Drift Velocities and Interactions of Cs+ Ions with Atmospheric Gases

Abstract

The drift velocities and interactions of the Cs+ ion in atmospheric gases have been measured at 300°K using an ion drift apparatus. Cs+ was found not to form clusters with N2, O2, and Ar at 300°K, implying a reaction rate less than 10−24 cm6/sec or an equilibrium constant less than 10−2 torr−1. Low field mobilities (μN) for the ion were 6.0, 5.9, and 5.6 × 1019 (V·sec·cm)−1 in O2, N2, and Ar, respectively. Cs+ was found to form clusters with water by the three-body reaction Cs+ + H2O+N2→(Cs·H2O)++N2, with a forward rate constant 9× 10−30cm6/sec and a reverse rate constant of 1× 10−15cm3/sec at 306°K. Ion clusters as large as (Cs· 4H2O)+ were observed and, by measuring the equilibrium constants between 300 and 650°K, the entropy and enthalpy of formation of the first, second, and third water clusters were determined. With SO2, Cs+ was found to cluster by the reaction Cs++SO2+M↔ (Cs· SO2)++M, with ΔH°= − 11 kcal/mole and ΔS°= − 19 eu. The forward rate constant for the formation of (Cs·SO2)+ was 3× 10−20 cm6/sec and the reverse rate constant 1× 10−13cm3/sec. The rates were the same with either SO2 or N2 as the third body. In CO2 the equilibrium amount of the clustered ion was too small to enable the measurement of the rate of formation, but the equilibrium constant measured between 200 and 400°K allowed the ΔH° and ΔS° values to be determined as −6 kcal/mol and − 14 eu, respectively. The low field mobility of Cs+ in CO2 was found to be 3.1× 1019 (V·sec·cm)−1. The interactions of the Cs+ ion with its large radius, low ionization potential, and almost inert electronic structure should be the minimum ion-molecule interaction for an ion of this size. The interactions of smaller and more reactive ions should be at least as great as that of Cs+. Evidently the formation of ion-molecule clusters with polar and polarizable molecules is a nearly universal property of small ions.