Femtosecond Wavepacket Spectroscopy: Influence of Temperature, Wavelength, and Pulse Duration

We discuss pump-probe signals from the wavepackets which are simultaneously generated on both the ground and excited electronic states by weak optical pulses shorter in duration than a vibrational period. A classical model of localized ground state bleaching and excited state population of the internuclear geometries where the photon energy matches the electronic energy gap predicts the pump and probe wavelength dependence and pulse duration dependence of the pump-probe signals. The classical signal contains higher harmonics of the fundamental vibrational frequency: in particular, if the wavepacket is probed near the middle of the well, the signal is dominated by the second harmonic of the vibrational frequency. We show that all pumpprobe signals can be calculated from de, the pump-induced change of the density operator, and present both quantum and classical pictures of the change in the ground state vibrational density operator, Computations of the iodine ground state contribution to the pump-probe signal agree with the classical predictions and show that deg can be narrower than the zero point level probability distribution. These computations are compared to the femtosecond transient dichroism signals observed for I2 in hexane reported by Scherer, Jonas, and Fleming (J. Chem. Phys. 1993,99, 153-168) and found to agree within experimental error. The quantum calculations also show the thermal simplification of the pump-probe signal toward the classical limit as temperature is increased. We show quantitatively that for an electronically resonant pump-pulse, deg tends toward a negative operator (a “pure ground state hole”) with increasing temperature, implying that certain types of pump-probe signals (e.g. photon-detected transient absorption) have signed ground state signals which do not oscillate through zero.