ISSN:
1089-7623
Source:
AIP Digital Archive
Topics:
Physics
,
Electrical Engineering, Measurement and Control Technology
Notes:
To find the residual gas pressure (P) suitable for operating a thermal positive ion source, a beam of molecule (MX) such as LiH, NaCl, or TlI was directed with a constant flux (N(approximate)1013 molecules cm−2 s−1) onto a surface of Re or W heated to a constant temperature (T(approximate)1000–2100 K), and the ion current (∼10−10–10−8 A, corresponding to ∼10−7–10−5 A cm−2) of M+ was measured as a function of P(∼1×10−7–5×10−5 Torr), which was increased by the introduction of air. These data thus obtained were analyzed to determine the ionization efficiency (β+) of MX and the work function (φ+) of each surface. Analysis of the relation among P, T, β+, and φ+ indicates that to produce a strong ion beam from MX, selection of P(approximate)1×10−6 Torr is recommended for keeping β+ as large as possible in a wide range of T(approximate)1100–1400 K where φ+〉6.3 eV is maintained by adsorption of O2, and hence where β+(approximate)1 even for TlX is attained, but to detect rapidly a neutral beam of MX in kinetic studies of very rapid reactions, P(approximate)1×10−5 should be selected together with ∼1600 K, thereby keeping φ+(approximate)5.8 eV, and hence, yielding β+≥0.9 for MX so long as the ionization energy of M is less than ∼5.2 eV. © 1998 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1148660
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