Different timescales during ultrafast stilbene isomerisation in the gas and liquid phases revealed using time-resolved photoelectron spectroscopy

Directly contrasting ultrafast excited-state dynamics in the gas and liquid phases is crucial to understanding the influence of complex environments. Previous studies have often relied on different spectroscopic observables, rendering direct comparisons challenging. Here, we apply extreme-ultraviolet (XUV) time-resolved photoelectron spectroscopy (TRPES) to both gaseous and liquid cis-stilbene, revealing the coupled electronic and nuclear dynamics that underlie its isomerisation. Our measurements track the excited-state wave packets from excitation along the complete reaction path to the final products. We observe coherent excited-state vibrational dynamics in both phases of matter that persist to final products, enabling the characterisation of the branching space of the S(1)-S(0) conical intersection. We observe a systematic lengthening of the relaxation time scales in the liquid phase and a red shift of the measured excited-state frequencies that is most pronounced for the complex reaction coordinate. These results characterise in detail the influence of the liquid environment on both electronic and structural dynamics during a complete photochemical transformation.