STICKING PROBABILITY OF METAL HALIDE MOLECULES INCIDENT UPON REFRACTORY-METAL SURFACES HEATED IN HIGH VACUA

The sticking probability (sigma) of metal halides (MX, for example, Na Br, CsF and TlCl) impinging with various fluxes (10(10)-10(14) molecul es cm(-2) s(-1)) upon polycrystalline surfaces of refractory metals (R e and W) heated to a high temperature (T-S, 900-2300 K) in readily att ainable high vacua (10(-5)-10(-3) Pa) was determined by a thermal posi tive ionization method. Namely, (1) a molecular beam of MX effusing fr om 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 (sigma P) of MX in the cell wa s evaluated from the data on I+ and T-C according to our method and (4 ) our data on sigma P was compared with literature values of P determi ned by various methods. In all the sample/surface systems thus studied , sigma was essentially unity irrespective of the condition whether th e surface was virtually clean or fully covered with residual gases bel ow 10(-3) Pa. (C) 1997 Elsevier Science B.V.