Comparing and Contrasting Vibrational Wavepacket Dynamics and Impulsive Stimulating Raman Scattering Descriptions of Pump-Probe Spectroscopy: A Theoretical Study

We simulate a third-order nonlinear signal in a pump-probe spectroscopy from the interference between first- and second-order wavepackets (WPs), as well as from a state-to-state transition for Stokes and coherent anti-Stokes pathways in the context of impulsive stimulated Raman scattering (ISRS) excitation. We present a detailed step-by-step description of both methods. The results obtained from these two methods are compared and contrasted through simulations of the excited-state absorption signal. While within the ISRS framework, vibrational dynamics is often attributed primarily to coherences between adjacent vibrational levels in the excited electronic states, our results show that coherences involving non-adjacent vibrational levels needs to be calculated for a better agreement with the WP approach. We also show that for the specific choice of pump/probe spectral bandwidths, the coherent anti-Stokes pathway majorly contributes to the observed signal.