Upon the irradiation of UV laser pulses at 193 nm or 248 nm, photodissociation, bimolecular photoreaction, and photodesorption of surface molecules were observed for the deposited molecular solids on a quartz plate at about 100K. The observed translational energy distribution of ejected particles made possible the analyses of the dynamical pathways of the detachment processes. Small molecules such as NH3 and CS2 showed photoejection of a hydrogen atom and a sulfur atom, respectively, similar to the gas phase processes, .: although the dissociation was found to occur through the lowest dissociation channel. Acetylene solids showed an addition reaction producing vinylacetylene at 193 nm with a quantum yield of 0.003 at 77K. Vinylacetylene was then converted to diacetylene after the excitation by the second photon through the disproportionation reaction with another acetylene: C4H*4 + C2 H2→C4H2+C2H4.
Butadiene solids covered with submonolayer Co2 (CO) 8 produced butene molecules at the interface for the excitation of Co2 (CO) 8 at 248 nm. CO fragments were also ejected from the surface, although the translational energy was very low suggesting the dissociation of the Co=CO bond in the interface region between the surface Co2 (CO) 8 and the inner butadiene layer. Three different mechanisms have been introduced for the photodesorption induced by UV laser irradiation of molecular solids: 1) Franck-Condon transition to a repulsive potential curve with respect to the surface bond, 2) thermal excitation of localized surface sites initiated by the generationof high energy atomic or diatomic fragments, and 3) optoacoustic shock propagation to the surface ejecting weakly bound molecules, which is induced by the electron-phonon coupling near the impurity sites. The last one was referred as a highly likely mechanism for the solids with the lowest valence excited states.
