">S0370-1573(00)X0221-9</identifier><part><date>199903</date><detail type="volume"><number>310</number><caption>vol.</caption></detail><detail type="issue"><number>5–6</number><caption>no.</caption></detail><extent unit="issue pages"><start>261</start><end>352</end></extent><extent unit="pages"><start>261</start><end>339</end></extent></part></relatedItem><identifier type="istex">CE5235C68496649D77502A841749C0A07CBBF079</identifier><identifier type="DOI">10.1016/S0370-1573(98)00057-X</identifier><identifier type="PII">S0370-1573(98)00057-X</identifier><accessCondition type="use and reproduction" contentType="">© 1999Elsevier Science B.V.</accessCondition><recordInfo><recordContentSource>ELSEVIER</recordContentSource><recordOrigin>Elsevier Science B.V., ©1999</recordOrigin></recordInfo></mods></metadata><enrichments><istex:catWosTEI uri="https://api.istex.fr/document/CE5235C68496649D77502A841749C0A07CBBF079/enrichments/catWos"><teiHeader><profileDesc><textClass><classCode scheme="WOS">PHYSICS</classCode><classCode scheme="WOS">PHYSICS, MULTIDISCIPLINARY</classCode></textClass></profileDesc></teiHeader></istex:catWosTEI></enrichments><serie></serie></istex></record>Pour manipuler ce document sous Unix (Dilib)
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