We have measured the initial (clean surface) reflectance of F2 and 0 2 normally incident on Si(100)-2x 1 and Si(l11)-7x7 as a function of incident kinetic energy Ei at different surface temperatures T,. For 02, the technique of King and Wells yields the initial sticking probability, SO, which increases monotonically with E, on both surfaces for nearly all Ei studied. For F2, the presence of abstractive chemisorption complicates the interpretation of the measurements. We find that the apparent sticking probability of F2 increases monotonically with E, on both Si(100)-2x 1 and Si(ll1)-7x7, consistent with the idea that F2 does not undergo precursormediated chemisorption on these clean surfaces. Using a crude model, we show that the data obtained with F2 on Si( 1 1 1)-7 x 7 are consistent with F2 abstractive chemisorption dominating for Ei < 0.1 eV and dissociative chemisorption becoming more probable as Ei is increased. We find that the apparent initial sticking probability of F2 depends linearly on the fluorine coverage, which is consistent with a stepwise chemisorption mechanism.For both F2 and 02, the sticking increases with T, for intermediate incident energies (0.1 eV < Ei 0.3 eV).The increase with T, in the case of F2 is consistent with a surface dynamical effect whereas for 0 2 the increase may be explained by the existence of a negative ion-like intermediate state.

SummaryReaction mechanisms and line shapes of energy levels in sBe have been studied in the bombardment of 9Be by protons of energies up to 5· 2 MeV. The (p, d) process was found to follow a pick·up mechanism, and with this information the detailed line shape of the sBe ground state was inferred. An R·matrix description using a conventional channel radius of 3·5 fm was found to be not in agreement with the results.