Tracing the vibrational dynamics of sodium iodide via the spectrum of emitted photofragments.

We study by real-time wave packet simulations the ultrafast photodissociation dynamics of the sodium iodide molecule with the aim to trace molecular vibrational motion in a bound electronic state. Applying a few-cycle infrared pump laser pulse, a nuclear wave packet is created in the ground electronic state via the dynamic Stark shift of the potential energy curves of the molecule. To probe this coherent motion in the ground state, we propose to use a series of ultrashort laser pulses with different photon energies that resonantly promote the spread-out wave packet to the repulsive excited state. As the kinetic energy release (KER) spectrum of the dissociating photofragments is sensitive to the shape of the vibrational wave packet, in our pump-probe scheme, KER-delay spectrograms generated for different probe photon energies are used to monitor the molecular motion in the bound state. In our numerical analysis supported by a simple analytical model, we show that for sufficiently long probe pulses the proposed mapping scheme reaches its limits as nuclear wave packet interferences wash out the observed images. The appearance of these interferences is attributed to nuclear wave packet amplitudes that are generated at the first and second half of the probe pulse with the same energy but with a certain time delay. In our detailed numerical survey on the laser parameter dependence of the presented scheme, we find that resonant probe pulses with a few femtosecond duration are suitable for a qualitative mapping of the bound-state molecular motion.