A mixed quantum-classical method is proposed to describe the dynamical behavior 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 symplectic, energy conserving and time-reversible algorithm is used for the propagation. As an application we treat the dynamics of I-2 molecules excited by femtosecond laser pulses moving in collision-free and high pressure rare-gas environments. For freely rotating I-2 molecules, the thermal average over rotational states leads to the decay of the pump-probe signal. For I-2 in inert gases, we show that dephasing by collisions with the buffer gas is a weak effect in comparison with the decay of the signal due to the anharmonicity of the potential energy curve. Therefore the oscillating structure of the pump-probe signals depends weakly on the mass and the pressure of the solvent, in agreement with experimental data.