Stimulated by experiments by Zewail and co-workers [Chem. Phys. Lett. 172, 109 (1990)], we perform quantum calculations on the long time behavior of a wave packet which moves on coupled electronic surfaces of the NaI molecule. Beat structures in the simulated femtosecond pump/probe signal are shown to depend critically on the parameters used in the calculation. The adiabatic motion is compared to the dynamics including the nonadiabatic coupling and the consequences for the signal are explored. The results are compared to a semiclassical analysis of Chapman and Child [J. Phys. Chem. 95, 578 (1991)].