A comparison of the selective excitation of molecular modes in gas and liquid phase using femtosecond pulse shaping

Abstract

Abstract A feedback‐controlled optimization in a femtosecond coherent‐anti Stokes Raman scattering (CARS) process is applied to selectively excite or suppress vibrational modes in the gas and liquid phase. The optimal control experiments are performed on carbon disulfide and toluene molecules. Here our aim is to understand whether the interaction of the molecules with the surrounding medium affects the optimization process. The CARS excitation was chosen to be not in resonance with an electronic transition in the molecule but to excite different vibrational modes coherently. A pure phase modulation of the Stokes pulse resulted in changes of the ratio of the Raman lines observed in the nonlinear scattering spectrum. This could also be achieved when no temporal shift between the pump and Stokes laser resulted in a simple change of the Raman resonances. The relative intensities of the Raman lines could be changed more effectively in the liquid phase than in the gas phase. The higher density in the condensed matter, which hinders free rotation and makes interactions between molecules an important factor, obviously seems to influence the control mechanism. Copyright © 2008 John Wiley & Sons, Ltd.