FEMTOSECOND CHIRPED PULSE EXCITATION OF VIBRATIONAL WAVE-PACKETS IN LD690 AND BACTERIORHODOPSIN

Chirped femtosecond pulses are used to selectively excite vibrationall y coherent wave packets in the ground and excited states of molecules in solution. Femtosecond chirped pump/transform-limited probe experime nts on both nonreactive and reactive systems (LD690 and bacteriorhodop sin) 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 mol ecular potential energy surfaces, we conclude that positively chirped pulses discriminate against the formation of an oscillatory ground-sta te component via impulsive stimulated Raman scattering, while negative ly chirped pulses enhance this process. The ability to separate the re lative contributions of either the ground- or excited-state vibrationa l coherence to the transient absorption signal by slightly modifying t he phase structure of the excitation pulse enables us to obtain state- specific information about the vibrational dephasing.