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

Abstract

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 B13 state.

Reaction Dynamics on the Formation of 1- and 3-Cyanopropylene in the Crossed Beams Reaction of Ground-State Cyano Radicals (CN) with Propylene (C3H6) and Its Deuterated Isotopologues

“…Information on the chemical dynamics was gained by fitting these TOF spectra and the angular distribution in the laboratory frame (LABORATORY) using a forward-convolution routine. [24][25][26] This approach initially assumed an angular distribution T(θ) and a translational energy distribution P(E T ) in the center-of-mass reference frame (CM). TOF spectra and the laboratory angular distribution were then calculated from these center-of-mass functions.…”

Photodissociation of Methane at Lyman Alpha (121.6 nm)

“…Some time ago, somewhat coarse-grained spectra of CH 4 and CD 4 were reported which showed that at 10.2 eV CD 4 absorbed about 20% more strongly than CH 4 . 12 Wang et al 9 have measured the spectra of CH 3 D and found that the spectrum is almost identical in shape and strength to that of CH 4 . When we used the same absorption cross section for CH 4 and CD 4 as 485 atm…”

“…Using the Lyα photolysis of H 2 O as a reference, Brownsword et al 7 were able to establish the absolute quantum yield for the H atom formation for CH 4 at Lyα, Φ H = 0.47 ± 0.11. More recently, Wang et al 9 reported a complete set of quantum yields for the different photodissociation channels of methane and its isotopomers. Comparing quantum yields from several research groups, the values were a little different.…”

Photochemistry and Photo-Induced Ultrafast Dynamics at Metal Surfaces

“…The excited-state orbital is essentially a 3s Rydberg-type orbital. Excitation to the excited states leads to dissociation of methane to CH 2 þ H 2 and CH 3 þ H, which has been studied experimentally 18,[36][37][38][39] and theoretically. [40][41][42] Recent ab initio calculations 42 and experiments 38,39 suggest that photodissociation of gaseous methane occurs after intersystem crossing to the T 1 state or internal conversion to the S 0 ground state.…”