Sticking probability of metal halide molecules incident upon refractory metal surfaces heated in high vacua

Abstract

The sticking probability (σ) of metal halides (MX, for example, NaBr, CsF and TlCl) impinging with various fluxes (10 10–10 14 molecules cm −2 s −1) upon polycrystalline surfaces of refractory metals (Re and W) heated to a high temperature ( T S, 900–2300 K) in readily attainable high vacua (10 −5–10 −3 Pa) was determined by a thermal positive ionization method. Namely, (1) a molecular beam of MX effusing from a Knudsen cell heated to various temperatures ( T C, 550–900 K) was directed onto the surface, (2) the ion current ( I +) of positive ions (M +) emitted from the surface was measured as a function of either T S or T C, (3) the apparent vapor pressure ( σP) of MX in the cell was evaluated from the data on I + and T C according to our method and (4) our data on σP was compared with literature values of P determined by various methods. In all the sample/surface systems thus studied, σ was essentially unity irrespective of the condition whether the surface was virtually clean or fully covered with residual gases below 10 −3 Pa.