A TDS study of D adsorption on terraces and terrace edges of graphite (0 0 0 1) surfaces

Adsorption of thermal (2000 K) D atoms on (0 0 0 1) surfaces of various highly oriented pyrolytic graphite (HOPG) and natural graphite substrates was studied under ultra high vacuum (UHV) conditions with thermal desorption spectroscopy (TDS). D chemisorption on terrace and terrace edge sites of graphite (0 0 0 1) surfaces was identified. Recombinative desorption of D adsorbed on terraces was observed between 400 and 600 K. The analysis of TD spectra from various graphite surfaces reveals the existence of three desorption states intrinsic to graphite (0 0 0 1), and proposed as being due to adsorbate structures composed of one (monomer) and two neighbouring (dimer) chemisorbed D atoms, and aggregates thereof (mixed). The dimer structure is supposed to exhibit higher stability than the monomer. Reaction of D with terrace edges leads to the formation of CD, CD₂ and CD₃-groups at edge C atoms. These groups decompose during heating between 790 and 1300 K via release of gaseous D₂ and CD_x, C₂D_x and C₃D_x-hydrocarbons.