STATE-SELECTIVE LASER PHOTOCHEMISTRY OF FORMALDEHYDE ON AG(111)

State-selective (VUV + UV) resonant multiphoton ionization is used to measure the state- and energy-distributions of the gas-phase products which accompany the UV photoinduced polymerization of formaldehyde on Ag(lll). At 355 nm, only direct desorption of H2CO is observed, wherea s both CO product formation and H2CO desorption are found at 266 nm. T he rotational state and translational energies of the desorbed CO prod ucts exhibit a ''fast'' channel associated with a prompt fragmentation process and a ''slow'' channel in near-thermal equilibrium with the A g surface. No gas-phase hydrogen products (H atoms, H-2) were detected during UV exposure, but H-2 desorption is observed during breakup of the polymer above 200 K. The time-dependent desorption yields for both slow CO and H2CO are strongly temperature dependent and reflect the r ate of polymerization. A simple kinetic model is used to describe the competing processes of desorption, fragmentation, and polymerization w hich ultimately result from the decay or fragmentation of an H2CO(a)(- ) Or other H2CO(a) excited-state species. In combination with depletio n rates measured over the temperature range 30-95 K, the kinetic model results in an activation energy for polymerization of 0.90 +/- 0.06 k cal/mol.