Selection of the residual gas pressure suitable for operating a positive ion source of thermal ionization type

Abstract

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≈1013 molecules cm−2 s−1) onto a surface of Re or W heated to a constant temperature (T≈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≈1×10−6 Torr is recommended for keeping β+ as large as possible in a wide range of T≈1100–1400 K where φ+>6.3 eV is maintained by adsorption of O2, and hence where β+≈1 even for TlX is attained, but to detect rapidly a neutral beam of MX in kinetic studies of very rapid reactions, P≈1×10−5 should be selected together with ∼1600 K, thereby keeping φ+≈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.