IR laser induced photodesorption of small alcohols (C1-C4) on silver (111)

Abstract

Infrared laser induced photodesorption of small alcohols (C1-C4) on Ag (111) was studied using C-H excitation (2800-3000cm^-1) and O-H excitation (3100-3500cm^-1). The photodesorption yield (PDY) depended on a number of factors including the mode of excitation, the annealing temperature, the wavelength of the laser, the laser intensity, and the film thickness. The PDY spectrum showed peaks at -3310cm^-1, -2880cm^-1 and -2970cm^-1. These were assigned as the O-H stretching mode, and the symmetric and antisymmetric C-H stretching modes, respectively. The PDY increased with the total amount of energy absorbed by the adsorbate film and decreased with the molecular weight of the alcohol molecules. The kinetic energies of the desorbing flux were determined by fitting the time-resolved photodesorption signal with a modified Maxwellian velocity distribution. Conditions which resulted in high PDY also gave high kinetic energies, which ranged from 10-25kJ/mol. The photodesorption signals resulting from C-H excitation were very similar to those from O-H excitation. However, the PDY was smaller, and the kinetic energy was higher when the C-H vibrational modes of the alcohol molecules were excited. The observed results were inconsistent with either the resonant heating or the direct mechanism. A non-thermal mechanism, which incorporates both the resonant heating and the direct mechanisms, is suggested.