Bridging the phase gap: directly comparing ultrafast dynamics in the gas and liquid phases*

Directly contrasting ultrafast dynamics of molecules in gas and liquid phases is crucial to understanding the influence of complex environments on (photo-)chemical processes. However, such direct comparisons have long remained out of reach because of a lack of suitable experimental techniques. Here, I will describe the application of two experimental techniques to this endeavor, i.e. extreme-ultraviolet time-resolved photoelectron spectroscopy (XUV-TRPES) [1] and water-window attosecond transient-absorption spectroscopy (WWATAS) [2,3]. TRPES has been applied to follow the ultrafast photoisomerization dynamics of stilbene in the gas and liqiud phases, revealing the nature of the involved conical intersections, intermediate states and vibrational coherences in both phases of matter [4]. The dominant influence of the liquid phase has been attributed to frictional forces that slow down the time scales by a factor of ~2. WWATAS has been applied to resolve femtosecond proton transfer in ionized urea solutions, revealing that the dominant pathway consists in proton transfer from an ionized urea molecule to a neutral one [5]. The intensity and the photon energy of the corresponding X-ray transition have been found to be mainly sensitive to the electronic-structure relaxation and the proton transfer, respectively, suggesting the ability of WWATAS to distinguish electronic from structural dynamics. Finally, the application of WWATAS to electronic relaxation of pyrazine in the gas and liquid phases will also be discussed. Whereas oscillatory population dynamics between excited states have been observed in the gas phase for the first time, these oscillations are absent in the liquid phase, indicating a dramatic effect of solvation on the electronic relaxation process. These methods reveal the potential of ultrafast spectroscopy based on high-harmonic-generation sources to understand the influence of solvation on the fastest electronic and nuclear dynamics underlying chemical dynamics.