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 the desorbed on Ag(111). At 355 nm, only direct desorption of H↓(2)CO is observed, whereas both CO product formati0 and H&(2)CO 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” channed in near-thermal equilibrium with the Ag surface. No gas-phase hydrogen products (H atoms, H2) 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↓(2)CO 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 H2CO-(a) 0r other H2CO(a) excited-state species. In combination with depletion rates measured over the temperature rang 30-95 K, the kinetic model results in an activation energy for polymerization of 0.90 ±0.06 kcal/mol.