Velocity Dependence of the Absolute Total Ionization Cross Section for Collisions of Argon Atoms With Singlet and Triplet Metastable Helium Atoms

Abstract:
Measurements of the velocity dependence of the absolute total ionization cross section of argon atoms upon impact with selected metastable states of helium atoms are reported.
A low voltage D. C. discharge was used as the source of the excited atoms, and a rotating slotted disk selector was used for velocity selection of the excited atoms. Selection of th electronic spin state of the excited atoms was accomplishe by irradiation of the excited atoms with radiation from a helium d i s cha r g e 1 amp .
Ionization of the argon target atoms by metastable. heliui atoms was studied by the gas cell technique in which all ionization products were collected. The ionization measurements were of sufficient precision to allow simultaneous determination of the cross section and the secondary electron ejection efficiency for each metastable state of helium. The secondary electron ejection efficiency of triplet me tas table helium atoms on an electroplated gold surface was determined to be 0.440 + 0.018 in the presence of argon gas. The secondary electron ejection efficiency of singlet: metastable atoms was determined to be 0.582 + 0.024 under similar conditions.
The local ionization cross section for the He(2 S)-Ar system was found to increase almost linearly from S.8 A to 2.1.95 £ with an increase of relative velocity from 1162 from 9.8 ?? to 26.2 K 2 with an increase in relative m/sec to 2787 in/ sec. After an initial increase, of the cross section velocity from 989 m/sec to 2058 m/sec, the velocity dependence of the cross: section of the He (2 S)-Ar system entered a saturation region in which the cress section changed very little with relative velocity. The collision energy dependence of the He(2"S)-Ar system was also used to determine the values of adjustable parameters present in a current theory based on the potential curve model for Penning and associative Pennine ionization.
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