THE COUPLED-CHANNEL DENSITY-MATRIX METHOD FOR OPEN QUANTUM-SYSTEMS - FORMULATION AND APPLICATION TO THE VIBRATIONAL-RELAXATION OF MOLECULESSCATTERING FROM NONRIGID SURFACES

The recently proposed coupled channel density matrix (CCDM) method for nondissipative dynamics [L. Pesce and P. Saalfrank, Chem. Phys. 219, 43 (1997)], is extended to open quantum systems. This method, which is the density matrix analogue of the coupled channel wave packet (CCWP) method in Schrodinger wave mechanics, allows for the solution of nucl ear Liouville-von Neumann equations in more than one dimension includi ng unbound modes. A semiphenomenological, Markovian, and trace-conserv ing dissipative model within the dynamical semigroup approach is sugge sted, and efficient numerical schemes for its implementation are prese nted. Using a two-mode model, we apply the dissipative CCDM method to the problem of vibrationally excited gas-phase hydrogen molecules, rel axing during the scattering from a cold, metallic, and nondissociative surface. The significance of a relaxation mechanism based on electron -hole pair creation in a metallic substrate is addressed. The dependen ce of the survival probability of the vibrationally excited molecules on the dissipative model parameters, on their initial translational en ergy, and on isotopic substitution is examined and rationalized on the basis of a simple classical kinetic model. (C) 1998 American Institut e of Physics.