Photodissociation dynamics of methyl formate at 193.3 nm: Branching ratios, kinetic-energy distributions, and angular anisotropies of products

We investigated the photodissociation dynamics of methyl formate-d (CH3OC(O)D) at 193.3 nm in a molecular-beam apparatus using undulator radiation as an ionization source. We measured kinetic-energy distributions, spatial angular anisotropies, and branching ratios of all photofragments. Fractions of energy release into product translation were calculated from the kinetic-energy distributions. Four primary dissociation pathways to asymptotes CH3O(X-tilde2E)+DCO(X-tilde2A'), CH3O(X-tilde2E)+DCO(A-tilde2A''), CH3OCO(X-tilde2A')+D(2S), and CH3OD(X-tilde1A')+CO(X 1Σ+) were identified; their branching ratios were determined to be 0.73, 0.06, 0.13, and 0.08, respectively. The former two dissociation paths were discernible in the time-of-flight spectra of fragment CH3O with a signal at m/z=29. Nominal products DCO (A-tilde2A'') and CH3OCO (X-tilde2A') were unobservable as DCO in state A-tilde dissociated to D(2S)+CO(X 1Σ+) and internally hot CH3OCO (X-tilde2A') decomposed to CH3(X-tilde2A2'')+CO2(X-tilde1A1g). Products DCO and CH3O have angular anisotropy parameter β≅-0.37 but other products have nearly isotropic angular distributions with |β|<0.1. Nonadiabatic transitions might play an important role in fragmentation of methyl formate irradiated at 193.3 nm.