Resonant inelastic x-ray scattering reveals hidden local transitions of the aqueous OH radical

Resonant inelastic x-ray scattering (RIXS) provides remarkable opportunities to interrogate ultrafast dynamics in liquids. Here we use RIXS to study the fundamentally and practically important hydroxyl radical in liquid water, OH(aq). Impulsive ionization of pure liquid water produced a short-lived population of OH(aq), which was probed using femtosecond x-rays from an x-ray free-electron laser. We find that RIXS reveals localized electronic transitions that are masked in the ultraviolet absorption spectrum by strong charge-transfer transitions-thus providing a means to investigate the evolving electronic structure and reactivity of the hydroxyl radical in aqueous and heterogeneous environments. First-principles calculations provide interpretation of the main spectral features.Ministry of Education (MOE)Published versionThis work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Chemical Sciences, Geosciences and Biosciences Division that supported the Argonne group under Contract No. DE-AC02- 06CH11357. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, and resources of the Center for Nanoscale Materials (CNM), Argonne National Laboratory, are supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES) under Contracts No. DEAC02-76SF00515 and No. DE-AC02-06CH11357. L. Y. acknowledges support from Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory for conceptual design and proposal preparation. L. K., J.-E. R. acknowledge support from the Swedish Science Council (Grant No. 2018-04088). L. K. was also supported by the European X-ray Free Electron Laser. Z.-H. L., T. D., and M. S. B. M. Y. acknowledge support from the Singapore Ministry of Education (MOE2014-T2-2- 052, RG105/17, and RG109/18). M. S. was supported by the CNRS GotoXFEL program. C. A. and R. S. were supported by the Cluster of Excellence “Advanced Imaging of Matter” of the Deutsche Forschungsgemeinschaft (DFG)—EXC 2056—Project ID No. 390715994. R. S. acknowledges support by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant No. DE-SC0019451. M. L. V. and S. C. acknowledge support from DTU Chemistry (start-up PhD grant). S. C. acknowledges support from the Independent Research Fund Denmark DFF-RP2 Grant No. 7014-00258B. At USC, this work was supported by the U.S. National Science Foundation (Grant No. CHE-1856342 to A. I. K.). A. I. K. is also a grateful recipient of the Simons Fellowship in Theoretical Physics and Mildred Dresselhaus Award from the Hamburg Centre for Ultrafast Imaging, which supported her sabbatical stay in Germany