Revealing ultrafast proton transfer dynamics in ionized aqueous urea through time-resolved x-ray absorption spectra and ab initio simulations

Probing the early dynamics of chemical systems following ionization is essential for our understanding of radiation damage. The advent of FELs and advances in high harmonic generation (HHG) has shown that time-resolved x-ray absorption spectroscopy (TRXAS) on a femtosecond timescale can provide crucial insights into the ultrafast processes occurring upon ionization due to its element-specificity. However, to get a clear interpretation of the dynamical features in the spectra, one often has to reply on theoretical simulations.

In this theoretical study, we investigate the response of urea in 10M aqueous solution to ionizing radiation and how it can be probed via TRXAS. We are able to interpret the temporal variation in the carbon (C) K-edge resonance signal as an effect of proton transfer between two hydrogen bonded ureas through our ab initio simulations. The ability of TRXAS to reveal rich insights into the ionization-induced dynamics of aqueous urea, in particular to follow the progress of proton transfer, at the C K-edges is hence shown. Our results are in good agreement with recent pump-probe experiments using strong field ionization, followed by x-rays generated from HHG on 10M aqueous urea solution.