A quantum mechanical study of predissociation dynamics of NaI excited by a femtosecond laser pulse

Abstract

This work was stimulated by the experiments of Rosker, Rose, and Zewail (RRZ) who used a pump–probe sequence of femtosecond pulses to study the predissociation of NaI. We calculate quantum mechanically the nuclear wave function created by the pump pulse and its subsequent evolution. To make contact with the experiments we assume that, depending on its wavelength, the probe pulse is absorbed by either the free sodium atoms or by the bound molecule in the neutral state, and that the measured fluorescence induced by the probe pulse is proportional to the populations of these two ‘‘species.’’ The similarity between computed populations and the observed signal confirms this conjecture. We study how various factors, such as the pulse length and shape, the initial vibrational state, the temperature, and the diabatic coupling strength affect the populations (and thus the LIF signal). We also show that the RRZ analysis based on classical trajectories and the Landau–Zener formula agrees semiquantitatively with the exact quantum results.