Controlling condensed-phase vibrational excitation with tailored infrared pulses

Vibrational population distributions within the CO-stretching T-1u manifold of W(CO)(6) in room-temperature n-hexane were created by using near-transform limited and linearly chirped picosecond infrared excitation pulses. These pulses were characterized using the second harmonic FROG (frequency-resolved optical gating) algorithm to determine the similar to 8 cm(-1)/ps chirp for both positively- and negatively-chirped 2 ps pulses. FROG and time-resolved transient difference spectra were obtained with an InSb focal plane array detector. While unchirped and positively-chirped excitation leads predominantly to upsilon = 1 population, negatively-chirped pulses produce excess population in the Li = 2 level. These results are compared to predictions from density matrix calculations for a model potential. (C) 1998 Elsevier Science B.V. All rights reserved.