Density matrix theory and computational aspects of quantum dynamics in active medium

This work provides an overview of dynamics in extended systems in terms of density operators and the Liouville–von Neumann equation. Starting with the equations for the whole system, a partition into primary and secondary regions is introduced to derive an equation for the primary region containing dissipation and fluctuation effects due to the medium. The theory is developed for phenomena involving active media, such as recently observed in femtosecond experiments. It is based on a molecular self-consistent field procedure and describes dissipation induced by light absorption. Computational aspects are briefly described, including methods suitable for nonlinear optical response. The treatment is applied to photodesorption of diatomic molecules from a metal surface due to excitation by femtosecond pulses, and in particular to CO/Cu(001). The theory provides a model for the measured nonlinear dependence of desorption yields on the fluence of the light pulse and shows how the system evolves in time. It also predicts that chirped pulses can enhance or suppress the yields.