Mechanism of low-energy electron stimulated desorption of O− from hydrogenated and hydrogen-free diamond surfaces exposed to activated oxygen

Abstract

In this work we report on a study of the mechanism of O− electron stimulated desorption (ESD) from hydrogenated and hydrogen-free polycrystalline diamond films exposed to thermally activated oxygen for incident electron energies in the 4–22 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes: (i) total O− and H− yields as a function of incident electron energy and (ii) kinetic-energy distribution (KED) of O− desorbed from the hydrogen-free diamond surface. The discussed ESD mechanism is referred to the information obtained from x-ray photoelectron spectroscopy, near-edge x-ray absorption fine structure, and core level H+ photodesorption measurements which reveal formation of C=O and C–O–C bonds on the hydrogen-free diamond surface, and C=O and C–O–H bonds on the hydrogenated one. Based on the maximum kinetic-energy value of O− and the ESD threshold measured for hydrogen-free surface, all low-energy (5–10 eV) O− ions are attributed to desorption by the dissociative electron attachment (DEA) to C–O–C, while DEA to C=O occurs at the incident electron energy higher than ∼10 eV. O− ESD from the hydrogenated diamond surface exposed to thermally activated oxygen is a more complicated process. Its threshold is substantially higher than for hydrogen-free diamond, and the line shape of the ESD yield curve is very similar to that of chemisorbed CO molecules. Several reaction pathways leading to production of O− by DEA are discussed for this sample. At incident electron energies higher than ∼15 eV, O− ESD proceeds also via dipolar dissociation processes for both hydrogenated and hydrogen-free diamond surfaces.