The effect of Duschinsky rotation on Femtosecond Coherence Spectra*

Ultra-short laser pulses can coherently excite molecular vibrations, which appear as oscillations in the signal of time-resolved spectroscopy measurements. In a transient-absorption spectroscopy measurement, there are typically several normal mode vibrations that contribute to the oscillations, and the amplitude and phase of each oscillation varies as a function of probe frequency. Femtosecond Coherence Spectroscopy (FCS) resolves the amplitude of the oscillations as a function of probe frequency and oscillation frequency creating mode specific amplitude profiles.  Prior work attempted to fit FCS profiles to a variety of one-dimensional models and showed promising results as a method to determine photo-physical parameters of laser dyes, but was unable to reproduce certain features in the data for physically relevant parameter values [1 - 2]. In particular the single-mode model could not reproduce the relative peak heights observed in the measured profiles. Here we show that a two-mode model incorporating Duschinsky rotation is able to reproduce the relative peak heights and is a promising approach to fit FCS profiles [3]. 

[1] Arpin and Turner, J. Phys. Chem. A, 125, 2425 (2021)

[2] Barclay et al., J. Phys. Chem. Lett., 13, 5413 (2022)

[3] Arpin et al., Appl. Math submitted