Electron-stimulated desorption of positive ions from physisorbed monolayers of H2, HD, and D2 on graphite and D2 on Kr-plated graphite.

Abstract

Angular distribution, time-of-flight, retarding field, and effective positive-ion cross-section measurements are presented for positive ions produced by electron-stimulated desorption from physisorbed monolayers of ${\mathrm{H}}_{2}$, HD, and ${\mathrm{D}}_{2}$ on graphite and from ${\mathrm{D}}_{2}$ on Kr-plated graphite. The electron energy was 60--63 eV, except as noted. The angular distributions are isotropic in the range of 140\ifmmode^\circ\else\textdegree\fi{}--170\ifmmode^\circ\else\textdegree\fi{} from the forward direction of the normally incident electron beam. Diatomic and triatomic ions are each less than 1% of the positive ions detected; the rest are monatomic ions (${\mathrm{H}}^{+}$ and/or ${\mathrm{D}}^{+}$). The proportion of higher energy ${\mathrm{H}}^{+}$ ions from ${\mathrm{H}}_{2}$ increases as the electron energy is raised above 30 eV, the threshold for dissociative ionization from the $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}^{+}$(2p\ensuremath{\sigma}) antibonding state of the diatomic positive ion. The proportion of low-energy ${\mathrm{D}}^{+}$ ions is larger for ${\mathrm{D}}_{2}$ on Kr-plated graphite than for ${\mathrm{D}}_{2}$ on bare graphite. Retarding field measurements were fit assuming one or two Gaussian components of monatomic-ion kinetic-energy distributions; the most probable monatomic-ion kinetic energies (without contact-potential corrections) were 5.7 eV for ${\mathrm{H}}^{+}$ from ${\mathrm{H}}_{2}$; 5.4 eV for ${\mathrm{D}}^{+}$ from ${\mathrm{D}}_{2}$; 7.9 eV for ${\mathrm{H}}^{+}$ from HD; 2.6 eV for ${\mathrm{D}}^{+}$ from HD; and 1.2 eV for ${\mathrm{D}}^{+}$ from ${\mathrm{D}}_{2}$ on Kr-plated graphite. The ratio of the number of ${\mathrm{H}}^{+}$ ions to the number of ${\mathrm{D}}^{+}$ ions from HD decreases from 3.1 to 2.9 as the coverage is increased from commensurate to incommensurate. The effective positive-ion cross sections are estimated within a factor of 2 to be 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$, 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$, and 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ ${\mathrm{nm}}^{2}$ for ${\mathrm{H}}_{2}$, HD, and ${\mathrm{D}}_{2}$.