LASER-INDUCED THERMAL-DESORPTION OF NO FROM A CRYOGENIC CLNO FILM - EVIDENCE FOR DESORPTION BARRIER FROM TIME-OF-FLIGHT DISTRIBUTIONS

The dynamics of the 193 nm laser-induced thermal desorption of NO mono mer from an irradiated ClNO film held at 80 K has been studied using s tate-resolved laser-induced fluorescence of NO. The rotational and ele ctronic (spin-orbit) populations of the desorbed NO(v=0) deduced from LIF excitation spectra are described well by a Boltzmann distribution with a temperature close to that of the film. Translational distributi ons with an asymmetric and forward peaked shape with a sharp onset wer e found and explained by a small desorption barrier of 27 +/- 5 meV. T he velocity distributions were only found to be Maxwellian provided th is activation energy for desorption is included in the usual expressio n for a Maxwell-Boltzmann distribution of desorbate velocities. Both t he internal and translational distributions support the conclusion tha t the desorption of cold NO(v=0) proceeds by a thermal desorption mech anism.