# Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis
## 1 Introduction
It was noted that the experimental spectra alone provided limited insight into the type of the relaxation processes contributing to the spectrum, and no information on their relative efficiencies could be inferred because the kinetic energy released in the different relaxation channels is similar--ab initio theoretical calculations are essential for interpretation. It was shown that ICD occurs at condensed interfaces containing water by examining the protonated water cluster ion, kinetic energy release, and threshold production energy. The latter was correlated with \(s\)-shell ionization of rare gas partners. Though these experiments could clearly reveal the occurrence of ICD in complex condensed phase targets, they could not determine the time scales of the proton transfer step vs. the initiating ICD event [14]. In general, the temporal evolution of the transient species can not be extracted from static experimental spectra [15]. Going beyond static photoelectron spectroscopy, one may consider time-resolved photoelectron spectroscopy (TRPES) to probe atom-specific mechanistic details of ionization-induced reactions in solution. Such studies have focused on the hydrated electron, where the modest vertical binding energy (VBE) of \(\sim\)3.5 eV allows detection in both the ground and excited states of e\({}^{-}\)(aq) with readily available UV probes. As done previously for non-time-resolved studies of ICD [8; 9], water cluster species introduced into vacuum in concert with ion/electron detection are often used to approximate bulk behavior [16; 17; 18; 19; 20]. TRPES studies in solution phase are challenging due to difficulties associated with introducing liquid jets [21] into the vacuum environment required by photoelectron spectrometers--space charge, electron escape depth issues render experimental realization and interpretation, respectively, a challenge [22]. The liquid jet studies have also focused on the dynamics of the hydrated electron; the e\({}^{-}\)(aq) is created via a photoinduced charge transfer to solvent (CTTS) from a solute anion (I\({}^{-}\),Br\({}^{-}\)) [23; 24] and its dynamics subsequently studied with IR pump/UV probe spectroscopy with photoelectron detection. Photon-in photon-out spectroscopies powered by soft X-ray free electron lasers (XFELs) provide a powerful window into aqueous phase chemical reactions in the physicochemical time domain. Not only is the few femtosecond time regime and the photon energy to probe inner-shell hole configurations of oxygen, the K-shell absorption edges of carbon (284 eV), nitrogen (410 eV), oxygen (543 eV) and aluminum (1560 eV) readily accessible, but the challenging vacuum requirement for photoelectron spectroscopy is markedly reduced. A further enabling breakthrough has been the development of stable liquid sheet jets of micron and sub-micron thickness [25; 26]. This combination of soft X-ray XFELs and the liquid sheet jets has enabled laser-pump/X-ray probe time-resolved absorption spectroscopy in the soft X-ray water window (280-530 eV) with milliOD sensitivity [5] and time-resolved resonant inelastic X-ray scattering (RIXS) [6] to study the fastest chemical processes in water radiolysis. Here we recap the optical pump/X-ray probe studies of radiolysis in pure liquid water in Section 2.1. In Section 2.2, we describe the prospects for extending these fundamental studies to the less \"ideal\" situation encountered in true radiolysis environments via X-ray pump/X-ray probe studies. In Section 2.3, we briefly describe methods used at longer timescales to probe radiolysis products, i.e., pulse radiolysis, scavenger, and stop-flow methods. In Section 3, we discuss the application to a practical problem of considerable importance: remediation of high-level nuclear waste (HLW). Finally in Section 4, we describe the outlook for ultrafast studies of radiolysis with XFELs.