Dynamics of multidissociation paths of acetaldehyde photoexcited at 157 nm: Branching ratios, distributions of kinetic energy, and angular anisotropies of products.

After the photolysis of acetaldehyde (CH(3)CHO) at 157.6 nm in a molecular-beam apparatus using photofragment translational spectroscopy and vacuum-ultraviolet photoionization to detect products, we observed 13 photofragments associated with six primary dissociation channels and secondary dissociation of products CH(3)CO and HCO. We measured time-of-flight spectra and spatial angular anisotropies of products and evaluated the branching ratios of products. All photoproducts have nearly isotropic angular distributions with an average /beta/ value less than 0.05. Primary dissociations to CH(3)CO+H and CH(3)+HCO are two major paths; most CH(3)CO subsequently decomposes spontaneously to CH(3)+CO and CH(2)CO+H and most HCO decomposes to H+CO. The ternary dissociation to CH(3)+CO+H thus accounts for approximately half of the total branching. Dissociations to CH(2)CO+H(2) and CH(2)+CH(2)O are observable, but the production of CH(4)+CO is ambiguous. The productions of C(2)H(3)+OH and C(2)H(2)+H(2)O indicate that isomerization from acetaldehyde to ethenol occurs before fragmentation. After photoexcitation to the n-3p state, most acetaldehyde converts into states T(1) and S(0) but a little isomerizes to ethenol followed by multichannel decomposition.