How to select the best condition for operating a positive ion source of molecular beam surface ionization type (abstract)a)

Abstract

In easily attainable high vacua (Pr≊20–0.2 μTorr), the efficiency (β+) for producing ions (M+) from diatomic molecules (MX) incident on a polycrystalline surface of W, Mo, or Re was measured as a function of surface temperature (T≊800–2300 K). Theoretical analysis of β+ elucidated the temperature dependence of the effective work function (φ+) for the ionization. Namely, (1) above ∼2000 K, each surface is kept virtually clean, having φ+≊5.2, 4.9, and 5.4 eV for W, Mo, and Re, respectively. (2) At ∼1400–1200 K, the range of which depends upon Pr, φ+ is increased up to ∼7 eV by adsorption of residual gas molecules (RGM), thereby yielding β+=1 for even Tl and the current density of ∼10 μA cm−2 for the flux of 1014 molecules cm−2 s−1. (3) Below ∼1200 K, however, φ+ is much decreased by coadsorption of MX and RGM. A theoretical diagram of T vs φ+ is devised to predict the best condition for β+=1. The best condition (φ+, T, and Pr) to make β+ as large as possible may readily be selected according to the diagram and the present data on φ+.