Broadband infrared and Raman probes of excited-state vibrational molecular dynamics; Simulation protocols based on loop diagram

TL;DR

This paper develops simulation protocols for vibrational dynamics in excited states using infrared and Raman techniques, emphasizing the influence of system dynamics on resolution limits and deriving correlation functions for different signal representations.

Contribution

It introduces three novel simulation protocols based on wavefunction propagation, sum over states, and semiclassical bath treatment for excited-state vibrational dynamics.

Findings

Resolution depends on system dynamics, not just experimental parameters.

Fourier uncertainty principle is upheld in all techniques.

Derived multipoint correlation functions for infrared and Raman signals.

Abstract

Vibrational motions in electronically excited states can be observed by either time and frequency resolved infrared absorption or by off resonant stimulated Raman techniques. Multipoint correlation function expressions are derived for both signals. Three representations for the signal which suggest different simulation protocols are developed. These are based on the forward and the backward propagation of the wavefunction, sum over state expansion using an effective vibration Hamiltonian and a semiclassical treatment of a bath. We show that the effective temporal ($\Delta t$) and spectral ($\Delta\omega$) resolution of the techniques is not controlled solely by experimental knobs but also depends on the system dynamics being probed. The Fourier uncertainty $\Delta\omega\Delta t>1$ is never violated.