Infrared spectroscopy and photodesorption of condensed phase ammonia

We have characterized the adsorption and thermal desorption of thick films of NH3 and ND3 condensed on Ag(111) with Fourier transform reflection absorption infrared spectroscopy and temperature programmed desorption. Over the coverage range of 1-50 monolayers, the asymmetric stretch (2513 cm(-1) for ND3 and 3380 cm(-1) for NH3) and symmetric deformation (832 cm(-1) for ND3 and 1078 cm(-1) for NH3) modes of ammonia are the most intense features in the vibrational spectra. Temperature programmed desorption yields an activation energy for thermal desorption of NH, and ND, multilayers of 23.8 kJ mol(-1) and 24.7 kJ mol(-1), respectively. At a coverage of 50 ML we used 193 nm light to stimulate molecular desorption from the multilayers and measured time-of-flight spectra. All time-of-flight spectra were bimodal; the average translational energies of the two components were 3150 +/- 100 K and 550 +/- 50 K. The fast channel is ascribed to ammonia molecules ejected from the vacuum-film interface while the slower channel is ascribed to ammonia molecules desorbing from deeper layers of the film. At 50 ML the angular distribution of photodesorbed ammonia peaks between 20 degrees and 40 degrees off the surface normal. (C) 1998 Elsevier Science B.V. All rights reserved.