Femtosecond transient-grating techniques: Population and coherence dynamics involving ground and excited states

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Abstract
Time-resolved transient grating techniques (TG) arising from four-wave mixing(FWM) processes are explored for the study of molecular dynamics in gas-phase systems ranging from single atoms to large polyatomic molecules. For atomic species such as Ar and Xe, each TG signal shows only a peak at zero time delay when all three incident pulses are overlapped temporally. For diatomic O 2 and N 2 and linear triatomic CS 2 molecules, the TG signals exhibit ground state rotational wave packet recurrences that can be analyzed to obtain accurate rotational constants for these molecules. With heavier systems such as HgI 2 , ground state vibrational and rotational wave packet dynamics are observed. Resonant excitation allows us to select between measurements that monitor wave packet dynamics, i.e., populations in the ground or excited states or coherences between the two electronic states. To illustrate these two cases we chose the X→B transition in I 2 . TG measurements yield dynamic information characteristic of vibrational and rotational wave packets from the ground and excited states. Reverse transient grating (RTG) experiments monitor the time evolution of an electronic coherence between the ground and excited states which includes vibrational and rotational information as well. Early time TG signal for the polyatomic samples CH 2 Cl 2 , CH 2 Br 2 , benzene, and toluene exhibit a coherence coupling feature at time zero followed by rotational dephasing. Differences in the amplitude of these two components are related to the contributions from the isotropic and anisotropic components of the molecular polarizability. A theoretical formalism is developed and used successfully to interpret and simulate the experimental transients. The measurements in this study provide gas-phase rotational and vibrational dephasing information that is contrasted, in the case of CS 2 , with liquid-phase measurements. This comparison provides a time scale for intramolecular dynamics, intermolecular collisions, and solvation dynamics.