Surface production of H{sup {minus}} ions by backscattering of low-energy protons
Measurements of the total yield of H{sup {minus}} ions, Y(H{sup {minus}}), were performed with a specially designed optical column which focused a low energy (5-25 eV) proton beam onto the target at normal incidence. All secondary negative ions and electrons scattered from the target were collected. Secondary negative ions produced in backscattering of protons from bare Mo targets were analyzed by quadrupole mass spectrometer. Above 50 eV incident proton energy O{sup {minus}} and OH{sup {minus}} ions were seen, indicating sputtering of adsorbed water vapor by the incident protons. The O{sup {minus}} and OH{sup {minus}} signals increase with increasing incident energy. In the energy range of interest, from 5-25 eV, only H{sup {minus}} ions and electrons were observed. The low work function surfaces investigated included Si (100) cesiated by ion bombardment and vapor deposition, and polycrystalline Mo, cesiated by vapor deposition. The H{sup {minus}} yield was determined by taking the ratio of the total secondary ion current to the incident proton current to the target. H{sup {minus}} yields interpolated to a work function of 1.45 eV were an optimum of 20% for 15 eV protons on cesiated molybdenum and 15% for 10 eV protons on cesiated Silicon. The maximum yield was 0.25 for 20 eV protons at 1.35 eV work function. A reduction of the yield by a factor of two is observed when the target is heavily exposed to hydrogen prior to deposition of cesium. The results for molybdenum were fit to our expression for Y(H{sup {minus}}) and compared to values of Y(H{sup {minus}}) calculated by integrating the calculated charge transfer probability, {eta}(H{sup {minus}}), over the outgoing energy distribution associated with a cosine distribution in scattering angle. The effects of the surface binding energy on particle reflection and energy loss are analyzed in terms of the corresponding effects on {eta}{sub 0} and E{sub th}.
- Research Organization:
- Stevens Inst. of Tech., Hoboken, NJ (United States)
- OSTI ID:
- 111262
- Resource Relation:
- Other Information: TH: Thesis (Ph.D.); PBD: 1993
- Country of Publication:
- United States
- Language:
- English
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