Vibrational polarization beats in femtosecond coherent anti-Stokes Raman spectroscopy: A signature of dissociative pump–dump–pump wave packet dynamics

Abstract

Knopp et al. [J. Raman Spectrosc. 31, 51 (2000)] have recently used resonant femtosecond coherent anti-Stokes Raman spectroscopy (CARS) to prepare and probe highly excited vibrational wave packets on the ground electronic potential surface of molecular iodine. The experiment uses a sequence of three resonant femtosecond pulses with two independently variable time delays. The first two pulses act as a pump and dump sequence to create a predefined, highly excited wave packet on the ground electronic state, whose amplitude is optimized by selecting the proper pump–dump (Raman) frequency difference and varying the time delay. The third pulse promotes the pump–dump wave packet to an excited electronic state, resulting in subsequent coherent emission of light at the anti-Stokes frequency. This fully-resonant CARS signal, measured as a function of time delay between the second and third pulses, oscillates at a frequency characteristic of the pump–dump wave packet. Due to anharmonicity, this frequency is a sensitive measure of the amount of vibrational excitation. Knopp et al. observed that under certain conditions the signal exhibits pronounced beating between the pump–dump wave packet frequency and the frequency characteristic of the bottom of the ground state well. In this paper we show that these beats arise only when the final pump–dump–pump wave packet is above the excited state dissociation threshold of the molecule. We derive analytical expressions showing that under these conditions, where the polarization is short-lived, there may be strong interferences between the contributions from molecules originally in different vibrational states of the thermal ensemble. In contrast, the CARS polarization in the below threshold case is long-lived, and these interferences cancel. Numerical evaluation of the CARS signal through vibrational wave packet propagation confirms the predictions of the analytical theory and reproduces the distinctive beating pattern observed in the experiments. Additional experiments and simulations demonstrate that these interferences can be turned on or off by carefully selecting the pulse frequencies. The experiments can also be viewed from a different perspective, as an extension of the pump–dump mechanism for selective bond breaking on the ground electronic state, to a pump–dump–pump sequence for selective bond breaking on the excited electronic state.