Pressure Dependence and End Effects in Precision Ion Mobility Studies

Abstract

The mobilities of K + ions have been measured by the Bradbury-Nielsen method in He, Ar, H2 and N 2 at 293 K at pressures and E/Nvalues in the range 1·4-190 torr and 1-28 Td respectively. Three drift tubes were used with drift lengths of 3· 395, 9· 076 and 50·00 cm. The anomalous variation of the reduced mobility with E/ N at low values of E/ N reported by Elford (1971) has been shown to be due to the presence of charged surface layers on the first grid of the time-of-flight system. The dependence of the reduced mobility on pressure also reported by Elford has been confirmed, and an explanation of the pressure dependence in He, Ar and H2 is proposed in terms of the formation of ion-atom or ionmolecule complexes in orbiting resonant states. The zero-field reduced mobilities in the zero-pressure limit have been derived by a fitting procedure and found to be 21 ·3 ± O' 2, 2·64 ± O· 02 and 12· 8 ± O' 1 cm2 y-1 S-l for He, Ar and H2 respectively. The pressure dependence of the reduced mobility for K + ions in N2 is shown to be of a different form from the other gases investigated and to be due to the formation of the cluster ion K + . N2. The present data are consistent with the equilibrium constant of Beyer and Keller (1971) for the reaction K + + N2 + N2 +± K + . N2 + N2. The zero-field reduced mobility for K+ ions in N2 in the zero-pressure limit has been found to be 2·50±0·02 cm2 Y-1 s-1.