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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Direct experimental observation of weakly-bound character of the attached electron in
europium anion</title><author><name sortKey="Cheng, Shi Bo" sort="Cheng, Shi Bo" uniqKey="Cheng S" first="Shi-Bo" last="Cheng">Shi-Bo Cheng</name><affiliation><nlm:aff id="a1"><institution>Department of Chemistry, The Pennsylvania State University</institution>, University Park, Pennsylvania 16802,<country>United States</country></nlm:aff></affiliation></author><author><name sortKey="Castleman, A W" sort="Castleman, A W" uniqKey="Castleman A" first="A. W." last="Castleman">A. W. Castleman</name><affiliation><nlm:aff id="a1"><institution>Department of Chemistry, The Pennsylvania State University</institution>, University Park, Pennsylvania 16802,<country>United States</country></nlm:aff></affiliation><affiliation><nlm:aff id="a2"><institution>Department of Physics, The Pennsylvania State University</institution>, University Park, Pennsylvania 16802,<country>United States</country></nlm:aff></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">26198741</idno><idno type="pmc">4510523</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4510523</idno><idno type="RBID">PMC:4510523</idno><idno type="doi">10.1038/srep12414</idno><date when="2015">2015</date><idno type="wicri:Area/Pmc/Corpus">000088</idno><idno type="wicri:explorRef" wicri:stream="Pmc" wicri:step="Corpus" wicri:corpus="PMC">000088</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Direct experimental observation of weakly-bound character of the attached electron in
europium anion</title><author><name sortKey="Cheng, Shi Bo" sort="Cheng, Shi Bo" uniqKey="Cheng S" first="Shi-Bo" last="Cheng">Shi-Bo Cheng</name><affiliation><nlm:aff id="a1"><institution>Department of Chemistry, The Pennsylvania State University</institution>, University Park, Pennsylvania 16802,<country>United States</country></nlm:aff></affiliation></author><author><name sortKey="Castleman, A W" sort="Castleman, A W" uniqKey="Castleman A" first="A. W." last="Castleman">A. W. Castleman</name><affiliation><nlm:aff id="a1"><institution>Department of Chemistry, The Pennsylvania State University</institution>, University Park, Pennsylvania 16802,<country>United States</country></nlm:aff></affiliation><affiliation><nlm:aff id="a2"><institution>Department of Physics, The Pennsylvania State University</institution>, University Park, Pennsylvania 16802,<country>United States</country></nlm:aff></affiliation></author></analytic><series><title level="j">Scientific Reports</title><idno type="eISSN">2045-2322</idno><imprint><date when="2015">2015</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p>Direct experimental determination of precise electron affinities (EAs) of lanthanides
is a longstanding challenge to experimentalists. Considerable debate exists in
previous experiment and theory, hindering the complete understanding about the
properties of the atomic anions. Herein, we report the first precise photoelectron
imaging spectroscopy of europium (Eu), with the aim of eliminating prior
contradictions. The measured EA
(0.116 ± 0.013 eV) of Eu is in
excellent agreement with recently reported theoretical predictions, providing direct
spectroscopic evidence that the additional electron is weakly attached.
Additionally, a new experimental strategy is proposed that can significantly
increase the yield of the lanthanide anions, opening up the best opportunity to
complete the periodic table of the atomic anions. The present findings not only
serve to resolve previous discrepancy but also will help in improving the depth and
accuracy of our understanding about the fundamental properties of the atomic
anions.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Drzaic, P S" uniqKey="Drzaic P">P. S. Drzaic</name></author><author><name sortKey="Marks, J" uniqKey="Marks J">J. Marks</name></author><author><name sortKey="Brauman, J X0a I" uniqKey="Brauman J">J.
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<front><journal-meta><journal-id journal-id-type="nlm-ta">Sci Rep</journal-id><journal-id journal-id-type="iso-abbrev">Sci Rep</journal-id><journal-title-group><journal-title>Scientific Reports</journal-title></journal-title-group><issn pub-type="epub">2045-2322</issn><publisher><publisher-name>Nature Publishing Group</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">26198741</article-id><article-id pub-id-type="pmc">4510523</article-id><article-id pub-id-type="pii">srep12414</article-id><article-id pub-id-type="doi">10.1038/srep12414</article-id><article-categories><subj-group subj-group-type="heading"><subject>Article</subject></subj-group></article-categories><title-group><article-title>Direct experimental observation of weakly-bound character of the attached electron in
europium anion</article-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>Cheng</surname><given-names>Shi-Bo</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Castleman</surname><given-names>A. W.</given-names><suffix>Jr.</suffix></name><xref ref-type="corresp" rid="c1">a</xref><xref ref-type="aff" rid="a1">1</xref><xref ref-type="aff" rid="a2">2</xref></contrib><aff id="a1"><label>1</label><institution>Department of Chemistry, The Pennsylvania State University</institution>, University Park, Pennsylvania 16802,<country>United States</country></aff><aff id="a2"><label>2</label><institution>Department of Physics, The Pennsylvania State University</institution>, University Park, Pennsylvania 16802,<country>United States</country></aff></contrib-group><author-notes><corresp id="c1"><label>a</label><email>awc@psu.edu</email></corresp></author-notes><pub-date pub-type="epub"><day>22</day><month>07</month><year>2015</year></pub-date><pub-date pub-type="collection"><year>2015</year></pub-date><volume>5</volume><elocation-id>12414</elocation-id><history><date date-type="received"><day>09</day><month>04</month><year>2015</year></date><date date-type="accepted"><day>22</day><month>06</month><year>2015</year></date></history><permissions><copyright-statement>Copyright © 2015, Macmillan Publishers Limited</copyright-statement><copyright-year>2015</copyright-year><copyright-holder>Macmillan Publishers Limited</copyright-holder><license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by/4.0/"><pmc-comment>author-paid</pmc-comment>
<license-p>This work is licensed under a Creative Commons Attribution 4.0
International License. The images or other third party material in this article are
included in the article’s Creative Commons license, unless indicated
otherwise in the credit line; if the material is not included under the Creative
Commons license, users will need to obtain permission from the license holder to
reproduce the material. To view a copy of this license, visit <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by/4.0/">http://creativecommons.org/licenses/by/4.0/</ext-link></license-p></license></permissions><abstract><p>Direct experimental determination of precise electron affinities (EAs) of lanthanides
is a longstanding challenge to experimentalists. Considerable debate exists in
previous experiment and theory, hindering the complete understanding about the
properties of the atomic anions. Herein, we report the first precise photoelectron
imaging spectroscopy of europium (Eu), with the aim of eliminating prior
contradictions. The measured EA
(0.116 ± 0.013 eV) of Eu is in
excellent agreement with recently reported theoretical predictions, providing direct
spectroscopic evidence that the additional electron is weakly attached.
Additionally, a new experimental strategy is proposed that can significantly
increase the yield of the lanthanide anions, opening up the best opportunity to
complete the periodic table of the atomic anions. The present findings not only
serve to resolve previous discrepancy but also will help in improving the depth and
accuracy of our understanding about the fundamental properties of the atomic
anions.</p></abstract></article-meta></front><body><p>Owing to the existence of abundant unpaired <italic>f</italic> electrons, lanthanide elements form
a very important group in the periodic table, which are highly valuable to many modern
technologies, including clean energy, consumer electronics, and advanced transportation,
etc. Unlike main group elements, however, the knowledge of the fundamental
physicochemical properties of the lanthanides is extremely limited, which hinders the
complete understanding about the properties of atoms. Specifically, one of the greatest
concerns that has puzzled the experimentalists and theoreticians for several decades is
the electron affinities (EAs), which can be viewed as one of the most important
properties in ionic chemistry<xref ref-type="bibr" rid="b1">1</xref><xref ref-type="bibr" rid="b2">2</xref><xref ref-type="bibr" rid="b3">3</xref>, of the lanthanide atoms. Note that,
as the simplest systems, research on the EAs of atoms can be traced to 1950s and
1960s<xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b5">5</xref><xref ref-type="bibr" rid="b6">6</xref>. Pioneered by Branscomb <italic>et al</italic>., the
photodetachment of the atomic H<sup>−</sup> and
D<sup>−</sup> have been performed in a modulated crossed-beam
experiment<xref ref-type="bibr" rid="b4">4</xref>. Subsequently, Lineberger and co-workers made a
significant contribution to this scientific field. Advanced dye lasers have been
employed by them to measure the EAs of elements via high-resolution threshold
photodetachment spectroscopy<xref ref-type="bibr" rid="b7">7</xref><xref ref-type="bibr" rid="b8">8</xref>, enabling them to obtain the total
photodetachment cross sections of a series of atomic anions. Although the negative ion
properties of many elements have been reported<xref ref-type="bibr" rid="b9">9</xref><xref ref-type="bibr" rid="b10">10</xref><xref ref-type="bibr" rid="b11">11</xref><xref ref-type="bibr" rid="b12">12</xref>, the
information about experimental EAs of the lanthanide atoms is still limited, or even
conflictive with respect to the theoretical predictions.</p><p>It is well-accepted that the experiments and theoretical calculations on lanthanides are
particularly challenging. Theoretically, the large number of electrons and presence of
open shells (<italic>d</italic> and/or <italic>f</italic>) in lanthanides result in extremely complicated
calculations on the electronic structures of these heavy elements<xref ref-type="bibr" rid="b12">12</xref><xref ref-type="bibr" rid="b13">13</xref>.
Experimentally, it is quite challenging to produce sufficient anions that can be used in
the photodetachment experiments. This situation is especially true for most of the
lanthanide anions except La<sup>−</sup> and
Ce<sup>−</sup> since the yields of the latter were found three
orders of magnitude greater than those of other lanthanide anions<xref ref-type="bibr" rid="b14">14</xref>. In
early 2000s, a series of measurements of EAs of lanthanides were attempted by Davis and
Thompson, including Eu<xref ref-type="bibr" rid="b15">15</xref>, Tm<xref ref-type="bibr" rid="b16">16</xref>, and Pr<xref ref-type="bibr" rid="b17">17</xref>.
EAs of ~1 eV were reported for these lanthanides, implying a
relatively strong interaction between the extra electron and the neutral. These findings
were considered as a breakthrough in atomic negative ions field. Subsequent high-level
theoretical calculations, however, raised questions about these measurements<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b19">19</xref><xref ref-type="bibr" rid="b20">20</xref>. Theoretical EAs of most lanthanides are only dozens or hundreds
of meV, much smaller than previous experimental results. In some cases, the experimental
EAs are one order of magnitude larger than theory, <italic>e.g</italic>., Eu. The EA of Eu was
measured to be 1.053 ± 0.025 eV
(strongly-bound)<xref ref-type="bibr" rid="b15">15</xref>, while the theoretical values are about 0.117 and
0.116 eV (weakly-bound), respectively<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b20">20</xref>. Note that only a
rough lower limit of the EA of Eu (≥0.05 eV) was estimated by
Nadeau <italic>et al</italic>. due to the limitations of the experimental technique<xref ref-type="bibr" rid="b14">14</xref>. Such a significant discrepancy between experiment and theory clearly shows the
challenge in obtaining accurate EAs of lanthanides. It is necessary to mention that some
recent studies measured the EA of another lanthanide, Ce, whose yield of the anion is
much higher than that of Eu<sup>−14</sup>. Although Davis and Thompson
suggested a 0.955 ± 0.026 eV EA for Ce
based on their LPES experiment<xref ref-type="bibr" rid="b21">21</xref>, a subsequent reinterpretation of the
LPES data claimed an EA of 0.660 eV<xref ref-type="bibr" rid="b22">22</xref>, which is consistent
with later experimental results along with the theoretical predictions<xref ref-type="bibr" rid="b19">19</xref><xref ref-type="bibr" rid="b23">23</xref><xref ref-type="bibr" rid="b24">24</xref><xref ref-type="bibr" rid="b25">25</xref><xref ref-type="bibr" rid="b26">26</xref>. Thus, no significant discrepancy exists in Ce, which is
completely different from the present case, Eu. As for Eu, therefore, a central and
important question is: can we increase the yield of Eu<sup>−</sup> to a
detectable level and then understand the true interaction between the additional
electron and the neutral in Eu<sup>−</sup> ion?</p><p>We explored this question by utilizing the photoelectron spectroscopy, which has been
proven to be a powerful approach to directly probe the electronic properties of atoms
and clusters<xref ref-type="bibr" rid="b27">27</xref><xref ref-type="bibr" rid="b28">28</xref><xref ref-type="bibr" rid="b29">29</xref><xref ref-type="bibr" rid="b30">30</xref><xref ref-type="bibr" rid="b31">31</xref><xref ref-type="bibr" rid="b32">32</xref><xref ref-type="bibr" rid="b33">33</xref><xref ref-type="bibr" rid="b34">34</xref><xref ref-type="bibr" rid="b35">35</xref><xref ref-type="bibr" rid="b36">36</xref><xref ref-type="bibr" rid="b37">37</xref><xref ref-type="bibr" rid="b38">38</xref><xref ref-type="bibr" rid="b39">39</xref><xref ref-type="bibr" rid="b40">40</xref>.
Herein, we present direct experimental observations on the features of the electron-atom
interaction in Eu<sup>−</sup>. The EA was measured to be
0.116 ± 0.013 eV, representing a
weakly-bound character between the extra electron and Eu, which is in outstanding
agreement with recently reported high-level theoretical calculations with the values of
0.117 and 0.116 eV, respectively<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b20">20</xref>. The present finding
reveals the first precise experimental EA of Eu, clearly eliminating the longstanding
discrepancy in previous experiment and theory. Also, the new experimental strategy
proposed herein has been found successful in producing detectable lanthanide anions,
providing the best opportunity in completing the periodic table of the negative
ions.</p><sec disp-level="1" sec-type="results"><title>Results</title><sec disp-level="2"><title>Mass spectrum of the europium (Eu) anion</title><p>The greatest challenge hindering the attainment of correct EAs of most of the
lanthanides is the difficulty of producing sufficient anions that can be used in
the photodetachment experiments utilizing conventional experimental method. In
photodetachment experiments, helium (He) or argon (Ar) is widely used as an
effective expansion or cooling gas to produce pure atomic or cluster anions<xref ref-type="bibr" rid="b41">41</xref>. For example, Ce<sup>−</sup> can just be
generated by using such experimental method<xref ref-type="bibr" rid="b25">25</xref>. However, in the
case of Eu<sup>−</sup>, employing these conventional carrier
gases did not produce any detectable atomic anions, probably because the yield
of Eu<sup>−</sup> is much lower than that of
Ce<sup>−</sup><xref ref-type="bibr" rid="b14">14</xref>. In one of our recent
studies, it has been established that the addition of N<sub>2</sub>O into helium
is beneficial to produce smaller oxide clusters, <italic>e.g</italic>.,
MgO<sup>−</sup><xref ref-type="bibr" rid="b40">40</xref>. Thus, a possible strategy
for synthesizing Eu<sup>−</sup> is proposed as follows:
utilizing N<sub>2</sub>O+ He as a reactant gas to increase the yield of
EuO<sup>−</sup> followed by increasing the output of the
ablation laser to provide sufficient energy to open the reaction channel
dissociating EuO<sup>−</sup> into Eu<sup>−</sup>and O. <xref ref-type="fig" rid="f1">Figure 1</xref> displays the mass spectrum of the
EuO<sub>x</sub><sup>−</sup>(x = 0–4) clusters using the abovementioned
method. It was found that the intensity of the Eu<sup>−</sup>signal is very sensitive to the power of the ablation laser, and the Eu anion
can only be observed at high laser power. The inset in <xref ref-type="fig" rid="f1">Fig.
1</xref> is an enlarged spectrum in the range of 140 to 190 m/z
to clearly show the peak distribution of Eu<sup>−</sup>, in
which two isotopes at 151 and 153 amu are evidenced. The assignment of the
Eu<sup>−</sup> peak is validated from both the
mass-to-charge ratio and the isotopic distribution. It is worth noting that
there may exist another dissociation channel, <italic>e.g</italic>.
EuO<sup>−</sup> → Eu + O<sup>−</sup>,
which is probably more favorable than the suggested channel forming
Eu<sup>−</sup> since atomic O has a higher EA than that of
Eu. This could be deduced from the relative low intensity of the
Eu<sup>−</sup> peak observed here, as shown in <xref ref-type="fig" rid="f1">Fig. 1</xref>. However, the encouraging experimental fact is that,
as will be shown in the following section, we were able to acquire the
photoelectron image of the Eu<sup>−</sup> by photodetaching the
experimentally produced anions. This indicates that the experimental strategy
used here successfully increased the yield of the Eu<sup>−</sup>ion to a detectable level, which could be viewed as a significant advance in
producing the gas-phase lanthanide anions. These findings open up great
opportunity for us to correctly understand the fundamental properties of these
heavy <italic>f</italic>-block atoms.</p></sec><sec disp-level="2"><title>Photoelectron imaging spectroscopy and EA of Eu</title><p><xref ref-type="fig" rid="f2">Figure 2</xref> depicts the photoelectron image and corresponding
binding energy spectrum of Eu<sup>−</sup> obtained at
532 nm photon energy. The double yellow arrow represents the
direction of the laser polarization. As shown in <xref ref-type="fig" rid="f2">Fig. 2</xref>,
three prominent rings can be identified, labeled X, B and D. Careful inspection
of the spectrum reveals many other resolved peaks at the binding energy range of
1.63–2.30 eV, which will be discussed below. The weak
ring X appears at the very edge of the camera, implying an extremely low binding
energy for this transition. Among the observed peaks in the photoelectron
spectrum, the ones lying at low binding energy region (up to 0.4 eV)
are more interesting since they contain the EA defined transition. To clearly
show the peak distributions of this region, an enlarged spectrum is included as
an inset in <xref ref-type="fig" rid="f2">Fig. 2</xref>. As shown in the inset of <xref ref-type="fig" rid="f2">Fig. 2</xref>, peak X is the most intense transition in the low
binding energy region, and the measured binding energy is
0.116 ± 0.013 eV. In most of the
photodetachment process, it is generally accepted that, among adjacent
transitions, the peak with the greatest intensity results from transition
between lowest-lying levels<xref ref-type="bibr" rid="b42">42</xref>. It is, therefore, reasonable to
temporarily assign X as the EA defined peak coming from the transition between
the ground state of Eu<sup>−</sup> to the corresponding neutral
ground state, and the EA of Eu is
0.116 ± 0.013 eV.</p><p>In order to validate the above identification, we have compared the energy
spacings of the observed peaks (<xref ref-type="fig" rid="f2">Fig. 2</xref>) with well-known Eu
neutral spectrum<xref ref-type="bibr" rid="b43">43</xref>, which can provide the most straightforward
and strongest support for our assignment of the EA defined peak. This is also
the reason that we utilized 532 nm (2.33 eV) laser
wavelength to detach Eu<sup>−</sup>, which is energetically
accessible to generate neutral Eu in excited states, while the
1064 nm (1.17 eV) photon energy is not sufficient to
produce excited Eu neutral. Recently, Beck <italic>et al</italic>. theoretically suggested
that the photodetachment channels from anionic ground state to
<sup>10</sup>P<sub>7/2</sub> and <sup>8</sup>P<sub>5/2</sub> neutral
thresholds of Eu will produce stronger peaks located at 1.862 and
2.088 eV, respectively, in the photoelectron spectrum<xref ref-type="bibr" rid="b18">18</xref>. This prediction is nicely reproduced in our spectrum (<xref ref-type="fig" rid="f2">Fig.
2</xref>) since the two strongest transitions B and D appear at 1.864 and
2.088 eV, respectively. Therefore, considering the energies of these
two transitions and the known term energies from previous atomic absorption
spectroscopy<xref ref-type="bibr" rid="b43">43</xref>, the EA of Eu can be calculated to be about
0.119 eV, which is closer to the suggested EA defined peak (X) than
any other adjacent peaks in the low binding energy region. This provides the
first experimental evidence that our assigned EA defined peak (X) is
reliable.</p><p><xref ref-type="fig" rid="f3">Figure 3</xref> shows the energy levels of neutral Eu<xref ref-type="bibr" rid="b43">43</xref> with those of the Eu anion sketched in. As shown in <xref ref-type="fig" rid="f3">Fig. 3</xref>, photodetachment with 532 nm photon
energy will raise the energy of the ground-state Eu anion by 2.33 eV
to an energy level from which it will be able to eject an electron. Taking the
suggested EA value
(0.116 ± 0.013 eV) of Eu into
account, the absorbed photon energy is capable of producing neutral atom either
in ground state (<sup>8</sup>S<sub>7/2</sub>) or in one of several excited
states (<sup>10</sup>D<sub><italic>J’</italic></sub>,
<sup>10</sup>P<sub><italic>J’</italic></sub>,
<sup>8</sup>D<sub><italic>J’</italic></sub>,
<sup>8</sup>P<sub><italic>J’</italic></sub>, or
<sup>6</sup>P<sub><italic>J’</italic></sub>). Therefore, the PES
can be expected to consist of six groups of peaks. Moreover, the <italic>j</italic>-level
fine structure resulting from the spin-orbit splitting of these levels should
produce structures in each of these peaks. To observe the fine structures
clearly, the higher binding energy region (1.63–2.30 eV)
of the spectrum (<xref ref-type="fig" rid="f2">Fig. 2</xref>) has been enlarged, and is shown
as <xref ref-type="fig" rid="f4">Fig. 4</xref>. Note that all peaks in <xref ref-type="fig" rid="f4">Fig.
4</xref> represent the transitions between the anionic Eu level and the
excited states of neutral Eu. As shown in <xref ref-type="fig" rid="f4">Fig. 4</xref>, five
groups of peaks are observed, labeled as A<sub><italic>i</italic></sub>,
B<sub><italic>i</italic></sub>, C<sub><italic>i</italic></sub>, D, and E, respectively,
corresponding to the transitions to different excited states of neutral Eu.
These fine structures allow us to further verify the assignment of the EA
defined peak suggested here by comparing them with the known neutral
excited-state term energies<xref ref-type="bibr" rid="b43">43</xref>. In <xref ref-type="table" rid="t1">Table
1</xref>, the binding energies of different peaks are listed along with the
energy levels of neutral Eu extracted from the present measurements. The known
excited states of neutral Eu are also summarized for comparison<xref ref-type="bibr" rid="b43">43</xref>. As shown in <xref ref-type="table" rid="t1">Table 1</xref>, good agreement between the
present measurements and the well-established electronic structures of neutral
Eu<xref ref-type="bibr" rid="b43">43</xref> is evidenced with the maximum deviation of only
0.018 eV, giving us further confidence that our assignment of the EA
defined peak (X) is correct. Note that the electron configuration of the
ground-state Eu<sup>−</sup> is
4<italic>f</italic><sup> 7</sup>6<italic>s</italic><sup>2</sup> 6p (vide infra). And, according to the electronic
structures of Eu<xref ref-type="bibr" rid="b43">43</xref>, the electron configurations for the final
neutral excited states corresponding to the peaks B<sub><italic>i</italic></sub>, D and E
are 4<italic>f</italic><sup> 7</sup> 6<italic>s</italic> 6<italic>p</italic>,
while those of the peaks A<sub><italic>i</italic></sub> and C<sub><italic>i</italic></sub> are
4<italic>f</italic><sup> 7</sup> 5<italic>d</italic> 6<italic>s</italic>.
Thus, the peaks B<sub><italic>i</italic></sub>, D and E could occur from direct
photodetachment of a 6<italic>s</italic> electron. In the case of peaks
A<sub><italic>i</italic></sub> and C<sub><italic>i</italic></sub>, they may be formed via
multi-step processes. Here, we provide one possible explanation about the
formation of peaks A<sub><italic>i</italic></sub> and C<sub><italic>i</italic></sub>, which is as
follows: the absorption of the photon energy (2.33 eV) may promote
the ground-state Eu<sup>−</sup> ion to an excited anionic state
probably with a 4<italic>f</italic><sup> 7</sup> 5<italic>d</italic>6<italic>s</italic><sup>2</sup> electron configuration followed by a 6<italic>s</italic>orbital detachment to form the final neutral thresholds since the photoelectron
angular distributions (PADs) of these peaks (see <xref ref-type="fig" rid="f2">Fig. 2</xref>)
are preferably oriented parallel to the laser polarization, which imply that the
photoelectron detachment occurs from atomic orbital of a mainly <italic>s</italic>-type
character. Lastly, Beck <italic>et al</italic>. suggested that the cross sections of the
4<italic>f</italic><sup> 7</sup>6<italic>s</italic><sup>2</sup>6<italic>p</italic> → 4<italic>f</italic><sup> 7</sup>6<italic>s</italic> 6<italic>p</italic> channels should be much larger than those in the
4<italic>f</italic><sup> 7</sup>6<italic>s</italic><sup>2</sup> 6<italic>p</italic> (electron configuration of ground-state
Eu<sup>−</sup>) → 4<italic>f</italic><sup> 7</sup>6<italic>s</italic><sup>2</sup> (electron configuration of ground-state Eu)
photodetachment channels<xref ref-type="bibr" rid="b18">18</xref>, which is also evidenced in the
present experiments since the intensities of peaks B and D (<italic>s</italic>-electron
detachment transitions) are much stronger than that of the <italic>p</italic>-electron
detachment band (X) (<xref ref-type="fig" rid="f2">Fig. 2</xref>). Therefore, based on all
these findings, it is reasonable to conclude that the peak X in <xref ref-type="fig" rid="f2">Fig. 2</xref> represents the transition from the ground state of
Eu<sup>−</sup> to the corresponding neutral ground state,
and the EA of Eu is determined to be
0.116 ± 0.013 eV. Additionally,
it is necessary to mention that the electron configuration of the ground state
of Eu<sup>−</sup> should be
4<italic>f</italic><sup> 7</sup>6<italic>s</italic><sup>2</sup> 6<italic>p</italic> (<sup>9</sup>P<sub>3</sub>) since
the measured EA of Eu is in excellent agreement with the theoretical value
calculated by Beck <italic>et al</italic>. with the basic assumption that it is the
<italic>p</italic>-electron attachment leading to the formation of ground-state
Eu<sup>−</sup> from neutral Eu
(4<italic>f</italic><sup> 7</sup>6<italic>s</italic><sup>2</sup>) atom<xref ref-type="bibr" rid="b18">18</xref>.</p><p>It is apparent that the newly established EA value
(0.116 ± 0.013 eV) of Eu here
differs considerably from Davis and Thompson’s result
(1.053 ± 0.025 eV)<xref ref-type="bibr" rid="b15">15</xref>, but is in outstanding agreement with recently reported
theoretical predictions<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b20">20</xref>. We believe our EA
(0.116 ± 0.013 eV) obtained here
is more reliable since, based on the above discussions, the present measurement
is not only consistent with the high-level calculations<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b20">20</xref> but
also in excellent agreement with the well-established neutral electronic
structures of Eu<xref ref-type="bibr" rid="b43">43</xref>. It was suggested that the significant
overestimation in previous measurement<xref ref-type="bibr" rid="b15">15</xref> may originate from
following reasons: (a) the transition is likely from the anionic ground state to
the excited state of neutral, or (b) from long-lived metastable states of anion
to the excited state of Eu<xref ref-type="bibr" rid="b18">18</xref>. The first possibility can be easily
ruled out since no peaks were found around 1 eV in our PES. Thus,
one possible explanation for the overestimation in previous measurement is that
the produced anions were not in their ground state, and the observed peaks may
originate from transitions between the anionic metastable states and the excited
states of Eu. To verify this suggestion, more accurate theoretical methods are
urgently desired to quantitatively locate the relevant excited states of
Eu<sup>−</sup>. Additionally, another possibility is that
the photodetached species were other species rather than the
Eu<sup>−</sup> ion, <italic>e.g</italic>.
EuH<sup>−</sup>. To testify this assumption, photodetachment
experiments on EuH<sup>−</sup> need to be done and compared with
previous spectrum.</p><p>Having presented the novel experimental strategy for increasing the yield of the
lanthanide anions and determined the EA of Eu, which are the focus of this
study, we now turn our attention to other spectroscopic features observed in the
photoelectron spectrum (<xref ref-type="fig" rid="f2">Fig. 2</xref>). As shown in <xref ref-type="fig" rid="f2">Fig. 2</xref>, a weaker band, marked as X’, appears at
lower binding energy with respect to peak X. The binding energy for this
transition is 0.039 eV, which is very close to the energy level of
one excited state (<sup>9</sup>P<sub>5</sub>) of
Eu<sup>−</sup> (0.041 eV) calculated by
O’Malley and Beck<xref ref-type="bibr" rid="b18">18</xref>. Thus, this peak most likely
originates from this excited state of the anion to the ground state of neutral,
establishing the splitting between the ground and excited state of
Eu<sup>−</sup> to be 0.077 eV. To rationalize
this identification, one may expect to find transitions coming from this anionic
excited state to the excited neutral states in higher binding energy region of
the spectrum. Therefore, observation of pair of peaks separated by about
0.077 eV would then be a strong indication of this anionic excited
state. After carefully inspecting the spectrum (<xref ref-type="fig" rid="f4">Fig. 4</xref>),
six additional peaks, marked as A', A<sub>1</sub>',
A<sub>2</sub>', C', C<sub>2</sub>', and
B<sub>2</sub>', are found to have such energy interval with
respect to their paired peaks originating from the anionic ground state, which
are shown in <xref ref-type="table" rid="t2">Table 2</xref>. This finding provides direct
experimental evidence about the existence of this excited state of
Eu<sup>−</sup>. In addition, there seem to be several other
peaks at higher energy side (up to 0.4 eV) to peak X, which probably
come from the transitions between the excited states of
Eu<sup>−</sup> and the excited states of the neutral. The
precise assignment of these peaks needs further high-level calculations
considering the excited states to make, which is beyond the scope of this study
and our ability.</p></sec></sec><sec disp-level="1" sec-type="discussion"><title>Discussion</title><p>The present study provides the first precise photoelectron imaging spectroscopy of
the Eu anion, revealing the character of the electron-atom interaction in
Eu<sup>−</sup>. By introducing a new experimental strategy,
Eu<sup>−</sup> with detectable intensity was produced, and the
EA was directly measured to be
0.116 ± 0.013 eV, which is in
outstanding agreement with the recent high-level theoretical results<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b20">20</xref>. Such a low EA reveals that the additional electron is attached
weakly to Eu neutral, resolving the longstanding and significant discrepancy between
previous experiment and theory. Moreover, the validation and accuracy of the EA is
further verified by comparing the fine structures observed here with the
well-established spectroscopic data for neutral. The present experimental results
also verify the power of recently advanced theoretical methods in predicting the
electronic properties of Eu<sup>−</sup>. For some of other
lanthanides, however, significant discrepancy still exists in different theoretical
methods with the deviation by factors varying from about 5 to 8<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b19">19</xref>. Thus, to obtain a complete and correct understanding about the lanthanide
chemistry, further experiments regarding other lanthanides are urgently desired,
which can provide a benchmark to test the accuracy of theory. In fact, we have
already acquired the images of several other lanthanides, which will be discussed in
other individual works. We believe our experimental findings highlighted here will
stimulate further interests and efforts in exploring the fundamental properties of
these challenging heavy elements in the periodic table.</p></sec><sec disp-level="1" sec-type="methods"><title>Methods</title><p>The Eu<sup>−</sup> was produced in our laser vaporization source,
where a 532 nm second harmonic Nd:YAG laser was used to ablate a
1/4″ Eu “rod” which was made by wrapping an Eu
foil around an Al rod. Helium seeded with 5% N<sub>2</sub>O (typically 50 psi) was
used as a carrier gas, and the generated Eu<sup>−</sup> was mass
analyzed using a time-of-flight mass spectrometer<xref ref-type="bibr" rid="b44">44</xref>. Another second
harmonic of a Nd:YAG laser (532 nm) was used for photodetaching excess
electrons from <sup>151</sup>Eu<sup>−</sup>. Photoelectrons were
accelerated toward position sensitive detectors where the resulting two-dimensional
velocity distribution was recorded with a charge-coupled device camera. Then, the
three-dimensional distribution was reconstructed from the photoelectron image using
the BASEX<xref ref-type="bibr" rid="b45">45</xref> and pBASEX<xref ref-type="bibr" rid="b46">46</xref> programs, which yielded
similar results. The photoelectron spectrum was calibrated against the known
Bi<sup>−</sup> binding energy spectrum<xref ref-type="bibr" rid="b47">47</xref>.</p></sec><sec disp-level="1"><title>Additional Information</title><p><bold>How to cite this article</bold>: Cheng, S.-B. and Castleman, A.W., Jr. Direct
experimental observation of weakly-bound character of the attached electron in
europium anion. <italic>Sci. Rep</italic>. <bold>5</bold>, 12414; doi: 10.1038/srep12414 (2015).</p></sec></body><back><ack><p>This material is based upon work supported by the Air Force Office of Science
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the paper. All authors reviewed the manuscript.</p></fn></fn-group></back><floats-group><fig id="f1"><label>Figure 1</label><caption><title>Time-of-flight mass spectrum of Eu<sup>−</sup> and
monoeuropium oxide clusters.</title><p>The inset shows the enlarged spectrum in the range of 140 to
190 m/z.</p></caption><graphic xlink:href="srep12414-f1"></graphic></fig><fig id="f2"><label>Figure 2</label><caption><title>Photoelectron image and corresponding photoelectron spectrum of
Eu<sup>−</sup>.</title><p>The spectrum was obtained at 532 nm photon energy. The inset
shows the enlarged spectrum in the range of 0 to 0.4 eV.</p></caption><graphic xlink:href="srep12414-f2"></graphic></fig><fig id="f3"><label>Figure 3</label><caption><title>Schematic of energy levels in neutral and anionic Eu observed in the
experiment.</title><p>The energy levels of the neutral Eu are obtained from Ref. <xref ref-type="bibr" rid="b43">43</xref>, while those of the anionic Eu are acquired from the
present experiment. The EA of Eu is defined as the energy difference between
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configurations and LS terms are included.</p></caption><graphic xlink:href="srep12414-f3"></graphic></fig><fig id="f4"><label>Figure 4</label><caption><title>Enlarged photoelectron spectrum of Eu<sup>−</sup> between 1.63 and 2.30 eV.</title><p>Transitions from anionic ground state to
different excited states of neutral Eu atom are labeled using different
letter series (A<sub><italic>i</italic></sub>, B<sub><italic>i</italic></sub>, …),
and are indicated by different color lines. Transitions from one excited
anionic state are marked with apostrophe.</p></caption><graphic xlink:href="srep12414-f4"></graphic></fig><table-wrap position="float" id="t1"><label>Table 1</label><caption><title>Atomic energy levels (in eV) in Eu<sup>0/−</sup>.</title></caption><table frame="hsides" rules="groups" border="1"><colgroup><col align="left"></col><col align="char" char="."></col><col align="char" char="."></col><col align="center"></col><col align="center"></col></colgroup><thead valign="bottom"><tr><th align="left" valign="top" charoff="50">Band</th><th align="center" valign="top" char="." charoff="50">Binding energy (eV)</th><th align="center" valign="top" char="." charoff="50">Atomic energy level (eV) (this work)</th><th align="center" valign="top" charoff="50">Atomic energy level (eV) (Ref. <xref ref-type="bibr" rid="b43">43</xref>)</th><th align="center" valign="top" charoff="50">Term of final state</th></tr></thead><tbody valign="top"><tr><td align="left" valign="top" charoff="50">X'</td><td align="center" valign="top" char="." charoff="50">0.039</td><td align="center" valign="top" char="." charoff="50">−0.077</td><td align="center" valign="top" charoff="50">—</td><td align="center" valign="top" charoff="50">—</td></tr><tr><td align="left" valign="top" charoff="50">X</td><td align="center" valign="top" char="." charoff="50">0.116</td><td align="center" valign="top" char="." charoff="50">0</td><td align="center" valign="top" charoff="50">0</td><td align="center" valign="top" charoff="50"><sup>8</sup><italic>S</italic><sub>7/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">A</td><td align="center" valign="top" char="." charoff="50">1.725</td><td align="center" valign="top" char="." charoff="50">1.609</td><td align="char" valign="top" char="." charoff="50">1.602</td><td align="center" valign="top" charoff="50"><sup>10</sup><italic>D</italic><sub>5/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">A<sub>1</sub></td><td align="center" valign="top" char="." charoff="50">1.746</td><td align="center" valign="top" char="." charoff="50">1.630</td><td align="char" valign="top" char="." charoff="50">1.618</td><td align="center" valign="top" charoff="50"><sup>10</sup><italic>D</italic><sub>7/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">A<sub>2</sub></td><td align="center" valign="top" char="." charoff="50">1.773</td><td align="center" valign="top" char="." charoff="50">1.657</td><td align="char" valign="top" char="." charoff="50">1.639</td><td align="center" valign="top" charoff="50"><sup>10</sup><italic>D</italic><sub>9/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">A<sub>3</sub></td><td align="center" valign="top" char="." charoff="50">1.797</td><td align="center" valign="top" char="." charoff="50">1.681</td><td align="char" valign="top" char="." charoff="50">1.668</td><td align="center" valign="top" charoff="50"><sup>10</sup><italic>D</italic><sub>11/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">A<sub>4</sub></td><td align="center" valign="top" char="." charoff="50">1.830</td><td align="center" valign="top" char="." charoff="50">1.718</td><td align="char" valign="top" char="." charoff="50">1.708</td><td align="center" valign="top" charoff="50"><sup>10</sup><italic>D</italic><sub>13/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">B</td><td align="center" valign="top" char="." charoff="50">1.864</td><td align="center" valign="top" char="." charoff="50">1.748</td><td align="char" valign="top" char="." charoff="50">1.744</td><td align="center" valign="top" charoff="50"><sup>10</sup><italic>P</italic><sub>7/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">B<sub>1</sub></td><td align="center" valign="top" char="." charoff="50">1.910</td><td align="center" valign="top" char="." charoff="50">1.794</td><td align="char" valign="top" char="." charoff="50">1.806</td><td align="center" valign="top" charoff="50"><sup>10</sup><italic>P</italic><sub>9/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">C</td><td align="center" valign="top" char="." charoff="50">1.995</td><td align="center" valign="top" char="." charoff="50">1.879</td><td align="char" valign="top" char="." charoff="50">1.877</td><td align="center" valign="top" charoff="50"><sup>8</sup><italic>D</italic><sub>3/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">C<sub>1</sub></td><td align="center" valign="top" char="." charoff="50">2.009</td><td align="center" valign="top" char="." charoff="50">1.893</td><td align="char" valign="top" char="." charoff="50">1.891</td><td align="center" valign="top" charoff="50"><sup>8</sup><italic>D</italic><sub>5/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">C<sub>2</sub></td><td align="center" valign="top" char="." charoff="50">2.024</td><td align="center" valign="top" char="." charoff="50">1.908</td><td align="char" valign="top" char="." charoff="50">1.912</td><td align="center" valign="top" charoff="50"><sup>8</sup><italic>D</italic><sub>7/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">B<sub>2</sub></td><td align="center" valign="top" char="." charoff="50">2.044</td><td align="center" valign="top" char="." charoff="50">1.928</td><td align="char" valign="top" char="." charoff="50">1.932</td><td align="center" valign="top" charoff="50"><sup>10</sup><italic>P</italic><sub>11/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">C<sub>3</sub></td><td align="center" valign="top" char="." charoff="50">2.063</td><td align="center" valign="top" char="." charoff="50">1.947</td><td align="char" valign="top" char="." charoff="50">1.944</td><td align="center" valign="top" charoff="50"><sup>8</sup><italic>D</italic><sub>9/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">D</td><td align="center" valign="top" char="." charoff="50">2.088</td><td align="center" valign="top" char="." charoff="50">1.972</td><td align="char" valign="top" char="." charoff="50">1.970</td><td align="center" valign="top" charoff="50"><sup>8</sup><italic>P</italic><sub>5/2</sub></td></tr><tr><td align="left" valign="top" charoff="50">E</td><td align="center" valign="top" char="." charoff="50">2.261</td><td align="center" valign="top" char="." charoff="50">2.145</td><td align="char" valign="top" char="." charoff="50">2.150</td><td align="center" valign="top" charoff="50"><sup>6</sup><italic>P</italic><sub>7/2</sub></td></tr></tbody></table><table-wrap-foot><fn id="t1-fn1"><p>Experimental binding energies (BEs) have an uncertainty of
±0.013 eV. The present atomic energy
levels are obtained by calculating the energy differences
between peak X and other peaks observed in the photoelectron
spectrum. The referenced spectroscopic data are obtained
from Ref. <xref ref-type="bibr" rid="b43">43</xref>.</p></fn></table-wrap-foot></table-wrap><table-wrap position="float" id="t2"><label>Table 2</label><caption><title>Observed transitions originating from one excited state of
Eu<sup>−</sup>, in eV.</title></caption><table frame="hsides" rules="groups" border="1"><colgroup><col align="left"></col><col align="char" char="."></col><col align="center"></col><col align="char" char="."></col><col align="char" char="."></col></colgroup><thead valign="bottom"><tr><th align="left" valign="top" charoff="50">Band</th><th align="center" valign="top" char="." charoff="50">Binding energy (eV)</th><th align="center" valign="top" charoff="50">Paired peak</th><th align="center" valign="top" char="." charoff="50">Binding energy (eV)</th><th align="center" valign="top" char="." charoff="50">ΔE (eV)</th></tr></thead><tbody valign="top"><tr><td align="left" valign="top" charoff="50">A'</td><td align="center" valign="top" char="." charoff="50">1.650</td><td align="center" valign="top" charoff="50">A</td><td align="center" valign="top" char="." charoff="50">1.725</td><td align="center" valign="top" char="." charoff="50">0.075</td></tr><tr><td align="left" valign="top" charoff="50">A<sub>1</sub>'</td><td align="center" valign="top" char="." charoff="50">1.672</td><td align="center" valign="top" charoff="50">A<sub>1</sub></td><td align="center" valign="top" char="." charoff="50">1.746</td><td align="center" valign="top" char="." charoff="50">0.074</td></tr><tr><td align="left" valign="top" charoff="50">A<sub>2</sub>'</td><td align="center" valign="top" char="." charoff="50">1.699</td><td align="center" valign="top" charoff="50">A<sub>2</sub></td><td align="center" valign="top" char="." charoff="50">1.773</td><td align="center" valign="top" char="." charoff="50">0.074</td></tr><tr><td align="left" valign="top" charoff="50">C'</td><td align="center" valign="top" char="." charoff="50">1.920</td><td align="center" valign="top" charoff="50">C</td><td align="center" valign="top" char="." charoff="50">1.995</td><td align="center" valign="top" char="." charoff="50">0.075</td></tr><tr><td align="left" valign="top" charoff="50">C<sub>2</sub>'</td><td align="center" valign="top" char="." charoff="50">1.950</td><td align="center" valign="top" charoff="50">C<sub>2</sub></td><td align="center" valign="top" char="." charoff="50">2.024</td><td align="center" valign="top" char="." charoff="50">0.074</td></tr><tr><td align="left" valign="top" charoff="50">B<sub>2</sub>'</td><td align="center" valign="top" char="." charoff="50">1.967</td><td align="center" valign="top" charoff="50">B<sub>2</sub></td><td align="center" valign="top" char="." charoff="50">2.044</td><td align="center" valign="top" char="." charoff="50">0.077</td></tr></tbody></table><table-wrap-foot><fn id="t2-fn1"><p>Experimental binding energies (BEs) have an uncertainty of
± 0.013 eV. ΔE represent
the energy differences between peaks A' and A,
A<sub>1</sub>' and A<sub>1</sub>,
A<sub>2</sub>' and A<sub>2</sub> and so
on.</p></fn></table-wrap-foot></table-wrap></floats-group></pmc></record>Pour manipuler ce document sous Unix (Dilib)
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