Excitation and ionization-excitation of helium by fast electron and proton impact: extreme ultraviolet (EUV) cross section measurements
Conference
·
OSTI ID:199636
- Univ. of Nevada, Reno. NV (United States); and others
Cross sections have been measured in the extreme ultraviolet for the excitation of helium, He I states (lsnp) {sup 1}P{sup o}, and the ionization-excitation of helium, He II states (np) {sup 2}P{sup o}, due to fast proton and electron impact for n = 2-5. Protons with velocities from 1.4-7.5 a.u. and electrons with velocities from 3.8-8.5 a.u. were used to excite helium gas in a differentially pumped target cell. A 2 MV Van de Graaff accelerator and an electron gun mounted in the target chamber provided the projectile beams, and a grazing incidence monochromator was used to measure the EUV radiation emitted by the target. A unique application of the Bethe approximation has been used to put the excitation results on an absolute scale. Cross sections for ionization-excitation from electron impact are substantially larger than those from proton impact above a velocity of 4 a.u., the difference reaching a maximum at approximately 6.5-7.0 a.u. for all principle quantum numbers. Cross section ratios for electron and proton impact, {sigma}{sup +}(np)/{sigma}{sup +} (lsnp) are compared as a function of projectile velocity and are compared with the ratio of double to single ionization {sigma}{sup ++}/{sigma}{sup +} as a limiting value for large n.
- OSTI ID:
- 199636
- Report Number(s):
- CONF-941129--
- Country of Publication:
- United States
- Language:
- English
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