Mobility of O+ in He and interaction potential of HeO+

Abstract

New experimental measurements are reported for the mobility of O + ions in He gas at 300 K. The accuracy of these new values is estimated as ±2.5%, which allows them to serve as a stringent test of a new ab initio potential that we have calculated using the RCCSD͑T͒ method. We employed the aug-cc-pV5Z basis set with counterpoise corrections and took spin-orbit coupling into account. The present experimental values lie below the calculated ones, but the difference becomes statistically significant only at moderate and high values of the ratio of the electric field strength to the gas number density; even there they are only marginally significant.

Interaction potential and transport properties of NeO+

“…It is possible that the values were affected by small parasitic charges on the guard rings of the drift tube, since such effects would be more pronounced when the drift voltage is very low, as was the case here. This explanation was suggested by our observation 17 that mobilities measured subsequent to the occurrence of an accidental gaseous discharge tended to be too low by several percent, even after several hours of recov-ery from the event. Since we cannot be certain that this was the true cause, we chose to include the data set at 0.96 Torr along with the others.…”

“…The same factor of 1.07 was applied in our previous study 17 of O + in He to account for a putative calibration error of the pressure gauge. We now know that the pressure gauge was accurate, but there is no need to revise the conclusions about the accuracy of the interaction potential.…”

“…Calibration measurements, in which we compared mobilities measured in the present apparatus to the very accurately known ͑uncertainties of ±1%͒ mobility of atomic neon ions in neon, 19 indicated that our measured mobilities were systematically too small by 6%-8%. It was thought previously 17 that the pressure gauge ͑a capacitance manometer͒ gave incorrect readings, but a recent comparison of the gauge to a freshly calibrated gauge of higher accuracy showed that the original gauge was accurate to better than 1%.…”

“…We have also considered some other open-shell systems, where the spin-orbit interaction is not important, owing to the electronic states involved: Hg + and Cd + ions, 15 as well as Tl + ions, 16 in all of the rare gases. Recently, 17 we reported new experimental measurements for the mobility of O + ions in He gas at 300 K. The accuracy of these new values was estimated as ±2.5%, which allowed them to serve as a stringent test of a new ab initio potential that we calculated using the RCCSD͑T͒ method and reported in the same paper. We employed the aug-cc-pV5Z basis set with the counterpoise correction and took into account coupling between the 4 ⌺ 3/2 − and the 2 ⌸ 3/2 spin-orbit states, arising from the interaction of He with O + ͑ 4 S͒ and O + ͑ 2 D͒, respectively.…”

“…In order to test if injection of ions with excess energy caused objectionable "injection effects" of the kind observed in our studies of O + mobilities in helium, 17 we conducted a series of measurements in which we varied the injection energy and carefully measured the arrival time of the ions. In contrast to the case of the light gas helium, no significant dependence of the arrival time on injection energy was found when oxygen ions were injected into neon.…”