Velocity Map Imaging Apparatus for Studies on the Photochemistry of Water Ice

Abstract

This work describes the design and development of a velocity map imaging apparatus that will be used to study the laser initiated photochemistry of water ice and other condensed phases. Experiments on methanol ice photolysis using a different apparatus at Kyoto University are described to give an appreciation of the photochemistry and the experimental parameters. Water deposited on a surface at temperatures below 140 K can form an amorphous solid. Amorphous solid water (ASW), which does not exhibit properties of a well-defined phase, is the most profuse phase of water found in astrophysical environments. Chemical characteristics of ASW - in particular its photochemistry - and the physical characteristics closely associated with the structure such as density and surface are reviewed. The correlations between the morphology and the growth conditions of ASW are also described. Methanol is also known to be a component on the icy mantle on interstellar grains. The effects of irradiating amorphous solid methanol by UV photons are discussed. Experiments at Kyoto University have been performed to detect state-selectively nascent OH and CH3 photofragments following photolysis at 157 nm. Information on the velocity distributions was obtained from time-of-flight measurements. At Queen’s University Velocity Map Imaging combined with resonance enhance multiphoton ionization (REMPI) will be used for quantum state-selective detection of the nascent photoproducts and their velocity distribution. To help automate the experiments “virtual instruments” have been created for the hardware components of the experiment using LabVIEW 8.6. The ion optics of the velocity map imaging spectrometer under construction at Queen’s have been characterized using the SIMION 7.0 software package, and the anticipated experimental image of nascent photoproducts has been simulated by a Monte-Carlo-type algorithm.