Electron-stimulated Desorption Of Positive Ions Research Articles

Electron-stimulated desorption of positive ions from Na nanoclusters adsorbed on Pt(111) and Ar,H2O,andSF6spacer layers

Electron-stimulated desorption of positive ions from supported Na nanoclusters has been investigated in terms of the localization or delocalization of valence holes, as well as the reaction of Na with the substrate or coadsorbed molecules. On the Ar substrate, the ${\mathrm{Na}}^{+}$ yield is enhanced markedly at the smallest coverage and decay steeply with increasing coverage. We interpret that ${\mathrm{Na}}^{+}$ is emitted via Coulomb explosion of the adsorbed Na nanoclusters in which valence holes are confined, i.e., localized. Very few ${\mathrm{Na}}^{+}$ ions are emitted from the Na nanoclusters adsorbed on the Pt(111) and ${\mathrm{H}}_{2}\mathrm{O}$ substrates. The complete suppression of the ${\mathrm{Na}}^{+}$ emission is interpreted as a result of neutralization of nanoclusters due to electron transfer from the substrate, i.e., the delocalization of valence holes. Na tends to interact with the ${\mathrm{H}}_{2}\mathrm{O}$ nanoclusters on the Ar substrate as revealed from the evolution of hydrated ${\mathrm{Na}}^{+}$ ions prior to bare ${\mathrm{Na}}^{+}$ ions. The formation of Na nanoclusters is suppressed on the ${\mathrm{SF}}_{6}$ substrate due to reactions, resulting in a much smaller ${\mathrm{Na}}^{+}$ yield than that from the Ar substrate.

Electron-Stimulated Desorption of Positive Ions from Methanol Adsorbed on a Solid Ar Substrate

Electron-stimulated desorption (ESD) of positive ions from weakly physisorbed molecules has been investigated. From methanol adsorbed on a solid Ar substrate, the protonated cluster ions of the type H+(CH3OH) n(n = 1 - 4) are emitted, together with the fragment ions such as [Formula: see text](n = 0 - 3), H3O+, CHO+, CH3O+, etc. The yields of these ions are markedly enhanced at the smallest coverage and decay steeply with increasing coverage. Coulomb explosion between valence holes confined in adsorbed nanoclusters is responsible for the enhanced ion yields. Very few ions except for H+ are emitted from a thick layer as well as nanoclusters adsorbed directly on a metal substrate due to the delocalization of valence holes.

Electron-stimulated desorption of positive ions from methane and fluoromethane nanoclusters

Electron-stimulated positive-ion desorption has been investigated for CH4, CF4, CHF3, and CH2F2 molecules adsorbed on a solid Ar substrate. The ion yields from CH4 are enhanced markedly at the smallest coverage and decay steeply with increasing coverage. In contrast, ion yields from CF4 increase monotonically with increasing coverage. The ions from hydrofluorocarbons exhibit both behaviors. Coulomb explosion associated with the delocalization of valence holes is responsible for the enhanced ion yields from nanoclusters, whereas the monotonic increase of the ion yields with coverage is caused by the dissociative ionization of individual molecules in which valence holes are localized. The localization (delocalization) of valence holes can be ascribed to the nature of the F 2p nonbonding (C–H bonding) orbitals.

Electron-stimulated desorption from LiF on Si(100)

Electron-stimulated desorption of positive ions from a LiF layer adsorbed on Si(100) is measured in the electron energy range 30–300eV. The relative ion yields change rapidly near the F2s and Li1s ionization thresholds. Emission of SiF+x ions (where x=1, 2, 3) is observed from LiF deposited at 100K. Emission of these fluorosilyls almost completely disappears after annealing at nearly 500K and is replaced by emission of SiLiF3 species. We suggest that for the low temperature growth, SiFx components are build in the area between LiF grains. Data presented indicate that adsorbate islands and uncovered Si(100) substrate channels can be obtained by LiF deposition above 500K or by annealing an LiF/Si system above this temperature. The presence of F2+ ions and their kinetic energy distribution dependence on the sample preparation temperature is also discussed.

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

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 ${\mathrm{D}}_{2}$.

Supression of electron-induced positive ion emission by a molecular overlayer: Ion-molecule charge exchange at a surface.

We report the suppression of electron-stimulated desorption of positive ions (${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$) from a ${\mathrm{TiO}}_{2}$ (110) surface caused by adsorption of a fractional monolayer of molecular ${\mathrm{NH}}_{3}$. A linear decrease of ${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$ emission with ${\mathrm{NH}}_{3}$ coverage is observed. This system allows us to distinguish between neutralization of desorbing ions via interaction with the substrate or with adsorbed molecules. We propose a novel charge exchange mechanism where electron transfer from the occupied orbitals of the adsorbed molecules to the desorbing ions causes the decrease in detected ion yield. Charge transfer cross sections of 2.8(\ifmmode\pm\else\textpm\fi{}0.5) and 2.7(\ifmmode\pm\else\textpm\fi{}1) (\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$) ${\mathrm{cm}}^{2}$ have been determined for ${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$ ions, respectively.

Electron-stimulated desorption of positive ions from hexagonal α-SiC

Electron-stimulated desorption (ESD) of positive ions (H +, O +, +OH, F + and Cl +) from the Si- and C-terminated surfaces of hexagonal α-SiC has been observed for electron energies in the 5–105 eV range. Comparison of these results with those for the ESD of the same ions from the surfaces of Si and condensed hydrocarbons leads to a model for the H +, +OH, F + and Cl + threshold desorption process based on transitions from deep valence Si and C “s-like” levels to states in the conduction band, followed by Auger decay to form a localized multiple valence-hole configuration. O + desorption, on the other hand, is initiated by O 2s ionization. Evidence is found for a strong dependence of the F + threshold on the local chemical bonding. The results indicate that the thresholds for ESD of these ions from SiC are determined more by the electronic excitation of the substrate than by direct excitation of the adsorbate bond.

Electron-stimulated desorption of positive ions from hexagonal α-SiC

Electron-stimulated desorption of positive ions from hexagonal α-SiC