Zhiyuan Min

Vacuum ultraviolet photochemistry of CH4 and isotopomers. II. Product channel fields and absorption spectra

In part I of this work the relative velocities and anisotropies of the atomic H and D fragments from methane photolysis at 10.2 eV were measured. In this paper the relative abundance of the methyl and methylene fragments are reported. A complete set of quantum yields for the different photodissociation channels of each isotopomer is obtained by combining the two sets of data. Previously it was found that H atoms are almost four times more likely than D atoms to be ejected; now it is found that hydrogen molecule photofragments are much richer in H atoms than in D. Overall, the heavier D atoms are more likely than the H atoms to remain attached to the carbon atom. An implication for astrophysics is discussed. The VUV absorption spectra of CH4 and CH3D are almost identical both at room temperature and 75 K. There is, as expected, no variation in the absorption spectrum with temperature. Evidence is given that all or almost all of the methylene is produced in the a 1A1 and not in the ground 3B1 state.

Photodissociation of acetaldehyde: The CH4+CO channel

Ab initio quantum chemical calculations for the molecular dissociation channel of acetaldehyde are reported. The enthalpy change for the dissociation of acetaldehyde into methane and carbon monoxide was calculated to be exoergic by 1.7 kcal/mol. The transition state for this unimolecular dissociation, confirmed by normal mode analysis, was found to have an activation energy of 85.3 kcal/mol. Experimental measurements are reported for the vibrational and rotational state distribution of the CO product. No v=1 CO is found and the rotational temperature is 1300±90 K. The reaction coordinate at the transition state implies that the CO product is vibrationally cold and rotationally hot. This conclusion, which requires quantum dynamics calculations to confirm definitively, does agree with and aids in explaining the experimental results.

The CO product of the reaction of O(3P) with CH3 radicals

The reaction of O(3P) atoms with CH3 radicals is shown to produce CO (in addition to the major product CH2O) which is detected by laser induced fluorescence. The rotational and vibrational temperatures of the CO product are about 2000 K. The results are explained by the assumption that the reaction takes place mainly by an indirect mechanism in which a methoxyl radical is formed and then dissociates unimolecularly.

Kinetic energies of hydrogen atoms photodissociated from alkyl radicals

Abstract A series of alkyl halides was photodissociated at 205.1 nm, forming halogen atoms and alkyl radicals. The latter were in turn dissociated at the same wavelength into alkenes and hydrogen atoms. The average kinetic energies of the hydrogen atoms are large (1.5–2.0 eV) and decrease only slowly with increasing chain length. The inference is that the dissociation mechanism involves crossing from the Rydberg state surface to a repulsive region of the ground state surface at a point near the center of a C–C bond. As had been previously found by Koplitz and coworkers, isotope scrambling was seen in CH 3 CD 2 radicals.

Hydrogen atom release from methyl groups of energized molecules

Three diverse molecules, 2,3,4,5,6-pentafluorotoluene (PFT), trimethylamine (TMA), and methyl bromide were irradiated by weak 193.3 nm light. In each case hydrogen atoms were released, as the major channel for PFT and as a minor channel for the two others. The low average translational energy release, the isotropic angular distribution, and the linear excimer power dependence all lead to the conclusion that the dissociation is from a very hot ground state. The unimolecular dissociation is less selective than in the more usual unimolecular processes at much lower energies. During these experiments an unusual multiphoton dissociation was found. Focusing 205.1 nm light on TMA produces H atoms with an average kinetic energy of 4.2±0.5 eV.