Temperature programmed desorption and reflection absorption infrared studies of photoinduced polymer film formation and degradation and hydrogen bromide/ice systems in ultra-high vacuum

Thin films of pure styrene, vinylpyrrolidinone and co-dosed mixtures were condensed on Ag(110) at 100 K. Polymerization was carried out using a 500 W mercury arc lamp and monitored using reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD).;Kinetics studies showed the photopolyrnerization reaction is second order with respect to monomer coverage. The rate constant is inversely proportional to the film thickness. Rates of copolymerization lie between those of the pure homopolymers. A model proposed by Adkin was used to model the rates of copolymerization, yielding a degree of correlation.;Post-irradiation TPD indicates formation of several species. In the pure monomeric films, these species can be identified as dimers, trimers and tetramers. In the mixed films crossed product species can also be identified. TPD data indicate the maximum chain length and thermal stability of the polymer films are dependent upon film thickness. RAIR spectra as a function of temperature provide evidence for the formation of polystyrene and polyvinylpyrrolidinone and show that thicker films are more thermally stable.;HBr uptake by thin ice films was examined at 90, 120 and 140 K. TPD of ice films exposed to HBr at 140 K formed a tetrahydrate. TPD shows a significant amount of pure water still present in the film following uptake at lower temperatures. Zero order desorption kinetics for pure ice and tetrahydrate were determined using a leading edge analysis yielding: Ed (pure) = 50.8 kJmol--1 and n0 (pure) = 7.5 x 1031 cm--2s --1 and Ed (tetrahydrate) = 37.7 kJmol--1 and n0 (tetrahydrate) = 2.0 x 1027 cm--2 s--1.;HBr/water films were co-deposited at 90 K. TPD data was collected for each film. These data were used to characterize the changes observed in the RAIR spectrum of a 0.5:1 film as a function of temperature. Marked changes occurred in the spectra at 140 K, and 155--160 K. The film stoichiometry was calculated as a function of temperature using the TPD data. and indicated no abrupt changes of stoichiometry. This analysis suggests contrary to previous assignments that the spectral changes observed upon heating are due to changes in the structure of the film.