A modification of the central force model (CFM) that describes the dissociation of water molecules into OHand H+ ions is proposed for molecular dynamics simulations of energetic particle bombardment of water ice.
The model keeps all the properties of the CFM but permits charge exchange between oxygen and hydrogen
atoms when the water molecule starts to dissociate after collision with an energetic projectile. The reaction
products, therefore, have the correct integer charges, -1 and +1 for hydroxyl and a proton. The threshold for
the ionic dissociation is corrected to be at the right value, 17.2 eV, in a vacuum. Using the proposed model,
total cross-sections for ionic dissociation as functions of the projectile energy are estimated for Ar and C
projectiles colliding with water molecules in a vacuum and water ice. Carbon projectiles are demonstrated to produce more dissociated ions at energies lower than 300 eV. Argon projectiles are more effective in breaking the molecules at higher energies.

By : Igor A. Wojciechowski and Barbara J. Garrison

A modification of the central force model (CFM) that describes the dissociation of water molecules into OHand H+ ions is proposed for molecular dynamics simulations of energetic particle bombardment of water ice.
The model keeps all the properties of the CFM but permits charge exchange between oxygen and hydrogen atoms when the water molecule starts to dissociate after collision with an energetic projectile. The reaction products, therefore, have the correct integer charges, -1 and +1 for hydroxyl and a proton. The threshold for the ionic dissociation is corrected to be at the right value, 17.2 eV, in a vacuum. Using the proposed model, total cross-sections for ionic dissociation as functions of the projectile energy are estimated for Ar and C projectiles colliding with water molecules in a vacuum and water ice. Carbon projectiles are demonstrated to produce more dissociated ions at energies lower than 300 eV. Argon projectiles are more effective in breaking the molecules at higher energies.

1. Introduction

The mechanism resulting in ion emission from solid targets during energetic ion bombardment remains elusive in a number of desorption techniques including secondary ion mass spectrometry (SIMS),1 matrix assisted laser desorption/ionization (MALDI),2 and electrospray ionization.3 In these techniques, quantitative information on elemental target composition relies on the measurement of the secondary ion yield. For the special matrix of water ice, the formation mechanism and structure of ionic water clusters is of interest to researchers from different fields.4 Ion emission from frozen samples bombarded by energetic ions is a commonly considered method for production of cluster ions of volatile liquids.5 The energetic particle bombardment of water ice is believed to be the basis for explaining the composition of the atmospheres of some Jupiter’s moons.

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