ELECTRON-STIMULATED DESORPTION OF HYDROGEN FROM W(110)

Adsorption of H on W(110) at 90 K gives rise to H+ desorption under el ectron impact with high disappearance cross section, sigma = 5 x 10(-1 7) cm2, while adsorption at 300 K does not yield H+. For saturated or nearly saturated layers formed at 90 K H+ persists to 300 K. The amoun t of H desorbed by ESD is very small, as shown by thermal desorption a nd work function measurements. It is postulated that the H+ yielding s tate corresponds to adsorption at imperfections which are filled rando mly at 90 K but for kinetic reasons not at 300 K. Once filled, however , they stay filled to 300 K for near-saturation coverages, but not whe n there are alternate sites available at lower 0, and stay empty on re cooling. The cross section for H+ production was estimated from the H/e- ratio as sigma(H+) = (5 +/- 1) x 10(-23) (W/H) cm2, where H/W is t he ratio of H+ yielding hydrogen to W atoms, the latter corresponding to 1.4 x 10(15) atoms/cm2. It was also found that submonolayer coverag es of Ag post-adsorbed on H-1/W(110) increase the yield and disappeara nce cross section of the H+ yielding state, but do not lead to appreci ably more total electron stimulated H desorption. Small amounts of CO kill H+ production.