In easily attainable high vacua (Pr≊20–0.2 &mgr;Torr), the efficiency (&bgr;+) 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 &bgr;+elucidated the temperature dependence of the effective work function (&fgr;+) for the ionization. Namely, (1) above ∼2000 K, each surface is kept virtually clean, having &fgr;+≊5.2, 4.9, and 5.4 eV for W, Mo, and Re, respectively. (2) At ∼1400–1200 K, the range of which depends uponPr, &fgr;+is increased up to ∼7 eV by adsorption of residual gas molecules (RGM), thereby yielding &bgr;+=1 for even Tl and the current density of ∼10 &mgr;A cm−2for the flux of 1014molecules cm−2 s−1. (3) Below ∼1200 K, however, &fgr;+is much decreased by coadsorption of MX and RGM. A theoretical diagram ofTvs &fgr;+is devised to predict the best condition for &bgr;+=1. The best condition (&fgr;+,T, andPr) to make &bgr;+as large as possible may readily be selected according to the diagram and the present data on &fgr;+.