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

Knopp [J. Raman Spectrosc. 31, 51 (2000)] have recently used resonant femto second 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 resona nt femtosecond pulses with two independently variable time delays. The firs t 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 opt imized by selecting the proper pump-dump (Raman) frequency difference and v arying the time delay. The third pulse promotes the pump-dump wave packet t o an excited electronic state, resulting in subsequent coherent emission of light at the anti-Stokes frequency. This fully-resonant CARS signal, measu red as a function of time delay between the second and third pulses, oscill ates at a frequency characteristic of the pump-dump wave packet. Due to anh armonicity, this frequency is a sensitive measure of the amount of vibratio nal excitation. Knopp observed that under certain conditions the signal exh ibits pronounced beating between the pump-dump wave packet frequency and th e 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 wav e packet is above the excited state dissociation threshold of the molecule. We derive analytical expressions showing that under these conditions, wher e the polarization is short-lived, there may be strong interferences betwee n the contributions from molecules originally in different vibrational stat es of the thermal ensemble. In contrast, the CARS polarization in the below threshold case is long-lived, and these interferences cancel. Numerical ev aluation of the CARS signal through vibrational wave packet propagation con firms the predictions of the analytical theory and reproduces the distincti ve 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 vie wed from a different perspective, as an extension of the pump-dump mechanis m for selective bond breaking on the ground electronic state, to a pump-dum p-pump sequence for selective bond breaking on the excited electronic state . (C) 2001 American Institute of Physics.