Hydrogen on polycrystalline diamond films: Studies of isothermal desorption and atomic deuterium abstraction

Abstract

Studies of hydrogen isothermal desorption and abstraction from polycrystalline diamond surfaces are presented. The surface H and D coverages (θH and θD) are measured in real time by mass analyzing the recoiled ions generated in a time-of-flight scattering and recoil spectroscopy (TOF-SARS) experiment. For surface temperatures (Ts) from 825 and 920°C, isothermal H2 desorption is 1st order in θH with a measured activation energy, ET, of 69±6 kcal/mol and a pre-exponential factor, ν, of 10 10.5±0.9 s-1. For H2 desorption from diamond, the estimated ΔET based on bond energy calculations is ≈88 kcal/mol, substantially higher than the experimentally measured E T. This difference suggests π-bonding of the surface after H 2 desorption is involved. Using a simple bond order argument, the π-bonding contribution is estimated to be ≈21 kcal/mol. The abstraction and replacement of absorbed H by atomic deuterium (Dat) is explained by three first-order reactions. Under a constant Dat flux, the rate of abstraction of adsorbed H by Dat is 1st order in θH, with an "apparent activation energy" (E a) of 0.8±0.2 kcal/mol and ν=(1.3±0.2)10 -3 s-1. The low Ea and 1st order kinetics imply that H is abstracted from the surface by a generalized Eley-Rideal or direct mechanism. Using the relative rate of Dat abstraction of H to D at adsorption on clean diamond, we estimate an upper limit for the abstraction activation barrier of 16 kcal/mol. Under identical reaction conditions, the rate for Hat abstraction of D is ≈1/3 the rate for Dat abstraction of H. We interpret this isotope effect using momentum transfer arguments. © 1995 American Institute of Physics.