Validity of the Bersohn–Zewail model beyond justification

The absorption of probe pulses in ultrafast pump–probe experiments can be determined from the Bersohn–Zewail (BZ) model. The model relies on classical mechanics to describe the dynamics of the nuclei in the excited electronic state prepared by the ultrashort pump pulse. The BZ model provides excellent agreement between the classical trajectory and the average position of the excited state wave packet. By investigating the approximations connecting the nuclear dynamics described by quantum mechanics and the BZ model, we conclude that this agreement goes far beyond the validity of the individual approximations.