# Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis
## 3 Discussion and Applications
**(b)** Na\({}_{2}\)O-Al\({}_{2}\)O\({}_{3}\)-H\({}_{2}\)O ternary phase diagram showing solubility region as well as regions saturated with respect to gibbsite, gibbsite/monosodium aluminate hydrate, and monosodium aluminate hydrate/nonsodium bis(hexahhydroxyaluminate) trihydroxide hexahydrate/sodium hydroxide. Data adapted from Refs. [67; 68; 69]. The composition of the tank waste promotes formation of some of these species. While previous radiolysis studies have focused on pure water, future studies will increase in complexity and explore compositions within the phase diagram. from X-rays and high energy gamma rays, demonstrated very efficient ICD and ETMD in micro-solvated boehmite (AIOOH) nanoplatelets [71]. This occurs when the density of states of the ionized chemisorbed species significantly overlaps the core hole states of the solid and is likely a primary process for producing ionized and radical species at the interfaces of particles in complex waste forms. This experimental and theoretical work demonstrated that ICD is a preferred decay mechanism upon single and double shallow core hole excitation of adsorbed interfacial water and the underlying nanoplatelet. ETMD from the water also occurs following ionization of the boehmite A(2\(s\)) level. ICD and ETMD both lead to the formation and ejection of H\({}_{2}\)O\({}^{+}\) with broad bimodal kinetic energy release (KER) distributions. The latter indicates the involvement of a manifold of complex multi-hole states with lifetimes that compete with proton transfer and hole delocalization. None of these interactions have been probed with time-resolved techniques,  
In solutions where solute species are present, there will be radiation-induced changes to the subsequent reaction pathways and reaction timelines [72]. As a example, see Figure 6. Production of these additional species takes place through relaxation processes, such as ETMD and ICD. For example, ICD can create final states of M\({}^{(q+1)+}\) + H\({}_{2}\)O\({}^{+}\) and ETMD can create M\({}^{q}\) + H\({}_{2}\)O\({}_{2}^{+}\) within the first solvent shell around the reactive site [72]. In the case of water, H\({}_{2}\)O\({}^{+}\) leads to a proton transfer reaction that generates an OH\({}^{+}\) radical. ICD and ETMD are competitive processes, with their relative importance in aqueous ionic solutions depending on the ion charge state, polarizability, and geometric structure of the solvent shell. Although the interactions are localized, the lifetimes depend on the number of water molecules involved and the distance from the central metal atom, and we hypothesize that there will be significant differences between alkaline systems and that observed in water; the central Na\({}^{+}\)--O distance aqueous solution is 2.377 A vs. 2.604 A for the O--O distance in water. We plan to apply methodology similar to that used to study ultrafast radiolysis in water to a series of NaOH-H\({}_{2}\)O solutions, and later, expand the technique to more complex aluminate-containing solutions. To explore the earliest time events in radiation effects, we propose initially to use X-ray pump-probe experiments in water and NaOH-H\({}_{2}\)O solutions to look at the formation of reactive species and transients in the water window near the O  
Figure 6: Ultrafast X-ray spectroscopies represent a new tool to understand the initial mechanisms that produce reactive species. Tunability throughout the soft-X-ray range allows access to important elemental species such as oxygen, nitrogen, and aluminum, of which, are of utmost importance for understanding these processes in high-level waste. For example, in complex solutions, alternative decay channels, such as ICD and ETMD, become operative. Shown here is the case for Al\({}^{3+}\) surrounded by H\({}_{2}\)O. In the case of concentrated solutions where ion-pairing may become prevalent, nearest neighbors may not be H\({}_{2}\)O, which could further serve to alter the production of reactive species through ionization. K-edge using the techniques developed for water ionization studies [5].