Vibrationally resolved electronic spectra including vibrational pre-excitation: Theory and application to VIPER spectroscopy

TL;DR

This paper develops theoretical methods to compute vibrationally resolved electronic spectra with vibrational pre-excitation, aiding the interpretation of VIPER spectroscopy experiments and predicting mode-specific effects.

Contribution

It introduces combined time-independent and time-dependent approaches for calculating spectra including vibrational pre-excitation, validated on medium-sized polyatomic systems.

Findings

Pre-excitation causes a red-shift in absorption spectra.

Excitation of ring distortion modes enhances VIPER effects.

Theoretical predictions match experimental observations.

Abstract

Vibrationally resolved electronic absorption spectra including the effect of vibrational pre-excitation are computed in order to interpret and predict vibronic transitions that are probed in the Vibrationally Promoted Electronic Resonance (VIPER) experiment [L. J. G. W. van Wilderen et al., Angew. Chem. Int. Ed. 53, 2667 (2014)]. To this end, we employ time-independent and time-dependent methods based on the evaluation of Franck-Condon overlap integrals and Fourier transformation of time-domain wavepacket autocorrelation functions, respectively. The time-independent approach uses a generalized version of the FCclasses method [F. Santoro et al., J. Chem. Phys. 126, 084509 (2007)]. In the time-dependent approach, autocorrelation functions are obtained by wavepacket propagation and by evaluation of analytic expressions, within the harmonic approximation including Duschinsky rotation effects. For several medium-sized polyatomic systems, it is shown that selective pre-excitation of particular vibrational modes leads to a red-shift of the low-frequency edge of the electronic absorption spectrum, which is a prerequisite for the VIPER experiment. This effect is typically most pronounced upon excitation of ring distortion modes within an aromatic pi-system. Theoretical predictions as to which modes show the strongest VIPER effect are found to be in excellent agreement with experiment.