State-Selective Laser Photochemistry of Formaldehyde on Ag(111)

摘要

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(111). At 355 nm, only direct desorption of H_2CO is observed, whereas both CO product formation and H_2CO desorption are found at 266 nm. The rotational state and translational energies of the desorbed CO products exhibit a "fast" channel associated with a prompt fragmentation process and a "slow" channel in near-thermal equilibrium with the Ag 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 H_2CO are strongly temperature dependent and reflect the rate of polymerization. A simple kinetic model is used to describe the competing processes of desorption, fragmentation, and polymerization which ultimately result from the decay or fragmentation of an H_2CO_((a))~- or other H_2CO_((a)) excited-state species. In combination with depletion rates measured over the temperature range 30-95 K, the kinetic model results in an activation energy for polymerization of 0.90 ± 0.06 kcal/mol.