LiF Surface Research Articles

Energy Loss and Secondary Electron Measurements in Collisions at Grazing Angle of 10 keV Oq+ Ions on LiF Surface

The energy loss and the mean number of secondary electrons are measured as a function of the projectile scattering angle for 10 keV Oq+ ions (q = 2–7) colliding on LiF target at grazing angle. These data are interpreted in terms of trajectory dependant properties, which are essentially governed by the neutralisation dynamics on the way in of the collision. The relative role of potential and kinetic emission is evaluated and discussed.

Analysis of MIES and UPS data of LiF and NaCl

Published electron energy spectra of LiF and NaCl surfaces taken by HeI UPS and by He*(2 3S) MIES are reanalyzed. By simulation of the spectra on the basis of the calculated bandstructure, it is possible to establish the energy position of the top of the valence band on the energy scale of the experimental spectra. It is found that the MIE spectra are shifted by several 100 meV to higher electron energies compared to UPS. Based on a hypothesis presented by Kuchitsu and coworkers in 1980, this can be explained by the localization of the created hole state in the topmost atomic layer of the crystal surface. The difference between surface and bulk Madelung energy plays two roles: firstly, it explains the observed energy shift in MIES, and secondly, it seems to explain the long lifetime of a surface hole state in ionic insulators.

Angular and velocity distributions of small cluster fragments in neutral (NH3)n scattering off LiF(100)

The scattering of neutral ammonia clusters off a LiF(100) surface is studied. A supersonic expansion of ammonia and ammonia seeded in Kr and He produces clusters of various sizes but uniform kinetic energies of 176, 57 and 285 meV per monomer molecule, respectively. The mass distribution of scattered particles is measured in a reflecting time-of-flight mass spectrometer using single-photon photoionization with vacuum ultraviolet (VUV) laser radiation at λ=118 nm (hν=10.49 eV). While in the incident beam (NH3)n clusters up to n=65 can be detected, the mass spectrum of the scattered particles is dominated by NH3+, and only a few small clusters are found. Angular distributions of these fragments show that the maximum of the scattered intensity shifts to greater detection angles for increasing fragment mass. Velocity distributions of released monomers are measured for all three cluster beams and found to be independent of the impact velocity of the clusters. The velocity distributions of scattered fragments larger than NH3+ show a decreasing width with increasing fragment size and a most probable velocity slightly higher than that of the monomer molecules.

Threshold in the Stopping of Slow Protons Scattered from the Surface of a Wide-Band-Gap Insulator

We have measured the energy loss of slow protons scattered with energies from 300 eV to 28 keV from a clean and flat LiF(001) surface under a grazing angle of incidence. Our data reveal a threshold behavior of stopping at low projectile energies. The effect on the outgoing charge state indicates that electron capture and loss are dominant mechanisms for the stopping of slow protons interacting with a wide-band-gap insulator. Our data allow one also to deduce information on charge transfer in front of the surface of an insulator.

Effect of the surface track potential on electron emission in 60–100 keVH+grazing bombardment of LiF

We have measured energy distributions of electrons emitted at forward angles during 60--100 keV ${\mathrm{H}}^{+}$ grazing bombardment of LiF surfaces. The electron energy spectra from the insulator are compared with those for Al(111), Si(111), and gas phase Ar. In the case of LiF, a broad peak appears with the maximum at an energy ${E}_{m}$ lower than the convoy electron energy ${(E}_{\mathrm{ce}})$. The absolute value of the energy shift $\ensuremath{\Delta}{E=E}_{m}\ensuremath{-}{E}_{\mathrm{ce}}$ increases with the ${\mathrm{H}}^{+}$ velocity, and depends on the sample topography. The results are discussed in terms of the attractive (track) and the repulsive surface-induced potentials.

N2 and HF vibrations on LiF(001): the effect of surface coverage

Adsorption energies and stretching vibrational frequency shifts for N2 and HF molecules adsorbed on a LiF(001) surface have been obtained from periodic Hartree–Fock calculations. Surface coverages between 6 and 100 investigated. A one-layer and a three-layer description of the LiF slab are found to yield virtually the same adsorption energies, adsorbate–substrate distances and vibrational frequencies at all coverages. The relative importance of the molecule–molecule and molecule–surface interactions at the optimized surface–adsorbate distance are presented. For an isolated N2 molecule physisorbed on Li+s, we obtain a shift upwards of +9cm−1, only one wavenumber away from the published experimental value. For low surface coverages, the frequency shift of N2 on Li+s stems entirely from the surface–adsorbate interaction, whereas for HF physisorbed on Li+s (F towards surface), the small upshift found is entirely due to the molecule–molecule interaction within the adlayer. HF molecules hydrogen-bonded on top of F−s are strongly downshifted in frequency.

Resonant coherent excitation and ionization of fast hydrogen atoms in front of a LiF(001) surface

Abstract We have scattered protons with energies ranging from 2 keV to 9 keV from a clean and flat LiF(001) surface under a grazing angle of incidence and studied the positive ion fractions of scattered projectiles. From a combined variation of projectile energy and azimuthal angle of the target we conclude a pronounced effect on the ion fractions by resonant coherent excitation and ionization of hydrogen atoms owing to the oscillating electric fields acting on fast projectiles in front of the insulator surface.

Determination of Debye–Waller factors from elastic diffraction peaks of thermal energy atomic scattering from solid surfaces

An efficient inverse procedure to solve the inverse scattering problem has been developed and the Debye–Waller factors have been determined from the elastic diffraction peaks of only one experimental setup (one data set) of thermal energy atomic scattering from solid scattering from solid surfaces. The new inverse procedure preserves the correlation between elastic scattering and thermal attenuation. The computations of the sophisticated He scattering from LiF(001) surface support the correctness of the new procedure. The computations have been compared with the results of the analysis of several different experimental setups.

Inelastic Rainbow Scattering of CH4 Molecules from a LiF(001) Surface

Angular intensity and velocity distributions of CH4 scattered from a LiF(001) surface have been measured using CH4 molecular beams with energies in the range of Ei=190–570 meV. Rainbow scattering of CH4 molecules has been obtained for the [100] incident azimuthal direction and the results are compared with the “Washboard” model proposed by Tully [J. Chem. Phys. 92 (1990) 680]. A qualitative agreement between our experimental results and the model has been observed as far as the dependence of angular intensity distribution of scattered CH4 molecules on both incident energy and surface temperature are concerned. It is found that the attractive potential acting on CH4 at the LiF surface plays an important role in determining the rainbow peak position. The model also explains the angular velocity distribution of scattered CH4 molecules.

An order–disorder phase transition in monolayer CO/LiF(001)

Monte Carlo simulations of CO physisorbed on a LiF(001) surface show that a monolayer of CO molecules forms an ordered p(2√×√)R45 herringbone structure which undergoes an order–disorder phase transition around 30 K. The CO molecules sit near the Li+ sites (C atom down) with a tilt of ∼40° from the surface normal.

Characterization of LiF and CaF 2 surfaces using MIES and UPS (HeI)

Metastable impact electron spectroscopy (MIES) and UPS (HeI) in combination with ab initio calculations (CRYSTAL-code) were applied to study surface and bulk defects in LiF and CaF 2. The investigated stoichiometric, defective and doped surfaces are LiF, LiF doped with Mg, and CaF 2. The experimental information obtained on the electronic structure of stoichiometric and defective surfaces of LiF (100), LiF on W (110) and CaF 2 (111) is discussed on the basis of the ab initio calculations. MIES spectra show features from Li agglomerates on the surface of electron bombarded LiF. The electronic structure of the LiF:Mg single crystal shows additional features above the valence band maximum caused by the Mg doping. These are identified as caused by the formation of (Mg O) surface bonds. Exposure of CaF 2 surfaces to X-rays (1486 eV) shows also formation of metallic regions on the surface.

Nonlinear effects in ion emission from LiF induced by N + and N 2+ MeV ion impact

Beams of N + and N 2 + in the 75–7500 keV/atom energy range have been used to bombard a LiF surface. Desorption yields and energy distributions of positive and negative secondary ions have been measured by the time-of-flight technique. Measurements of the energy distributions revealed the relative contribution to desorption due to collision cascades and electronic excitations in the solid. Experiments have shown that F − ions are desorbed as a result of elastic collision cascades and that the corresponding ion yield depends linearly on the number of constituents of the projectile, i.e. Y(N 2 +) > 2Y(N +). In contrast, emission of clusters, such as Li 2F +, were found to be caused by electronic excitation in the solid with the respective yields revealing a nonlinear dependence: Y(N 2 +) > 2Y(N +). Both processes were found to contribute to Li + desorption. Nonlinear effects are discussed in terms of effective stopping power which depends not only on the electronic stopping power of the primary ion, but also on the track size.

Neutralization and electronic excitation during low-energy H and He scattering from LiF

Low-energy H + and He + ions, together with their neutrals in the ground state, have been scattered from a polycrystalline LiF surface in order to investigate the mechanism of ion neutralization and electronic excitation. The surface peak of F obtained with He + incidence is composed of three subpeaks assignable to elastic scattering and inelastic scattering due to single and double electron-hole pair excitation. In the case of H + scattering, the surface peak of F consists only of the elastic peak, and a background due to multiple scattering from the deeper layers is remarkable. Almost the same H + spectrum without the surface peak is obtained for H 0 incidence. These results are analyzed on the basis of molecular-orbital energy calculations. It is found that the F 2p orbital has antibonding character with respect to the He 1s orbital during collision and, hence, one or two F 2p electrons can be excited preferentially. In the case of H + scattering from F −, on the other hand, the H 1s state appears in the band gap due to antibonding interaction with the F 2p orbital, leading to excitation of the H 1s electron rather than the F 2p valence electrons.

Ionic and electronic processes at ionic surfaces induced by atomic-force-microscope tips

Several ion and electron processes near contact between two insulators are predicted, which should be observable following recent developments in atomic force microscopy. When two solids approach each other closer than about two interatomic distances, instabilities and strong outward displacement of the surface atoms occur. This has been known for metals, and is shown in this paper for ionic systems. Using periodic density-functional calculations, we demonstrate that the ionic model holds for relatively large $(<1.5\AA{})$ displacements of individual ${\mathrm{Mg}}^{2+}$ and ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ions from the MgO surface. It breaks at further displacements where the electron redistribution is strong, and neutralizes both the displaced ion and the vacancy left at the surface. The interaction of the sharp and blunt MgO tips with the LiF, NaCl, and MgO surfaces was studied using an embedded-cluster Hartree-Fock method. Conditions for trapping of surface ions on the tip during the tip and surface separation are determined. It is demonstrated that this trapping can initiate formation of one-dimensional strings of ions stretching out from the surfaces as they separate. The latter result is confirmed by the classical molecular-dynamics simulations of the contact formation and separation of the plane MgO and LiF surfaces. These simulations have also demonstrated that the junction between two dissimilar ionic surfaces breaks not at the original but at a new interface accompanied by contamination of one of the surfaces. Analysis of electronic processes reveals that electrons can tunnel from the tip to the anion vacancies left in the surface.

Resonant Coherent Excitation of Fast Hydrogen Atoms in Front of a LiF(001) Surface

We have scattered protons and hydrogen atoms with energies of some keV from a LiF(001) surface under a grazing angle of incidence. From the intensity of Lyman-\ensuremath{\alpha} radiation (transition from $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$ to $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1,\ensuremath{\lambda}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}121.6\mathrm{nm}$) as a function of projectile energy for different azimuthal orientations of the crystal surface, we find clear evidence for a resonant coherent excitation of $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$ states of hydrogen atoms in the oscillating electric field in front of the insulator surface.

A further theoretical exploration of the surface-aligned photo-initiated H+CO2 reaction: Surface motion and temperature dependence

We report a further theoretical investigation of a model surface-aligned photoreaction with a moving surface approximation. The reaction is initiated by the photodissociation of a well-aligned HBr adsorbed on the LiF(001) surface. The collision of the dissociating H fragment with a coadsorbed CO2 leads to the OH and CO products. In an earlier theoretical study with a static surface model, it has been shown that the reactivity can be significantly enhanced relative to the corresponding gas phase reaction for some adsorption alignments. In this work, we address the roles played by surface motion and temperature. Our results indicate that some (∼0.3 eV) energy can be lost to the surface either from the adsorbed HOCO complex or from one of the products when it collides with the surface during the final disintegration of the HOCO complex. However, the energy transfer has a minor effect on the reactivity. The final state distributions of the products are found to be similar to those produced with the static surface model. On the other hand, a significant temperature effect is predicted for one adsorbate configuration. Apparently, the lowering of temperature for a well-aligned system results in a more narrowly focused alignment and higher reactivity.

Self-lubrication in scanning-force-microscope image formation on ionic surfaces

The mechanisms of and conditions required for true atomic resolution remain the focus of scanning force microscopy (SFM). We present a theoretical model of SFM using a molecular-dynamics method for the calculation of the interaction between a crystalline sample and a tip nanoasperity, combined with a semiempirical treatment of the mesoscopic van der Waals attraction between tip and surface, and the macroscopic parameter of cantilever deflection. The main features of the SFM experiment were modeled, including force versus distance curves at various tip positions on the surface, and scanning of a perfect LiF surface in contact regime with a repulsive force of 1--2 nN. It is shown that tip contamination due to adhesion to the surface atoms may promote periodic SFM imaging, if the adsorbed surface material makes stable structures on the tip. We demonstrate that the adsorbed cluster can adjust itself to conditions of scanning by exchanging atoms with the surface and changing its structure. We believe that this dynamic ``self-organization'' of the surface material on the tip during scanning could be a general effect which may explain why periodic surface images are often obtained using a variety of tips and large tip loads.

Axial energy distributions of Li + and F − desorbed from LiF surfaces by fast ion impact

Initial axial kinetic energy ( E z ) distributions of Li + and F − secondary ions desorbed from LiF surfaces, bombarded by low current (∼500 particles/s) N 3+ beams in the 0.075–7.5 MeV primary ion energy ( E PI) range, were measured by using the time-of-flight technique. In this energy range, the electronic energy loss, S e, increases from 25 to 160 eV/Å, while the nuclear stopping power, S n, decreases from 8.0 to 0.25 eV/Å. The observed F − initial axial kinetic energy distribution can be described by the linear collision cascade theory and no other contribution was found. The F − total yield decreases proportionally to S n. The Li + distribution presents a remarkable deviation from the cascade prediction, indicating the existence of additional mechanisms related to the electronic energy-loss process. For the range of E PI studied, these mechanisms produce a Li + energy distribution with a Maxwell-Boltzman-like shape, which vanishes above E z ∼ 10 eV and presents a maximum at E z ∼ 1.2 eV. The Li + yield is nonlinear with the electronic energy loss, S e. A simple desorption model, based on the spatial distribution of the energy deposited by the projectile and on the effective energy-loss concept, is presented. This spatial distribution of the deposited energy is due to secondary electron cascades and is connected with a ESD-like mechanism on the surface.

Rotational- and vibrational-state resolved HF-surface interactions investigated by surface light-induced drift

Experiments using surface light-induced drift are performed to yield information on the rotational (J) and vibrational (v) state dependence of molecule–surface interactions. Data are presented for the change in accommodation coefficient for tangential momentum transfer α upon excitation of HF interacting with a polycrystalline LiF surface (on a Cu substrate) and a hydrophobic stearic-acid monolayer (on a stainless-steel substrate). We employed both P- and R-branch excitation of HF in the fundamental vibrational band (v=0→1) with J=0–4, using a continuously tunable color-center laser (λ≈2.5 μm). By combining the results for the P- and R-branch, we find that the influences of J and v upon the molecule–surface interaction can be considered independent to a good approximation. It is found that α decreases upon vibrational excitation v=0→1, whereas it increases with increasing J. The J and v dependences of α are discussed in the framework of a unified kinetic theory of molecule-surface interaction.

Epitaxial growth of alkaline earth fluorides on the (0 0 1) surface of lithium fluoride. III. The system — epitaxial growth of LiBaF3

Abstract The development of overlayers during the interaction of molecular beams of BaF 2 with (0 0 1) surfaces of LiF is studied for crystal temperatures T = 573–673 K and for impinging BaF 2 fluxes j on = 1 × 10 12 −1.5 × 10 13 cm −2 s −1 . As long as the LiF supply from the substrate is not hindered formation and epitaxial growth of LiBaF 3 dominates strongly, and growth of BaF 2 in the orientation BaF 2 (0 0 1)[1 0 0]|LiF(0 0 1)[1 1 0] with a misfit m = + 8.8% occurs only as a transient and altogether negligible phenomenon. Grwoth of LiBaF 3 proceeds in the orientation with parallel axes, i.e. with LiBaF 3 (0 0 1)[1 0 0]|LiF(0 0 1)[1 0 0], with a misfit m = − 0.82%. As growth mode, island growth with nucleation, growth and coalescence of three-dimensional islands is found. The formation of closed layers can be accelerated by decreasing the crystal temperature or increasing the impinging molecular beam flux. With a BaF 2 flux only, layer thicknesses, because of the necessary supply of LiF from the substrate, are limited, while with congruent LiF and BaF 2 fluxes, layers of virtually unlimited thicknesses can be grown. By evaporation of LiF onto already grown LiBaF 3 layers, epitaxial growth of LiF in an orientation corresponding to that of the underlying LiF crystal is found. Closed LiF layers are achieved for T = 573 K and j on (LiF) = 1 × 10 12 cm −2 s −1 at a thickness of ≈ 30 nm. This would, in principle, enable the fabrication of superlattices LiF/LiBaF 3 /LiF etc. with layer thicknesses suitable for optical applications.