Regarding the transient spectroscopy of I-2 in rare-gas environments, it is shown how to calculate femtosecond pump-probe signals with the help of the stochastic Langevin equation. The method is very fast due to the introduction of an additional parameter which describes the energy flow between the system and environment. The stochastic trajectory approach is tested by comparison with full-scale molecular dynamics calculations and good agreement is obtained for the signals. The stochastic trajectory simulation can be applied to the interpretation of experimental signals and adjustment of external parameters in theoretical models.