Laser-induced thermal desorption of NO from a cryogenic C1NO film. Evidence for desorption barrier from time-of-flight distributions

Abstract

The dynamics of the 193 nm laser-induced thermal desorption of NO monomer from an irradiated C1NO film held at 80 K has been studied using state-resolved laser-induced fluorescence of NO. The rotation and electronic (spin-orbit) populations of the desorbed NO(ν=0) deduced from LIF excitation spectra are described well by a Boltzmann distribution with a temperature close to that of the film. Translational distributions with an asymmetric and forward peaked shape with a sharp onset were found and explained by a small desorption barrier of 27±5 meV. The velocity distributions were only found to be Maxwellian provided this activation energy for desorption is included in the usual expression for a Maxwell-Boltzmann distribution of disorbate velocities. Both the internal and translational distributions support the conclusion that the desorption of cold NO(ν=O) proceeds by a thermal desorption mechanism.