Femtosecond-time-resolved highly selective molecular-vibration excitation using a novel vibrationally synchronized pumping technique with frequency difference resonance

Selective stimulated Raman scattering using a single-color femtosecond N-pu lse train of finite pulse duration leads to a low excitation efficiency in the high frequency region. This low efficiency of Raman mode excitation is due to spectral modulation which stems from the finite duration of the puls es in each train. To overcome the low efficiency of excitation in the high frequency region, a vibrationally synchronized pumping technique with frequ ency difference resonance is proposed. In this technique, both the pulse-re petition rate and the center frequency difference of carrier-phase-locked t wo-color beams consisting of a femtosecond N-pulse train are chosen to be r esonant with the frequency of a specific vibrational or rotational mode of interest. In addition, a formula describing the optimum pulse number N for efficient selective excitation is derived. (C) 2001 Elsevier Science B.V. A ll rights reserved.