The role of pulse sequences in controlling ultrafast intramolecular dynamics with four-wave mixing

This article seeks to provide a fundamental understanding of time-resolved four-wave mixing (FWM) processes based on a large body of experimental meas urements on a model system consisting of isolated iodine molecules. The the oretical understanding is based primarily on a diagrammatic approach. Doubl e-sided Feynman diagrams are used to classify and describe the coherent FWM processes involved in the signal obtained with each pulse sequence. Differ ent pulse sequences of degenerate femtosecond pulses are shown to control t he optical phenomena observed, that is transient grating, reverse-transient grating, photon echo and virtual photon echo. The experimental data reveal clear differences between the nonlinear optical phenomena. We find that th e virtual photon echo sequence k(1) - k(2) + k(3) is the most efficient for controlling the observation of ground-or excited-state dynamics. The strat egy followed to make this assessment was to compare transients when the tim e delay between two of the three pulses set in or out of phase with the exc ited-state vibrational dynamics. We have obtained a signal from pulse seque nces k(1) + k(2) - k(3) for which FWM signal generation for this two-electr onic-level system is forbidden. This signal can be explained by the cascadi ng of a first-order polarization and a second-order process to generate the FWM signal. The implications of our findings are discussed in the context of multiple-pulse methods for the control of intramolecular dynamics.