PHOTOFRAGMENT IMAGING OF METHANE

The photolysis of methane is studied using photofragment imaging techn iques. Our study reveals that the photolysis of methane proceeds via m any different pathways. The photofragment imaging technique is used to resolve and characterize these various pathways and provides therefor e unique insight into the dynamical processes that govern this photodi ssociation. The formation of H-atom photofragments following absorptio n of a Lyman-alpha photon, and H-2 photofragments following absorption of two ultraviolet photons (lambda=210-230 nm) are studied. The measu red H-atom photofragment images reveal that a channel that produces fa st H atoms concomitant with methyl fragments is dominant in the Lyman- alpha photolysis of methane. This channel leads to an anisotropic reco il of the fragments. A secondary channel is observed leading to the fo rmation of somewhat slower H atoms, but an unique identification of th is second channel is not possible from the data. At least part of thes e slower H atoms are formed via a channel that produces H atoms concom itant with CH and H-2 photofragments. The recoil of these slower H ato ms appears to be isotropic. The measured, state-resolved H-2(v,J), pho tofragment images reveal that two channels lead to H-2 photofragments from the two-photon photolysis of methane: a channel that leads to H-2 products concomitant with methylene fragments; and a channel that lea ds to H-2 products concomitant with CH and H fragments. H-2(v,J) rotat ional and vibrational distributions are measured for each of these two channels separately, The H-2 products formed via the H-2+CH2 channel are rotationally and vibrationally highly excited, whereas those forme d via the H-2+CH+H channel are rotationally and vibrationally cooler. Rotational distributions of H-2 formed via the H-2+CH+H channel are we ll reproduced by Boltzmann distributions. Results on D-2 elimination f ollowing two-photon photolysis. of CD4 are in general similar and in q ualitative agreement with the results on CH4. (C) 1996 American Instit ute of Physics.