Making a MESS: a Multi-Experiment Spectral Suite for studying hydrated electrons

Abstract

In this work, the design and implementation of a Multi-Experiment Spectral Suite (MESS) is described. The MESS currently contains two time-resolved experiments - Transient Absorption (TA), and Second Harmonic Generation (SHG) – but has been designed with a view to expansion. The primary species of interest to be studied is the hydrated electron. As an electron is generated during water ionization, its hydrated form is significant within many scientific processes including, but not limited to, biological chemistry and tissue damage, atmospheric chemistry and nuclear chemistry. However, the majority of what is known about the hydrated electron is its behaviour in a bulk solvation environment with little understanding known about its behaviour at interfaces where it reacts in most instances. The MESS provides the ability to build a bottom up picture of solvation environment on molecular dynamics in going from the bulk (TA) to the interface (SHG).

SHG enables the study of interfacial dynamics due to its inherent surface sensitivity. The method described here is an improvement on a previous phase-sensitive SHG technique that varied the phase of the interference between the SHG from a sample (ESHG) and a local oscillator (ELO) manually by incrementally changing the distance between the sample and reference sample. The new technique presented here is able to record the entire interference pattern in a single shot by using a phase varying unit comprised of a Reference Surface sandwiched between two glass wedges. The technique is characterised in this work by studying the excited state dynamics of malachite green at the air/water interface, exhibiting excellent phase stability and sensitivity and comparatively short acquisition times.

In comparison, the TA technique presented is experimentally simpler than SHG. The excited state dynamics of molecules are studied in which a pump pulse induces an excitation which is then probed by a white light pulse spanning 520-950 nm continuously. In combination with 30 kHz modulation of the laser, shot-to-shot pump-probe measurements are presented for both SHG and TA techniques.