Theoretical and experimental studies of ion transport and ion-molecule interactions (1988)
AuthorsSimpson, Richard W.show all
Mobilities of positive and negative atomic ions in helium have been determined using both two- and three-temperature theories. These calculations require that an interaction potential be supplied which corresponds to the molecular ion HeX⁺ or HeX-, where X⁺ or X- is the ion of interest. The interaction potential must have the correct separation behaviour to spectroscopic states of the particles involved including ¹S He. Molecular orbital and valence bond methods have been employed to calculate interaction potentials for use in mobility calculations. A drift-tube mass spectrometer has been used for the experimental determination of ion mobilities in helium. These, and other experimental mobility data, have been compared to the calculated mobilities. The development of computer programs for mobility calculations, experimental control, data acquisition, and data analysis are described. An important feature of the ion mobility program used in this work is its ability to accept a tabulated interaction potential. This enables the result of an ab-initio calculation to be used directly in a mobility calculation. The differences between experimental measurements of the mobilities of ions of the same mass are explained in terms of differences in the ion neutral interaction potential. The mobilities of F-, O⁺, Q⁺*, B⁺, N⁺, F⁺ and Li⁺ ions in helium are calculated using the mobility program with ab-initio interaction potentials. Where two or more molecular states can arise from the spectroscopic states of helium and the ion involved, the ion mobility is determined from an average of the results of mobility calculations for each of the possible molecular states. An accurate measurement of the mobility of N⁺ in helium from 35 Td to 140 Td has been made which agrees with an earlier mobility measurement. The rate constant for the reaction of positive helium ions, obtained from this work and subsequently measured independently, disagrees with previous work. Valence bond calculations are reported for a series of diatomic molecules and ions; the principal valence structures found correspond to those expected on the basis of traditional bonding theories. The further application of the methods used and developed in this work is discussed.