The role of pulse sequences in controlling ultrafast intramolecular dynamics with four-wave mixing
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This article seeks to provide a fundamental understanding of time-resolved four-wave mixing (FWM) processes based on a large body of experimental measurements on a model system consisting of isolated iodine molecules. The theoretical understanding is based primarily on a diagrammatic approach. Double-sided Feynman diagrams are used to classify and describe the coherent FWM processes involved in the signal obtained with each pulse sequence. Different pulse sequences of degenerate femtosecond pulses are shown to control the 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 the 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 strategy followed to make this assessment was to compare transients when the time delay between two of th...e three pulses set in or out of phase with the excited-state vibrational dynamics. We have obtained a signal from pulse sequences k(1) + k(2) - k(3) for which FWM signal generation for this two-electronic-level system is forbidden. This signal can be explained by the cascading 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.