# Signatures of Antisymmetric Vibrations in the Ultrafast Dynamics of Quadrupolar Dyes

## TL;DR

This paper shows how to detect antisymmetric vibrations in molecules using ultrafast spectroscopy, revealing new insights into their excited state dynamics.

## Contribution

A new method to identify antisymmetric vibrations in ultrafast 2DES through asymmetric peak patterns and simulations.

## Key findings

- Sub-50 fs 2DES maps show asymmetric peak patterns linked to antisymmetric vibrations.
- Simulations confirm that these peaks arise from an anharmonic excited state potential.
- The method tracks wavepacket motion before relaxation obscures the signal.

## Abstract

Antisymmetric molecular vibrations are central to ultrafast,
nonadiabatic
photophysical and photochemical processes such as conical intersection
dynamics, Herzberg–Teller couplings and, potentially, singlet
fission. Direct spectroscopic identification of such vibrations is,
however, challenging, since they are typically Raman inactive and
affect optical transitions only weakly. Here, we report experimental
signatures of vibronic couplings to a high-frequency antisymmetric
vibration in the excited state dynamics of a quasi-quadrupolar molecule
by ultrafast two-dimensional electronic spectroscopy (2DES). The early
time, sub-50 fs 2DES maps reveal an asymmetric peak pattern with characteristic
low-energy cross-peaks. We show that these peaks arise from stimulated
emission transitions from an anharmonic, double-minimum excited state
potential energy surface formed by vibronic coupling to a high-frequency
antisymmetric mode. Simulations based on a phenomenological essential
state model support the results. Our findings offer a new approach
for identifying antisymmetric vibrations in ultrafast 2DES and track
excited state wavepacket motion before relaxation washes out the spectra.