Time-resolved high-harmonic spectroscopy of ultrafast photo-isomerization dynamics

TL;DR

This study demonstrates the use of time-resolved high-harmonic spectroscopy to track ultrafast electronic and nuclear dynamics during the photo-isomerization of 1,3-cyclohexadiene to 1,3,5-hexatriene, revealing reaction timescales and pathways.

Contribution

First application of TR-HHS to monitor bond-making chemical reactions, providing insights into ultrafast isomerization dynamics with combined electronic and vibrational information.

Findings

Electronic excited state relaxes within 80 fs to ground state.

Ring-opening occurs around 400 fs after excitation.

TR-HHS effectively tracks ultrafast photo-chemical reactions.

Abstract

We report the first study of time-resolved high-harmonic spectroscopy (TR-HHS) of a bond-making chemical reaction. We investigate the transient change of the high harmonic signal from 1,3-cyclohexadiene (CHD), which undergoes ring-opening and isomerizes to 1,3,5-hexatriene (HT) upon photoexcitation. By associating the change of the harmonic yield with the changes of the ionization energy and vibrational frequency of the molecule due to the isomerization, we find that the electronic excited state of CHD created via two-photon absorption of 3.1 eV photons relaxes almost completely within 80 fs to the electronic ground state of CHD with vibrational excitation. Subsequently, the molecule isomerizes abruptly to HT, i.e., ring-opening occurs, around 400 fs after the excitation. The present results demonstrate that TR-HHS, which can track both the electronic and the nuclear dynamics, is a powerful tool for unveiling ultrafast photo-chemical reactions.