Water Cluster Ions: Formation and Decomposition of Cluster Ions in the Oxygen-Water System

Drift-tube techniques have been employed to determine rate constants in the reaction sequence leading to formation of the hydronium ion hydrates H3O+(H2O)n starting from O2+ 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 O2+ and H3O+(H2O)n in oxygen are given as a function of E/Po in the range 5–60 V cm−1· torr−1 and for O2+ and H3O+ in argon from 4 to 20 V cm−1· torr−1. In oxygen, quasiequilibrium studies were conducted in which a cluster ion H3O+(H2O)n loses one or more water molecules during drift, successive ions from n=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.