Selective Excitation of Wavepacket Motion in the Condensed Phase Using Chirped Pulses

Abstract

Recent theory and experiments have shown that chirped ultrashort pulses may be used to control the dynamics of wavefunctions evolving on potential energy surfaces of isolated molecular systems [1]. In this work we show that the phase structure of the excitation pulse can substantially affect the coherent vibrational dynamics of molecules in room temperature liquids. The interaction of a short pulse with a two electronic state system can induce vibrational motion on both the excited and ground state potential energy surfaces, which in turn can lead to ambiguity in the interpretation of pump-probe experiments [2,3]. Figure 1 illustrates the impulsive stimulated Raman scattering process that gives rise to motion on the S0 state and shows how a negatively chirped (NC) pulse enhances this contribution through a single photon “pump-dump” sequence of field interactions. Conversely, a positively chirped (PC) pulse discriminates against this process. In accord with previous theoretical work [4], one can to some extent turn on or off the ground state contribution to the transient absorption signal by using an appropriately chirped excitation pulse.. In this way chirping the pump pulse can help determine the origin of experimentally observed coherent vibrational dynamics.