Classical theory for real-time femtosecond probing of the NaI* photodissociation

Abstract

Femtosecond pump-probe experiments [Rosker et al., Chem. Phys. Lett. 146, 175 (1988)] on the dissociation of NaI* are modelled classically to obtain the absorption transients as a function of the pump and probe wavelengths. The initial ground state, the optically pumped predissociating state, and the third surface that is coupled by the probe pulse are explicitly included. The classical model can almost quantitatively explain all the features of the experimental results. The oscillations in the transients are shown to be due to molecules trapped in the adiabatic well, with most of the intensity coming from the covalent region of the well before the crossing point rx. The rising troughs in the transients are due to successive leaks out of the well into the covalent region after the crossing point rx, leading to dissociation. The difference in the absorption transient between on-resonance 589 nm probing as compared to off-resonance 612 nm and 580 nm probing is shown to arise from the difference in Lorentzian absorption widths between the trapped (500 cm−1) and dissociating (100 cm−1) covalent molecules, with the transition dipole moment to the third surface having about the same magnitude over the range of the covalent potential.