# Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis
## 1 Introduction  
Understanding the elementary steps following ionization in aqueous systems will provide a framework for radiation-matter interactions in chemistry and biology. It is of central importance for radiobiological physics and chemistry and impacts cancer therapies, space travel, nuclear reactors, and environmental management [1, 2]. Impact of an ionizing particle (X-ray, gamma, ion) leads to ejection of energetic primary electrons, followed by inelastic and non-adiabatic processes that produce highly damaging slower secondary electrons during the so-called physical and physico-chemical stages, that is, within a time scale of \(10^{-12}\) s [1]. During these stages, reactive radical species are produced, e.g., the hydrated electron [3, 4] and the chemically aggressive hydroxyl radical [5, 6]. These radical species as well as H atoms and the molecular products H\({}_{2}\) and H\({}_{2}\)O\({}_{2}\) will undergo reactions amongst themselves and with any added solutes, interfaces, etc. in the subsequent chemical stage to induce radiation damage that may not be manifest until even decades following the initial passage of the ionizing radiation. Although these latter reactions are nominally the observed outcome of radiolysis, it is the very fast processes on the sub-picosecond time scale that initiate this radiation damage. Most studies in the ultrafast physico-chemical time regime of liquid water radiolysis have focused on the hydrated electron as summarized in a recent comprehensive review [4]. The primary ionization produces H\({}_{2}\)O\({}^{+}\) and a quasifree electron. Only recently has the elementary proton transfer reaction H\({}_{2}\)O\({}^{+}\)+H\({}_{2}\)O \(\rightarrow\) H\({}_{3}\)O\({}^{+}\) + OH, which occurs in \(\sim\)50-fs to produce the hydroxyl radical, been observed [5]. Generally speaking, experimental data probing the mechanisms of more complex chemical reactions on ultrafast timescales are extremely limited, despite being critical input for modeling of radiation chemistries, particularly in condensed phases. Inner-shell vacancy decay represents a first step in radiation chemistries and in the condensed phase differs significantly from that of isolated atoms in the gas phase, where non-radiative Auger decay and fluorescence are well characterized. Following theoretical predictions [7] for inner-valence excitation, studies of gas-phase clusters have revealed a novel, non-local decay mechanism, Intermolecular Coulombic Decay (ICD) first in neon and water dimers [8; 9]. In ICD, when the energy of the initially created hole is greater than the double ionization potential of the system, energy transfer following Auger-decay can eject an \"ICD\" electron from the neighbor of the originally excited molecule, leading to a Coulomb explosion and slow electrons that can be captured in coincidence with recoiling ions. Studies with larger clusters suggest that the propensity toward non-local decay increases with size [10]. Extensions to condensed phases have been pioneered using electron spectroscopy in liquid microjets by [PERSON] and co-workers [11; 12] and have been initiated by core level ionization. These studies have revealed other non-local decay mechanisms, e.g., electron transfer mediated decay (ETMD). Static electron spectroscopy measurements identified ETMD and ICD by lineshape analysis-observing subtle changes in Auger spectra and inferring time resolution using the internal Auger clock, \(\sim\)4 fs for the oxygen 1 s hole. For example, comparison of the Auger spectra of normal and heavy water revealed that the inner-shell triggered autoionization processes are affected by proton transfer (OH stretching frequency is \(\sim\)9 fs) [13].