Femtosecond chirped pulse excitation of vibrational wave packets in LD690 and bacteriorhodopsin

Chirped femtosecond pulses are used to selectively excite vibrationally coherent wave packets in the ground and excited states of molecules in solution. Femtosecond chirped pump/transform-limited probe experiments on both nonreactive and reactive systems (LD690 and bacteriorhodopsin) are presented, showing that slight pulse chirps can lead to large differences in the observed amplitudes and damping times of the wave packet oscillations. By comparing the experimental data with numerical simulations based on multimode harmonic oscillator models for the molecular potential energy surfaces, we conclude that positively chirped pulses discriminate against the formation of an oscillatory ground-state component via impulsive stimulated Raman scattering, while negatively chirped pulses enhance this process. The ability to separate the relative contributions of either the ground- or excited-state vibrational coherence to the transient absorption signal by slightly modifying the phase structure of the excitation pulse enables us to obtain state-specific information about the vibrational dephasing.