Photolysis of methane revisited at 121.6 nm and at 118.2 nm: quantum yields of the primary products, measured by mass spectrometry
Identifieur interne : 004333 ( PascalFrancis/Curation ); précédent : 004332; suivant : 004334Photolysis of methane revisited at 121.6 nm and at 118.2 nm: quantum yields of the primary products, measured by mass spectrometry
Auteurs : Berenger Gans [France] ; Severine Boye-Peronne [France] ; Michel Broquier [France] ; Maxence Delsaut [Belgique] ; Stephane Douin [France] ; Carlos E. Fellows [Brésil] ; Philippe Halvick [France] ; Jean-Christophe Loison [France] ; Robert R. Lucchese [États-Unis] ; Dolores Gauyacq [France]Source :
- PCCP. Physical chemistry chemical physics : (Print) [ 1463-9076 ] ; 2011.
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- Wicri :
- topic : Pompe, Atmosphère.
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- KwdEn :
Abstract
Methane photolysis has been performed at the two Vacuum UltraViolet (VUV) wavelengths, 121.6 nm and 118.2 nm, via a spectrally pure laser pump-probe technique. The first photon is used to dissociate methane (either at 121.6 nm or at 118.2 nm) and the second one is used to ionise the CH2 and CH3 fragments. The radical products, CH3(X), CH2(X), CH2(a) and C('D), have been selectively probed by mass spectrometry. In order to quantify the fragment quantum yields from the mass spectra, the photoionisation cross sections have been carefully evaluated for the CH2 and CH3 radicals, in two steps: first, theoretical ab initio approaches have been used in order to determine the pure electronic photoionisation cross sections of CH2(X) and CH2(a), and have been rescaled with respect to the measured absolute photoionisation cross section of the CH3(X) radical. In a second step, in order to take into account the substantial vibrational energy deposited in the CH3(X) and CH2(a) radicals, the variation of their cross sections near threshold has been simulated by introducing the pertinent Franck-Condon overlaps between neutral and cation species. By adding the interpolated values of CH quantum yields measured by Rebbert and Ausloos [J. Photochem., 1972, 1,171-176], a complete set of fragment quantum yields has been derived for the methane photodissociation at 121.6 nm, with carefully evaluated 1σ uncertainties: ϕ[CH3(X)] = 0.42 ± 0.05, ϕ[CH2(a)] = 0.48 ± 0.05, ϕ[CH2(X)] = 0.03 ± 0.08, ϕ[CH(X)] = 0.07 ± 0.01. These new data have been measured independently of the H atom fragment quantum yield, subject to many controversies in the literature. From our results, we evaluate ϕ(H) = 0.55 ± 0.17 at 121.6 nm. The quantum yields for the photolysis at 118.2 nm differ notably from those measured at 121.6 nm, with a substantial production of the CH2(X) fragment: ϕ[CH3(X)] = 0.26 ± 0.04, ϕ[CH2(a)] = 0.17 ± 0.05, ϕ[CH2(X)] = 0.48 ± 0.06, ϕ[CH(X)] = 0.09 ± 0.01, ϕ(H) = 1.31 ± 0.13. These new data should bring reliable and essential inputs for the photochemical models of the Titan atmosphere.
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<term>Atmosphere</term>
<term>Cross section (collision)</term>
<term>Fragment</term>
<term>Laser</term>
<term>Mass spectrometry</term>
<term>Mass spectrum</term>
<term>Methane</term>
<term>Models</term>
<term>Photodissociation</term>
<term>Photoionization</term>
<term>Photolysis</term>
<term>Photon</term>
<term>Pump</term>
<term>Quantum yield</term>
<term>Theoretical study</term>
<term>Uncertainty</term>
<term>Vibrational energy</term>
<term>Wavelength</term>
</keywords>
<keywords scheme="Pascal" xml:lang="fr"><term>Photolyse</term>
<term>Méthane</term>
<term>Rendement quantique</term>
<term>Spectrométrie masse</term>
<term>Longueur onde</term>
<term>Laser</term>
<term>Pompe</term>
<term>Photon</term>
<term>Fragment</term>
<term>Spectre masse</term>
<term>Photoionisation</term>
<term>Section efficace</term>
<term>Calcul ab initio</term>
<term>Energie vibrationnelle</term>
<term>Photodissociation</term>
<term>Incertitude</term>
<term>Modèle</term>
<term>Atmosphère</term>
<term>Etude théorique</term>
<term>3115A</term>
<term>3380G</term>
</keywords>
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<front><div type="abstract" xml:lang="en">Methane photolysis has been performed at the two Vacuum UltraViolet (VUV) wavelengths, 121.6 nm and 118.2 nm, via a spectrally pure laser pump-probe technique. The first photon is used to dissociate methane (either at 121.6 nm or at 118.2 nm) and the second one is used to ionise the CH<sub>2</sub>
and CH<sub>3</sub>
fragments. The radical products, CH<sub>3</sub>
(X), CH<sub>2</sub>
(X), CH<sub>2</sub>
(a) and C('D), have been selectively probed by mass spectrometry. In order to quantify the fragment quantum yields from the mass spectra, the photoionisation cross sections have been carefully evaluated for the CH<sub>2</sub>
and CH<sub>3</sub>
radicals, in two steps: first, theoretical ab initio approaches have been used in order to determine the pure electronic photoionisation cross sections of CH<sub>2</sub>
(X) and CH<sub>2</sub>
(a), and have been rescaled with respect to the measured absolute photoionisation cross section of the CH<sub>3</sub>
(X) radical. In a second step, in order to take into account the substantial vibrational energy deposited in the CH<sub>3</sub>
(X) and CH<sub>2</sub>
(a) radicals, the variation of their cross sections near threshold has been simulated by introducing the pertinent Franck-Condon overlaps between neutral and cation species. By adding the interpolated values of CH quantum yields measured by Rebbert and Ausloos [J. Photochem., 1972, 1,171-176], a complete set of fragment quantum yields has been derived for the methane photodissociation at 121.6 nm, with carefully evaluated 1σ uncertainties: ϕ[CH<sub>3</sub>
(X)] = 0.42 ± 0.05, ϕ[CH<sub>2</sub>
(a)] = 0.48 ± 0.05, ϕ[CH<sub>2</sub>
(X)] = 0.03 ± 0.08, ϕ[CH(X)] = 0.07 ± 0.01. These new data have been measured independently of the H atom fragment quantum yield, subject to many controversies in the literature. From our results, we evaluate ϕ(H) = 0.55 ± 0.17 at 121.6 nm. The quantum yields for the photolysis at 118.2 nm differ notably from those measured at 121.6 nm, with a substantial production of the CH<sub>2</sub>
(X) fragment: ϕ[CH<sub>3</sub>
(X)] = 0.26 ± 0.04, ϕ[CH<sub>2</sub>
(a)] = 0.17 ± 0.05, ϕ[CH<sub>2</sub>
(X)] = 0.48 ± 0.06, ϕ[CH(X)] = 0.09 ± 0.01, ϕ(H) = 1.31 ± 0.13. These new data should bring reliable and essential inputs for the photochemical models of the Titan atmosphere.</div>
</front>
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<sZ>3 aut.</sZ>
<sZ>5 aut.</sZ>
<sZ>10 aut.</sZ>
</fA14>
<fA14 i1="02"><s1>Centre Laser de l'Université Paris-Sud, Université Paris-Sud. LUMAT FR2764</s1>
<s2>91405 Orsay</s2>
<s3>FRA</s3>
<sZ>3 aut.</sZ>
</fA14>
<fA14 i1="03"><s1>Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles CP 160/09, 50 Avenue F.D. Roosevelt</s1>
<s2>1050 Bruxelles</s2>
<s3>BEL</s3>
<sZ>4 aut.</sZ>
</fA14>
<fA14 i1="04"><s1>Laboratorio de Espectroscopia e Laser, Instituto de Fisica, Universidade Federal Fluminense</s1>
<s2>24210-340, Niteroi, RJ</s2>
<s3>BRA</s3>
<sZ>6 aut.</sZ>
</fA14>
<fA14 i1="05"><s1>Institut des Sciences Moléculaires, CNRS UMR 5255, Université Bordeaux I</s1>
<s2>33405 Talence</s2>
<s3>FRA</s3>
<sZ>7 aut.</sZ>
<sZ>8 aut.</sZ>
</fA14>
<fA14 i1="06"><s1>Texas Department of Chemistry, Texas A&M University, P.O. Box 30012</s1>
<s2>College Station, Texas 77842-3012</s2>
<s3>USA</s3>
<sZ>9 aut.</sZ>
</fA14>
<fA15 i1="01"><s1>Università degli Studi di Perugia, Dipartimento di Chimica, via Elce dio Sotto, 8</s1>
<s2>06123 Perugia</s2>
<s3>ITA</s3>
<sZ>1 aut.</sZ>
</fA15>
<fA15 i1="02"><s1>Oxford University, Department of Chemistry, The Physical and Theoretical Chemistry Laboratory, South Parks Road</s1>
<s2>Oxford, OX1 3QZ</s2>
<s3>GBR</s3>
<sZ>2 aut.</sZ>
</fA15>
<fA15 i1="03"><s1>Université Bordeaux 1/CNRS UMR 5255, Institut des Sciences Moléculaires</s1>
<s2>33405 Talence</s2>
<s3>FRA</s3>
<sZ>3 aut.</sZ>
</fA15>
<fA15 i1="04"><s1>JILA/NIST, Department of Chemistry and Biochemistry, University of Colorado,</s1>
<s2>Boulder, CO, 80309</s2>
<s3>USA</s3>
<sZ>4 aut.</sZ>
</fA15>
<fA15 i1="05"><s1>University of Melbourne, School of Chemistry</s1>
<s3>AUS</s3>
<sZ>5 aut.</sZ>
</fA15>
<fA15 i1="06"><s1>University of Sydney, School of Chemistry</s1>
<s3>AUS</s3>
<sZ>6 aut.</sZ>
</fA15>
<fA20><s1>8140-8152</s1>
</fA20>
<fA21><s1>2011</s1>
</fA21>
<fA23 i1="01"><s0>ENG</s0>
</fA23>
<fA43 i1="01"><s1>INIST</s1>
<s2>26801</s2>
<s5>354000191573960080</s5>
</fA43>
<fA44><s0>0000</s0>
<s1>© 2011 INIST-CNRS. All rights reserved.</s1>
</fA44>
<fA45><s0>51 ref.</s0>
</fA45>
<fA47 i1="01" i2="1"><s0>11-0290996</s0>
</fA47>
<fA60><s1>P</s1>
<s3>PR</s3>
</fA60>
<fA61><s0>A</s0>
</fA61>
<fA64 i1="01" i2="1"><s0>PCCP. Physical chemistry chemical physics : (Print)</s0>
</fA64>
<fA66 i1="01"><s0>GBR</s0>
</fA66>
<fC01 i1="01" l="ENG"><s0>Methane photolysis has been performed at the two Vacuum UltraViolet (VUV) wavelengths, 121.6 nm and 118.2 nm, via a spectrally pure laser pump-probe technique. The first photon is used to dissociate methane (either at 121.6 nm or at 118.2 nm) and the second one is used to ionise the CH<sub>2</sub>
and CH<sub>3</sub>
fragments. The radical products, CH<sub>3</sub>
(X), CH<sub>2</sub>
(X), CH<sub>2</sub>
(a) and C('D), have been selectively probed by mass spectrometry. In order to quantify the fragment quantum yields from the mass spectra, the photoionisation cross sections have been carefully evaluated for the CH<sub>2</sub>
and CH<sub>3</sub>
radicals, in two steps: first, theoretical ab initio approaches have been used in order to determine the pure electronic photoionisation cross sections of CH<sub>2</sub>
(X) and CH<sub>2</sub>
(a), and have been rescaled with respect to the measured absolute photoionisation cross section of the CH<sub>3</sub>
(X) radical. In a second step, in order to take into account the substantial vibrational energy deposited in the CH<sub>3</sub>
(X) and CH<sub>2</sub>
(a) radicals, the variation of their cross sections near threshold has been simulated by introducing the pertinent Franck-Condon overlaps between neutral and cation species. By adding the interpolated values of CH quantum yields measured by Rebbert and Ausloos [J. Photochem., 1972, 1,171-176], a complete set of fragment quantum yields has been derived for the methane photodissociation at 121.6 nm, with carefully evaluated 1σ uncertainties: ϕ[CH<sub>3</sub>
(X)] = 0.42 ± 0.05, ϕ[CH<sub>2</sub>
(a)] = 0.48 ± 0.05, ϕ[CH<sub>2</sub>
(X)] = 0.03 ± 0.08, ϕ[CH(X)] = 0.07 ± 0.01. These new data have been measured independently of the H atom fragment quantum yield, subject to many controversies in the literature. From our results, we evaluate ϕ(H) = 0.55 ± 0.17 at 121.6 nm. The quantum yields for the photolysis at 118.2 nm differ notably from those measured at 121.6 nm, with a substantial production of the CH<sub>2</sub>
(X) fragment: ϕ[CH<sub>3</sub>
(X)] = 0.26 ± 0.04, ϕ[CH<sub>2</sub>
(a)] = 0.17 ± 0.05, ϕ[CH<sub>2</sub>
(X)] = 0.48 ± 0.06, ϕ[CH(X)] = 0.09 ± 0.01, ϕ(H) = 1.31 ± 0.13. These new data should bring reliable and essential inputs for the photochemical models of the Titan atmosphere.</s0>
</fC01>
<fC02 i1="01" i2="X"><s0>001C01F01</s0>
</fC02>
<fC03 i1="01" i2="X" l="FRE"><s0>Photolyse</s0>
<s5>01</s5>
</fC03>
<fC03 i1="01" i2="X" l="ENG"><s0>Photolysis</s0>
<s5>01</s5>
</fC03>
<fC03 i1="01" i2="X" l="SPA"><s0>Fotolisis</s0>
<s5>01</s5>
</fC03>
<fC03 i1="02" i2="X" l="FRE"><s0>Méthane</s0>
<s2>NK</s2>
<s2>FX</s2>
<s5>02</s5>
</fC03>
<fC03 i1="02" i2="X" l="ENG"><s0>Methane</s0>
<s2>NK</s2>
<s2>FX</s2>
<s5>02</s5>
</fC03>
<fC03 i1="02" i2="X" l="SPA"><s0>Metano</s0>
<s2>NK</s2>
<s2>FX</s2>
<s5>02</s5>
</fC03>
<fC03 i1="03" i2="X" l="FRE"><s0>Rendement quantique</s0>
<s5>03</s5>
</fC03>
<fC03 i1="03" i2="X" l="ENG"><s0>Quantum yield</s0>
<s5>03</s5>
</fC03>
<fC03 i1="03" i2="X" l="SPA"><s0>Rendimiento quántico</s0>
<s5>03</s5>
</fC03>
<fC03 i1="04" i2="X" l="FRE"><s0>Spectrométrie masse</s0>
<s5>04</s5>
</fC03>
<fC03 i1="04" i2="X" l="ENG"><s0>Mass spectrometry</s0>
<s5>04</s5>
</fC03>
<fC03 i1="04" i2="X" l="SPA"><s0>Espectrometría masa</s0>
<s5>04</s5>
</fC03>
<fC03 i1="05" i2="X" l="FRE"><s0>Longueur onde</s0>
<s5>05</s5>
</fC03>
<fC03 i1="05" i2="X" l="ENG"><s0>Wavelength</s0>
<s5>05</s5>
</fC03>
<fC03 i1="05" i2="X" l="SPA"><s0>Longitud onda</s0>
<s5>05</s5>
</fC03>
<fC03 i1="06" i2="X" l="FRE"><s0>Laser</s0>
<s5>06</s5>
</fC03>
<fC03 i1="06" i2="X" l="ENG"><s0>Laser</s0>
<s5>06</s5>
</fC03>
<fC03 i1="06" i2="X" l="SPA"><s0>Láser</s0>
<s5>06</s5>
</fC03>
<fC03 i1="07" i2="X" l="FRE"><s0>Pompe</s0>
<s5>07</s5>
</fC03>
<fC03 i1="07" i2="X" l="ENG"><s0>Pump</s0>
<s5>07</s5>
</fC03>
<fC03 i1="07" i2="X" l="SPA"><s0>Bomba</s0>
<s5>07</s5>
</fC03>
<fC03 i1="08" i2="X" l="FRE"><s0>Photon</s0>
<s5>08</s5>
</fC03>
<fC03 i1="08" i2="X" l="ENG"><s0>Photon</s0>
<s5>08</s5>
</fC03>
<fC03 i1="08" i2="X" l="SPA"><s0>Fotón</s0>
<s5>08</s5>
</fC03>
<fC03 i1="09" i2="X" l="FRE"><s0>Fragment</s0>
<s5>09</s5>
</fC03>
<fC03 i1="09" i2="X" l="ENG"><s0>Fragment</s0>
<s5>09</s5>
</fC03>
<fC03 i1="09" i2="X" l="SPA"><s0>Fragmento</s0>
<s5>09</s5>
</fC03>
<fC03 i1="10" i2="X" l="FRE"><s0>Spectre masse</s0>
<s5>10</s5>
</fC03>
<fC03 i1="10" i2="X" l="ENG"><s0>Mass spectrum</s0>
<s5>10</s5>
</fC03>
<fC03 i1="10" i2="X" l="SPA"><s0>Espectro masa</s0>
<s5>10</s5>
</fC03>
<fC03 i1="11" i2="X" l="FRE"><s0>Photoionisation</s0>
<s5>11</s5>
</fC03>
<fC03 i1="11" i2="X" l="ENG"><s0>Photoionization</s0>
<s5>11</s5>
</fC03>
<fC03 i1="11" i2="X" l="SPA"><s0>Fotoionización</s0>
<s5>11</s5>
</fC03>
<fC03 i1="12" i2="X" l="FRE"><s0>Section efficace</s0>
<s5>12</s5>
</fC03>
<fC03 i1="12" i2="X" l="ENG"><s0>Cross section (collision)</s0>
<s5>12</s5>
</fC03>
<fC03 i1="12" i2="X" l="SPA"><s0>Sección eficaz</s0>
<s5>12</s5>
</fC03>
<fC03 i1="13" i2="3" l="FRE"><s0>Calcul ab initio</s0>
<s5>13</s5>
</fC03>
<fC03 i1="13" i2="3" l="ENG"><s0>Ab initio calculations</s0>
<s5>13</s5>
</fC03>
<fC03 i1="14" i2="X" l="FRE"><s0>Energie vibrationnelle</s0>
<s5>14</s5>
</fC03>
<fC03 i1="14" i2="X" l="ENG"><s0>Vibrational energy</s0>
<s5>14</s5>
</fC03>
<fC03 i1="14" i2="X" l="SPA"><s0>Energía vibracional</s0>
<s5>14</s5>
</fC03>
<fC03 i1="15" i2="X" l="FRE"><s0>Photodissociation</s0>
<s5>15</s5>
</fC03>
<fC03 i1="15" i2="X" l="ENG"><s0>Photodissociation</s0>
<s5>15</s5>
</fC03>
<fC03 i1="15" i2="X" l="SPA"><s0>Fotodisociación</s0>
<s5>15</s5>
</fC03>
<fC03 i1="16" i2="X" l="FRE"><s0>Incertitude</s0>
<s5>16</s5>
</fC03>
<fC03 i1="16" i2="X" l="ENG"><s0>Uncertainty</s0>
<s5>16</s5>
</fC03>
<fC03 i1="16" i2="X" l="SPA"><s0>Incertidumbre</s0>
<s5>16</s5>
</fC03>
<fC03 i1="17" i2="X" l="FRE"><s0>Modèle</s0>
<s5>17</s5>
</fC03>
<fC03 i1="17" i2="X" l="ENG"><s0>Models</s0>
<s5>17</s5>
</fC03>
<fC03 i1="17" i2="X" l="SPA"><s0>Modelo</s0>
<s5>17</s5>
</fC03>
<fC03 i1="18" i2="X" l="FRE"><s0>Atmosphère</s0>
<s5>18</s5>
</fC03>
<fC03 i1="18" i2="X" l="ENG"><s0>Atmosphere</s0>
<s5>18</s5>
</fC03>
<fC03 i1="18" i2="X" l="SPA"><s0>Atmósfera</s0>
<s5>18</s5>
</fC03>
<fC03 i1="19" i2="X" l="FRE"><s0>Etude théorique</s0>
<s5>19</s5>
</fC03>
<fC03 i1="19" i2="X" l="ENG"><s0>Theoretical study</s0>
<s5>19</s5>
</fC03>
<fC03 i1="19" i2="X" l="SPA"><s0>Estudio teórico</s0>
<s5>19</s5>
</fC03>
<fC03 i1="20" i2="X" l="FRE"><s0>3115A</s0>
<s4>INC</s4>
<s5>32</s5>
</fC03>
<fC03 i1="21" i2="X" l="FRE"><s0>3380G</s0>
<s4>INC</s4>
<s5>33</s5>
</fC03>
<fN21><s1>192</s1>
</fN21>
<fN44 i1="01"><s1>OTO</s1>
</fN44>
<fN82><s1>OTO</s1>
</fN82>
</pA>
</standard>
</inist>
</record>
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