Competition between adsorbate fragmentation and desorption on laser-heated surfaces

We present a theoretical study of the consequences of rapid laser heating of a model adsorbate-substrate system. The adsorbate in this study contains one weak bond and a number of strong bonds. The impetus for our theoretical model comes from the desorption/ dissociation experiments on labile organic molecules adsorbed on heated surfaces. Among the numerous interesting results of these experiments are nonstatistical processes where depending on the heating rate, stronger bonds break before the weaker ones. We model these processes by means of a stochastic trajectory approach using the generalized Langevin method. Results are given for model polyatomics of different lengths and configurations as well as different heating rates. The strengths of the weak bonds inside the molecule are changed and reactivities of adsorbates in parallel and perpendicular adsorption geometries are contrasted. The branching ratio between desorption and dissociation is found to depend sensitively on a number of these variables. We also find that small changes in the relative bond strengths of the weak bonds can change the reactivity from statistical to nonstatistical.