Experimentally measured values of the ratio. O,/ K, of the transverse diffusion coetlicient to the mobility for Kf ions drifting in argon gas are reponed. Results were obtained with estimated accuracies of+3% at E I N values ranging from 10-600Td ( 1 Td = IO-" Vm') and have been adjusted to 29R K. These experimental values are compared with values obtained from Monte Carlo simukatLms using several prgposed interaction potentials as input. The comparisons suggest that none of the potentials examined is a fatally accurate representation of the K+-Ar interaction potential.
Measurements are made of the velocity distributions of He+, Ne+ and Ar' ions drifting in their respective parent gas under the action of a uniform electric field at room temperature (297-300 K). Considerable improvements have been implemented in the present apparatus in comparison with older systems. The drift tube is greatly enlarged and its pressure accurately controlled by a regulating servo-valve. A new ion optical system coupled with a wider quadrupole analyser head ensures a high collection efficiency of ions transmitted through the retarding potential grids. A novel in-drift obstacle and a 90" off-axis detector effectively suppress interference from uv photons and metastable species. Experimental results for E / N ranging from 60 to 320 Td are compared with both the Wannier equation for the RMS ion velocity and the corresponding Maxwellian distribution. The typical experimental curve is found to possess only a single maximum with its peak displaced towards the low velocity end and an enhanced high velocity tail in comparison with the Maxwellian.
Room temperature measurements of the ratio & / p between the transverse diffusion coefficient and the mobility of mass-identified ions in a neutral gas are reported for Na+ ions in argon and neon. A new apparatus was used which, in contrast to previous methods, does not involve mechanical movement which alters the geometry of the drift tube assembly during a measurement scan, or variation of the experimental conditions inside it. Results with an estimated accuracy of better than 3% were obtained at E/N values ranging from 10 to 260 Td with argon and 10 to 200Td with neon.
Several commonly measured ion transport coefficients were investigated in order to determine their sensitivity for testing and comparing proposed ion-neutral interaction potentials. A variety of positive ions, negative ions, neutrals, and temperatures were included in order to draw as general a conclusion as possible. All transport coefficients considered were found to be sufficiently sensitive to be used to clearly distinguish between less and more accurate interaction potentials. It was also found that the longitudinal diffusion coefficient is the most sensitive test, followed by both the transverse diffusion coefficient and the ratio of the longitudinal diffusion coefficient to mobility, followed by the ratio of the transverse diffusion coefficient to mobility and that the mobility is the least sensitive test. When presently achievable levels of experimental error were also taken into account, however, there was no significant difference in the sensitivities.
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