Arresting doubly excited electronic state mediating lightning speed ring-opening reaction of 1,3-cyclohexadiene

Abstract

The photoinduced ring-opening reaction of 1,3-cyclohexadiene to produce 1,3,5-hexatriene is a well-known example of the Woodward-Hoffmann rule for stereochemical reactions governed by molecular orbital symmetry, and it plays an essential role in photobiological synthesis of vitamin D3 in the skin. Since the photoexcited 11B state of 1,3-cyclohexadiene is not electronically correlated to the ground state of 1,3,5-hexatriene, the reaction is expected to proceed via non-adiabatic transitions through a doubly excited 21A electronic state. However, spectroscopic observation of this elusive state has been difficult. Here we present the results of a photoelectron spectroscopy study using table-top ultrafast deep and extreme UV lasers, based on filamentation four-wave mixing and high-harmonic generation, which enabled us to arrest the 21A state. It is shown that the ring-opening reaction takes only 60 fs to complete, which is a considerably shorter time than previous experimental and theoretical estimates. The ballistic reaction creates vibrational coherence in the reaction products.

Identification of an ultrafast internal conversion pathway of pyrazine by time-resolved vacuum ultraviolet photoelectron spectrum simulations

“…Time-resolved pump-probe photoelectron spectroscopic measurements have recently been conducted with an extreme UV probe pulse to track the entire process of ultrafast radiationless decay ultimately to the ground state in photochemical reactions subject to bond formation or cleavage. 58,59 Simulations of time-resolved PESs incorporated into ab initio quantum or semiclassical nonadiabatic molecular dynamics methods are promising to reveal the dynamics of full valence (and even inner-shell) electrons and highly excited vibrations involved in such experiments.…”