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 : 006830 ( Main/Exploration ); précédent : 006829; suivant : 006831Photolysis 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.
Descripteurs français
- Pascal (Inist)
- Wicri :
- topic : Pompe, Atmosphère.
English descriptors
- 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.
Affiliations:
- Belgique, Brésil, France, États-Unis
- Aquitaine, Nouvelle-Aquitaine, Vienne (Autriche), État de Rio de Janeiro, Île-de-France
- Bordeaux, Orsay, Talence, Vienne (Autriche)
- Université Bordeaux I, Université Paris-Sud, Université de Bordeaux
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Le document en format XML
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<profileDesc><textClass><keywords scheme="KwdEn" xml:lang="en"><term>Ab initio calculations</term>
<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>
<keywords scheme="Wicri" type="topic" xml:lang="fr"><term>Pompe</term>
<term>Atmosphère</term>
</keywords>
</textClass>
</profileDesc>
</teiHeader>
<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>
</TEI>
<affiliations><list><country><li>Belgique</li>
<li>Brésil</li>
<li>France</li>
<li>États-Unis</li>
</country>
<region><li>Aquitaine</li>
<li>Nouvelle-Aquitaine</li>
<li>Vienne (Autriche)</li>
<li>État de Rio de Janeiro</li>
<li>Île-de-France</li>
</region>
<settlement><li>Bordeaux</li>
<li>Orsay</li>
<li>Talence</li>
<li>Vienne (Autriche)</li>
</settlement>
<orgName><li>Université Bordeaux I</li>
<li>Université Paris-Sud</li>
<li>Université de Bordeaux</li>
</orgName>
</list>
<tree><country name="France"><region name="Île-de-France"><name sortKey="Gans, Berenger" sort="Gans, Berenger" uniqKey="Gans B" first="Berenger" last="Gans">Berenger Gans</name>
</region>
<name sortKey="Boye Peronne, Severine" sort="Boye Peronne, Severine" uniqKey="Boye Peronne S" first="Severine" last="Boye-Peronne">Severine Boye-Peronne</name>
<name sortKey="Broquier, Michel" sort="Broquier, Michel" uniqKey="Broquier M" first="Michel" last="Broquier">Michel Broquier</name>
<name sortKey="Broquier, Michel" sort="Broquier, Michel" uniqKey="Broquier M" first="Michel" last="Broquier">Michel Broquier</name>
<name sortKey="Douin, Stephane" sort="Douin, Stephane" uniqKey="Douin S" first="Stephane" last="Douin">Stephane Douin</name>
<name sortKey="Gauyacq, Dolores" sort="Gauyacq, Dolores" uniqKey="Gauyacq D" first="Dolores" last="Gauyacq">Dolores Gauyacq</name>
<name sortKey="Halvick, Philippe" sort="Halvick, Philippe" uniqKey="Halvick P" first="Philippe" last="Halvick">Philippe Halvick</name>
<name sortKey="Loison, Jean Christophe" sort="Loison, Jean Christophe" uniqKey="Loison J" first="Jean-Christophe" last="Loison">Jean-Christophe Loison</name>
</country>
<country name="Belgique"><region name="Vienne (Autriche)"><name sortKey="Delsaut, Maxence" sort="Delsaut, Maxence" uniqKey="Delsaut M" first="Maxence" last="Delsaut">Maxence Delsaut</name>
</region>
</country>
<country name="Brésil"><region name="État de Rio de Janeiro"><name sortKey="Fellows, Carlos E" sort="Fellows, Carlos E" uniqKey="Fellows C" first="Carlos E." last="Fellows">Carlos E. Fellows</name>
</region>
</country>
<country name="États-Unis"><noRegion><name sortKey="Lucchese, Robert R" sort="Lucchese, Robert R" uniqKey="Lucchese R" first="Robert R." last="Lucchese">Robert R. Lucchese</name>
</noRegion>
</country>
</tree>
</affiliations>
</record>
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