Ion identity and transport properties in CO2 over a wide pressure range

Abstract

We have investigated in drift tube mass spectrometers the identity and the transport properties of ions formed in CO2 gas at pressures ranging from 10−4 to 762 torr. Under bombardment by low energy (20–100 eV) electrons in the ion source, the primary positive ion is predominantly CO+2, with traces of C+, O+, and CO+. The predominant ion becomes O+2 at pressures above 100 μ (0.1 torr), and clustering of CO2 molecules to the O2+ occurs even at pressures below 1 torr. Break-up of the clusters also occurs, the ion identity changing many times in the drift region. The zero-field reduced mobility of the O+2⋅ (CO2)n charge carrier is a function of pressure, and varies from (1.30±0.03) cm2/V⋅sec at 0.2 torr to (1.18±0.03) cm2/V⋅sec at 1 torr. The sole negative ion produced directly by the electron bombardment is O−, which clusters to form the stable ion CO−3, whose reduced mobility is (1.27±0.06) cm2/V⋅sec for E/N ?60 Td at all pressures below 1 torr. At much higher pressures and under somewhat different conditions (above 30 torr and using beta source ionization), ions in CO2 are observed to form multiple clusters, the population distribution among the clusters being strongly dependent on gas temperature. The zero-field reduced mobility of the charge carrier is largely independent of the identity of the core ion, and varies with pressure from roughly 1.2 cm2 /V⋅sec at 35 torr to about 1.0 cm2/V⋅sec at 762 torr.