Multiphoton ionization studies of IR multiphoton dissociation: direct C-H bond cleavage in methanol

The multiphoton ionization (MPI) spectra of products generated by infrared multiphoton dissociation (IRMPD) of CH/sub 3/OH and CD/sub 3/OH are reported. Over the 445-510-nm dye laser range covered, the MPI spectral features are predominantly due to hydroxymethyl radicals, CH/sub 2/OH and CD/sub 2/OH, with a very minor contribution from CH/sub 3/. At short reaction times no formaldehyde is detected. The lowest energy methanol decomposition pathway, to CH/sub 3/ + OH, is not explored because resonantly enhanced CH/sub 3/, MPI is inefficient within the wavelength range available. Selective IRMPD of CD/sub 3/OH in 1 Torr of a 1:1 CH/sub 3/OH:CD/sub 3/OH mixture yields exclusively CD/sub 2/OH at short (200 ns) reaction times demonstrating direct C-H bond cleavage does occur in competition with the lower enthalpy reaction (C-O bond breaking). The upper bound for the branching ratio to CH/sub 2/OH + H (relative to CH/sub 3/ + OH) is 20% based on comparison with shock tube experiments. The time scale for CH/sub 2/OH produced bimolecularly is consistent with the rate constant for H-atom abstraction by OH from methanol. The excitation proceeds via a resonant two-photon absorption to a 3p Rydberg level (screening constant delta = 0.65) followed by one more photon to exceed the ionization threshold. Failure to observe two-photon fluorescence is consistent with this assignment of the initial step as a p-to-p type excitation.