Adsorption of NO on Diamond C(111)-(2×1) by Band Gap Excitation*

Abstract

The adsorption of NO on the diamond C(111)-(2x1) surface was studied in the presence of 226nm laser radiation and in the absence of such excitation. Without laser radiation, sticking was not observed on either the diamond C(111)-(1x1) or the reconstructed (2x1) surfaces for ambient (background) NO exposure in an ultra-high vacuum apparatus. The adsorption probability was estimated to be less than 10-8 based on the detection limit of oxygen in Auger electron spectroscopy of 0.3% of a monolayer. Diamond surfaces exposed to ambient NO (purified at 150 K) and 600 μJ/cm2 laser pulses significantly increased the adsorption of NO (see figure 1). Exposures were performed on and off the A2Σ1/2 (v'=0, J'=13/2) ← X2∏3/2(v"=0,J"=13/2) resonance; no significant differences were found. 226 nm photons were of sufficient energy to excite electrons across the ~5.5 eV band gap in diamond. However, the reaction saturated at an oxygen adatom coverage of less than 2% of a monolayer which suggested the adsorption proceeded on "defect sites" on the surface. The natural type IIa diamond was cleaved in air to expose the (111) surface and mechanically polished with 0.25 μm diamond abrasive in olive oil to produce a hydrogen-saturated surface with a mirror finish. Heating the sample to 800 K gave sharp (1x1) LEED patterns. Heating the sample to above 1275 K to remove the hydrogen produced three rotated domains of the (2x1) reconstructed surface.