Ultra-fast Laser Induced Processes in Aqueous Solutions

When ionising radiation interacts with water (a polar medium) it leads to the generation of free electrons and parent-cations/anions, also called radical ions. This process is commonly referred to as radiolysis. The exact physics underlying the generation, evolution and equilibration of these species, however, has been the topic of much debate since they were first observed in the 1960's [1].
One thing that is clear is that the earliest stages of this interaction are dominated by processes linked to the thermalisation and solvation of the free electrons. This involves the screening of these electrons by the reorientation of the surrounding molecular dipoles. As a result they can no longer be classed as being available/free to satisfy the needs of the electron-hungry radical species to return to equilibrium. As a result these radicals can remain free and go on to seed a rich physical chemistry in water that dominates a multitude of applications including electrochemistry, radiobiology and ultimately radiotherapy. Until now, however, limitations in the temporal resolution of experiments undertaken to investigate ion-induced radiolysis have prevented a clear picture of this to be determined [2].
This project aims to perform the first real-time experimental investigation of ion induced damage and the solvation time of the resulting free electrons in water. These experiments will be performed using the TARANIS laser system (picosecond (10-12 s) scale resolution) and the new TARANIS-X upgrade (femtosecond (10-15 s) scale resolution) to produce pulses of laser accelerated ions with highly synchronised optical probe radiation. Since there are absorption bands linked to both the solvated electron and hydroxyl ion at optical wavelengths it will be possible to simultaneously study the formation and return to equilibrium of these two reactive species. This will give brand new insight into the origin of the physical chemistry that takes place in ion irradiated water.
Recently we have developed the experimental techniques required to work in this inherently multiscale environment and to overcome challenges that in the past have prevented such studies [3]. As part of this project these techniques will be improved and then employed to study the very earliest stages of ion induced dynamics in water and how the inclusion of controlled dopants can change these dynamics. These results will be quantitatively compared with those obtained for high energy photon radiolysis measurements to be made at the Diamond Light Source and from ab initio simulations to see if different ionising species seed different response and recovery cycles within the medium.
This project will take place within the Centre for Applied and Interdisciplinary Radiation Research (CAIRR), a new research Centre established at QUB with the aim of covering radiation processes both theoretically and experimentally from the most fundamental of processes all the way through to a full-patient description of radiotherapy.