A novel mixed quantum/classical method for the dynamics of molecules in a gas environment with an application to femtosecond time-resolved spectroscopy

A mixed quantum/classical method is proposed to describe the dynamical behaviour of a diatomic molecule in a gas environment. The vibrational coordinate is treated quantum-mechanically and all other degrees of freedom classically. Within the present approach the classical equations for the rotational motion have no singularities. A simplectic algorithm is used for the propagation. As an application we treat collisions of rare-gas atoms with I2 excited by femtosecond pulses. The decay of the pump–probe signal via the thermal average over rotational states and by collisions with the buffer gas is considered.