Competing atomic and molecular hydrogen pathways in the photodissociation of methanol at 157 nm

Photofragment translational spectra at m/e=1(H), 2(H2,D), 3(HD), and 4(D2) have been obtained for CH3OH, CH3OD, and CD3OH at 157 nm excitation. Analysis of the time-of-flight spectra reveals two different atomic H loss channels: hydroxyl H elimination, and methyl H elimination. While the hydroxyl H elimination seems to be a single fast process, the methyl H loss exhibits clearly two significantly different mechanisms: one fast and one slow. Experimental results also show two molecular hydrogen elimination channels: three-center elimination from the methyl group, which displays two different micropathways, and four-center elimination involving hydrogen atoms on both the C and O sites. The relative branching of the atomic versus molecular hydrogen elimination channels was found to be 1:0.15. These results present a uniquely clear picture of methanol photodissociation at 157 nm, and thus provide an excellent case for quantitative theoretical investigations.