A classical hamiltonian method for calculating IR-Resonant photodesorption rates

Summary In this work, classical nonlinear dynamical methods are used to study photodesorption induced by IR-laser resonant excitation of an internal vibrational mode of a physisorbed molecule. Starting from the large disagreement between the experimental values of the photodesorption rates and the theoretical quantum perturbative ones for CH₃F/NaCl, the Elastic-Resonant-State Decay (ERSD) model is analysed focusing attention on the nonexponential time decay behaviour of the survival probability of the system, that shows two different time scales. This is obtained by a suitable average of classical trajectories calculated by integrating Hamilton’s equations of motion. The Poincaré sections are used to clarify the details of the microscopic dynamics of the IR-photodesorption, pointing out the dynamical role of cantori as responsible for desorption occurring on a longer time scale. A method to build up the separatrix for the ERSD system is proposed, stressing that such a separatrix acts as a bottleneck against the energy flux exchanged between the physisorption and the intramolecular bond of the adsorbate. A calculation of the photodesorption rate when a condition of fast relaxation between the two bonds is verified, is performed using the Transport Theory in Hamiltonian System. The values of the photodesorption rate obtained by this approach are in good agreement with those obtained by integrating numerically the equations of motion.