Photoelectron Spectra of Methane, Silane, Germane, Methyl Fluoride, Difluoromethane, and Trifluoromethane

Pullen, B. P.; Carlson, Thomas A.; Moddeman, W. E.; Schweitzer, George K.; Bull, William E.; Grimm, Frederick A.

doi:10.1063/1.1674057

Cited by 226 publications

(66 citation statements)

References 47 publications

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“…Many groups have measured He I and He II photoelectron spectra. [20][21][22][23][24][25][26][27][28] Selected values are listed in Table I. The energies of the key dissociation channels of CF 2 X 2 (X= H,Cl,Br) are also listed in Table I. The ground state energies were generally calculated from enthalpies of formation at 0 K given in the JANAF tables.…”

Section: Energetics Of the Key Dissociation Channelsmentioning

confidence: 99%

Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV

Seccombe

Chim

Tuckett

et al. 2001

The Journal of Chemical Physics

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The vacuum-UV absorption and fluorescence spectroscopy of CF 2 X 2 (X=H,Cl,Br) in the range nm is reported. Tunable vacuum-UV radiation in the range 8-22 eV from synchrotron sources at either Daresbury, UK or BESSY1, Germany is used to excite the titled molecules. Fluorescence excitation spectra, with undispersed detection of the fluorescence, were recorded at Daresbury with a resolution of 0.1 nm. VUV absorption spectra at a resolution of 0.08 nm, and dispersed emission spectra with an optical resolution of 8 nm were recorded at BESSY1. Action spectra, in which the VUV energy is scanned with detection of the fluorescence at a specific wavelength, were also recorded at BESSY1 with a resolution of 0.3 nm ; appearance energies for production of a particular emitting state of a fragment are then obtained. Using the single-bunch mode of BESSY1, lifetimes of all emitting states that fall in the range ca. 3-80 ns have been measured. The peaks in the VUV absorption spectra of CF 2 X 2 are assigned to Rydberg transitions. For CF 2 H 2 below 11 eV, there is good agreement between the absorption and the fluorescence excitation spectra, whereas above 11 eV and for the whole range 8-22 eV for CF 2 Cl 2 and CF 2 Br 2 there is little similarity. This suggests that photodissociation to emitting states of fragment species represent minor channels. In the range 8-15 eV, emission is due mainly to CF 2 Ã 1 B 1 − X 1 A 1 and weakly to CFX Ã 1 A" − X 1 A'. These products form by photodissociation of Rydberg states of CF 2 X 2 , and the thresholds for their production therefore relate to energies of the Rydberg states of the parent molecule. For CF 2 H 2 below 11.8 eV CF 2 Ã 1 B 1 can only form with H 2 , whereas for CF 2 Cl 2 and CF 2 Br 2 it is not possible to say whether the other products are 2X or X 2 . For energies above ca. 15 eV, emission is due to diatomic fragments ; CF B 2 ∆ and A 2 Σ + , CCl A 2 ∆, CH B 2 Σ -and A 2 ∆, Cl 2 and Br 2 D' 2 3 Π g , and possibly CBr A 2 ∆. From their appearance energies, there is evidence that, with the exception of CF B 2 ∆ / CF 2 H 2 where the ground state of HF must form , the excited state of CF, CCl or CH forms in association with three atoms. Our results yield no information whether the three bonds in CF 2 X 2 * break simultaneously or sequentially. We suggest that the anomalous behaviour of CF 2 H 2 , in forming H-H or H-F bonds in unimolecular photofragmentation processes, relates to the small size of the hydrogen atom, and hence the unimportance of steric effects in the tightlyconstrained transition state. In no cases is emission observed from excited states of either the CF 2 X free radical or the parent molecular ion, CF 2 X 2 + .4

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Section: Energetics Of the Key Dissociation Channelsmentioning

confidence: 99%

Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV

Seccombe

Chim

Tuckett

et al. 2001

The Journal of Chemical Physics

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“…With this exception, the peak positions of the TPES are in good agreement with the He I data. [14][15][16][17] The relative intensities of the TPES and He I spectra, however, differ significantly. In the He I spectrum of Potts et al, 17 photoelectron band of CF 2 D 2 recorded at the same resolution as the CF 2 H 2 spectrum.…”

Section: Iva3 Fixed-energy Tpepico Experimentsmentioning

confidence: 99%

“…The fragmentation of VUV-excited CF 2 Cl 2 + and CF 2 H 2 + has been extensively studied both by electron impact mass spectrometry (EIMS) and photoionisation mass spectrometry (PIMS), [3][4][5][6][7][8][9][10] but studies have not been performed on CF 2 Br 2 + . The He I, and in some cases He II, photoelectron spectra of all three molecules have been measured by several groups, [11][12][13][14][15][16][17][18][19][20] but no threshold photoelectron spectra (TPES) have been reported. TPES can be used to probe the effects of autoionisation to near-threshold electrons in the titled molecules.…”

Section: Introductionmentioning

confidence: 99%

Fragmentation of the valence states of CF2Cl2+, CF2H2+, and CF2Br2+ studied by threshold photoelectron–photoion coincidence spectroscopy

Seccombe¹,

Tuckett²,

Fisher³

2001

The Journal of Chemical Physics

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Using tunable vacuum-ultraviolet radiation from a synchrotron, the decay pathways of the valence electronic states of CF 2 X 2 + (X=Cl,H,Br) At higher photon energies, smaller fragment ions are formed following cleavage of more than one bond.With CF 2 Cl 2 and CF 2 Br 2 , the appearance energies of the fragment ions are close to the thermochemical energy for production of that ion with neutral atoms, suggesting that these ions form by bond-fission processes only. With CF 2 H 2 , the one ion unambiguously assigned, CFH + , can only form at certain energies with molecular neutral fragments (i.e. CFH + + HF), involving simultaneous bond-breaking and bond-making processes. The t values for cleavage of a single C−F or C−X bond suggest a relationship between the part of the molecule where ionisation occurs and the bond that breaks ; impulsive values of t are more likely to be obtained when the breaking bond lies close to the part of the molecule from which ionisation occurs, statistical values when ionisation occurs further away from the breaking bond. Furthermore, for all CF 2 X 2 + cations there is a trend from impulsive to statistical behaviour as the photon energy is increased.4

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“…In order of increasing energy the character of these molecular orbitals is predominantly of F (1e 4 ), C-F (3a 1 2 ), and CH 3 (2e 4 ) type. Removing one electron from these orbitals leads to the fluoromethane cation in the three lowest electronic states: X ( 2 E), A ( 2 A 1 ), and B ( 2 E).…”

Section: Introductionmentioning

confidence: 99%

“…The electron configuration of neutral fluoromethane is commonly described as (1e 4 ) (3a 1 2 ) (2e 4 ). In order of increasing energy the character of these molecular orbitals is predominantly of F (1e 4 ), C-F (3a 1 2 ), and CH 3 (2e 4 ) type.…”

Section: Introductionmentioning

confidence: 99%

Statistical and non-statistical reactions in energy selected fluoromethane ions

et al. 1995

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The unimolecular reactions of the fluoromethane ion have been investigated by the threshold photoelectron photoion coincidence technique (TPEPICO). The breakdown curves have been measured in the energy range between the adiabatic ionization potential of 12.53 eV and 21 eV. In this energy range the formation of the CH 2 F + , CHF + , CH 2 + , CH 3 + , and CF + fragment ions is observed. The appearance energies (AE) for these ions at 300 K are 13.2, 13.91, 13.93, 14.51 and < 17.7 eV respectively. The 0 K threshold energy for the formation of CH 2 F + ions is 13.37 eV. Two different pathways for the unimolecular reaction of the fluoromethane ion can be distinguished. The reaction proceeding through the electronic ground state of the ion is entirely statistical. In contrast to this the reaction occurring through the first electronic excited state shows nonstatistical behaviour. The two pathways are distinguished via the kinetic energy released in the respective dissociation channels. While all thermochemically allowed reaction channels are observed on the ion ground state potential, the first excited state seems to decay predominantly by F loss reaction.

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Photoelectron Spectra of Methane, Silane, Germane, Methyl Fluoride, Difluoromethane, and Trifluoromethane

Cited by 226 publications

References 47 publications

Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV

Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV

Fragmentation of the valence states of CF2Cl2+, CF2H2+, and CF2Br2+ studied by threshold photoelectron–photoion coincidence spectroscopy

Statistical and non-statistical reactions in energy selected fluoromethane ions

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