Femtosecond spectrally dispersed three-pulse four-wave mixing: the role ofsequence and chirp in controlling intramolecular dynamics

The roles of pulse sequence and pulse chirp were explored using femtosecond three-pulse four-wave mixing (FWM), The experiments were carried out on ga s-phase It and the degenerate laser pulses are resonant with the transition between the X ((1)Sigma g+) ground and B ((3)Pi(0u+)) excited electronic s tates. Impulsive excitation leads to the observation of vibrational coheren ce in the ground and the excited states. Control over the observed populati on and vibrational coherence is achieved by using specific pulse sequences. Using chirped pulses results in changes in vibrational coherence. When the PWM signal is spectrally dispersed, the two-dimensional data (wavelength a nd time delay) provide important spectroscopic information about the intram olecular dynamics of both electronic states. This information is not typica lly available in time or spectrally integrated measurements. A theoretical foundation for these observations based on the density matrix formalism is briefly discussed. Copyright (C) 2000 John Wiley & Sons, Ltd.