Evidence for a Twisted Intermediate in Gaseous Uracil Revealed by Photoelectron Quantum Beats

Recent theoretical and experimental investigations have identified the formation of a previously unrecognized intermediate characterized by a strongly twisted C5=C6 double bond in the ground electronic state of pyrimidine nucleobases in an aqueous solution. This intermediate is produced by ultrafast internal conversion, representing a critical pathway in the photophysics of nucleobase relaxation. Nevertheless, the existence of such a species under gas-phase conditions has not yet been confirmed. In this study, we present the first direct experimental evidence for the formation of twisted intermediates in isolated molecules. Analysis of vibrational quantum beats observed using extreme ultraviolet time-resolved photoelectron spectroscopy reveals the presence of this transient species in the gas phase, further confirming that its formation is an intrinsic property of the nucleobase chromophore.