Molecular photoexcitation in a medium: Density operator approach

The excitation of a molecule in a medium, by absorption of a Light pulse, i s described in terms of density operators and the Liouville-von Neumann equ ation for the whole system. This is divided into a primary region containin g the molecule and its close neighbors to which it may be bonded or strongl y interacting, plus a secondary region including the remaining medium. A ti me-dependent self-consistent field procedure factors the density operator i nto primary and secondary components, each of which interacts with the appl ied light field. Direct absorption and indirect absorption are described wi th superoperators in a Liouville space formalism. Equations for the respons e of the primary region are developed assuming a stochastic medium. The tre atment allows for dissipation due to collective electronic or vibrational e xcitations in the medium. Applications to photodesorption of molecules from metal surfaces are briefly discussed. (C) 2000 John Wiley & Sons, Inc.