Infrared-laser-induced photodesorption of NH3 and ND3 adsorbed on single-crystal Cu(100) and Ag film

Abstract

Desorption is obtained when ir laser pulses at 33203400 cm-1 excite the N H stretching modes of NH3 and/or ND3 adsorbed on single-crystal Cu(100) and Ag film surfaces at low temperatures and under ultrahigh-vacuum conditions. While resonant photodesorption occurs with a quantum efficiency of less than 10-4 for laser fluence up to 20 mJ/cm2, no significant isotope selectivity in photodesorption is observed when NH3 is vibrationally excited in co-adsorbed mixtures of NH3 and ND3. Surface coverage, composition, and heat of adsorption of ammonia are determined by conventional thermal desorption, x-ray photoemission, and laser-induced thermal desorption excited by 532-nm light pulses. Molecular desorption is detected by time-of-flight quadrupole mass spectrometry. The desorption behavior is studied as a function of laser frequency and fluence as well as the surface coverage at different substrate temperatures. Calculations of the photodesorption rates based on a master equation including phonon and electronic damping mechanisms are performed. It is shown that the resonant or indirect heating effect caused by the energy damping can explain some but not all of the major experimental observations. The possibility of internal molecular excitation during the desorption process is suggested. This and other dynamic aspects of the ir photodesorption process are also discussed. © 1985 The American Physical Society.