Water cluster ions: Formation and decomposition of cluster ions in the oxygen-water system

Abstract

Drift-tube techniques have been employed to determine rate constants in the reaction sequence leading to formation of the hydronium ion hydrates H 3O+(H2O)n starting from O 2+ ions in O2-H2O gas mixtures at 310°K. Total pressures used ranged from 0.5 to 2 torr with water partial pressures from 0.15 to 1.2%. Measured three-body clustering rate constants declined in all cases as E/P0 increased in the range 5-30 V cm -1- torr-1. Thermal rate constants were deduced from the low E/Po data. Mobilities for Oî+ and H3O+(H20)n in oxygen are given as a function of E/Po in the range 5-60 V cm-1, torr-1 and for Oz+ and HaO+ in argon from 4 to 20 V cm-1- torr-1. In oxygen, quasiequilibrium studies were conducted in which a cluster ion H3O+(H22O)n loses one or more water molecules during drift, successive ions from = 3 to 0 becoming dominant as E/Po increased from 5 to 60 V cm-1. torr-1. A simple analytical model for treating sequential reactions in drift experiments is presented.