Negative ion formation from SF/sub 6/ on hot surfaces
Positive surface ionization is a widely used technique for the isotopic analysis of a number of elements. The corresponding negative ion technique has found much less use. One of the main reasons is that hot filaments emit electrons, which are accelerated by the potentials on the negative ion lens, and repulsed by the potentials on the positive ion lens. Negative surface ionization (NSI) must then be limited to conditions under which the resulting electron current does not become large enough to disrupt the operation of the lens due to arcing, charge density defocusing or heating of the focus plates. Hot filaments operated in the negative ion mode have the potential of ionizing not only by NSI, but also by electron attachment. Many molecules attach thermal electrons (<0.5 volts) to form negative ions. The electron attachment rate drops off very rapidly at electron energies above 0.5 volts. The region in a surface ionization source where electrons have such low energy is limited to just in front of the filament. The hexafluorides are a group of molecules that are of considerable interest for this type of ion source. Hexafluorides of S, Se, Te, Mo, W, Re, Ir, Pt, Os, Au and U readily form negative ions. All except SF/sub 6/ (ea <0.5 volts) have high electron affinities (3 to 10 volts), and SF/sub 6/ has a high thermal electron attachment rate. Thus SF/sub 6/ will form negative ions from an electron attachment process, while the others will form negative ions from an NSI process.
- Research Organization:
- Exxon Nuclear Idaho Co., Inc., Idaho Falls (USA)
- DOE Contract Number:
- AC07-79ID01675
- OSTI ID:
- 6419674
- Report Number(s):
- CONF-810548-6; TRN: 81-011666
- Resource Relation:
- Conference: 29. annual conference on mass spectrometry, Minneapolis, MN, USA, 24 May 1981
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GENERAL PHYSICS
SULFUR FLUORIDES
SURFACE IONIZATION
ANIONS
ELECTRON ATTACHMENT
FILAMENTS
ION SOURCES
MOLECULAR IONS
CHARGED PARTICLES
FLUORIDES
FLUORINE COMPOUNDS
HALIDES
HALOGEN COMPOUNDS
IONIZATION
IONS
SULFUR COMPOUNDS
640301* - Atomic
Molecular & Chemical Physics- Beams & their Reactions