LiF Surface Research Articles

Epitaxial growth of alkaline earth fluorides on the (0 0 1) surface of lithium fluoride. II. The system [formula omitted

Epitaxial growth of CaF 2 on the (0 0 1) surface of LiF is studied for crystal temperatures between 573 and 773 K and for molecular beam fluxes of 1 × 10 12–1 × 10 13 CaF 2 molecules per cm 2 and s. For T = 573 K layer growth with the orientation CaF 2(0 0 1)[1 0 0]∥LiF(0 0 1)[1 1 0] is found. The layer growth mode is interpreted by a restricted mobility of CaF 2 molecules at this low temperature. LEED studies suggest a rearrangement of the (0 0 1) plane of CaF 2 leading to the formation of nm-scaled fourfold pyramids with (1 1 1) facets or, alternatively, an ab initio formation of such pyramids accompanied by a fast coalescence to a closed layer. For temperatures T ⩾ 673 K growth of islands occurs. With increasing temperature the orientation changes to CaF 2(0 0 1)[1 0 0]∥LiF(0 0 1)[1 0 0]. This latter orientation may be attributed to an increasing mixing of Li + and Ca 2+ ions, enforcing in this way a continuation of the cation arrangement of the underlying LiF crystal into the growing CaF 2 islands. As for MgF 2 LiF(0 0 1) , for the present system growth of LiF crystallites — presumably on already grown CaF 2 layers — is found. The epitaxial growth of these crystallites with the (1 1 1) surface parallel to the original (0 0 1) surface of the LiF crystal is interpreted by a space filling arrangement of F − anions above Ca 2+ ions.

Scattering of vibrationally and electronically excited CO molecules from a LiF(100) surface

Experiments are performed in which vibrationally and electronically excited CO(a 3II, v = 1) molecules are scattered from a LiF(100) surface. As there is originally no population in the vibrationless level of the metastable state, this experiment gives the unique possibility of probing the vibrationally inelastic channel in the scattering of vibrationally excited molecules. The vibrational deactivation probability can thus be accurately determined and is found to be below 10 −3 for the system under study.

State-to-state scattering of metastable CO molecules from a LiF (100) surface

Scattering of electronically excited, state-selected metastable CO(aΠ3) molecules from a cleaved LiF(100) surface is studied experimentally. Internal state distributions, fluorescence profiles, time-of-flight (TOF) profiles and angular distributions of the surviving metastable CO molecules are measured. Relative and absolute survival probabilities are determined for various impact velocities. The dependence of translation and rotational temperature on the velocity of the incoming beam unambiguously indicates a direct inelastic scattering process, even though the angular distributions are broad, both in plane and out of plane. The internal state distribution after scattering shows an overpopulation of the initially prepared Ω=1-component relative to the other spin components.

Photoinitiated Reaction Dynamics between Aligned Adsorbates on Solid Surfaces: A Theoretical Exploration of the H + CO2System on LiF(001)

We report a quasi-classical trajectory study of a chemical reaction between H and CO2 at the LiF(001) surface. The reaction is initiated by photodissociation of well-aligned HBr(ad) at 193 nm, which produces a “hot” H atom directed toward a nearby CO2(ad). Single molecules of each reactant are placed on a static surface, and a full-dimensional HCO2 potential derived from ab initio calculations is used. The adsorbate−substrate and the adsorbate−adsorbate potentials consist of both nonelectrostatic and electrostatic contributions. Several energetically favorable adsorption configurations are determined by a Monte Carlo method. Quasi-classical trajectories are calculated at 80 K for four different adsorption configurations. We find that the reactivity at some configurations is significantly enhanced compared with the corresponding gas-phase simulation. The calculated impact parameters and incident angles of the surface-aligned collisions indicate that the enhanced reactivity can be largely attributed to the ...

The surface electronic structure of stoichiometric and defective LiF surfaces studied with MIES and UPS in combination with ab-initio calculations

UPS (He I) and metastable impact electron spectroscopy (MIES) spectra of the LiF(100) single crystal surface and stoichiometric LiF films are presented. The spectra are interpreted on the basis of ab-initio electronic structure calculations. Defective surfaces, produced by electron dosing, were studied in the same manner. The MIES spectra reveal that the electron dosing produces metallic patches on the surface, but no uniform Li adlayer. The calculation show that the F-center contribution to the electron emission is very close in energy to that from the metallic patches; thus, the two contributions cannot be distinguished by the present experimental techniques.

MeV nitrogen bombardment of LiF: From the nuclear to the electronic sputtering regimes

The time-of-flight technique was used to measure the axial kinetic energy (Ez ) distribution of secondary ions emitted from a LiF surface bombarded with N3+ ions in the 0.30–6.0 MeV energy range. Desorption yields were also measured as function of the projectile energy. The analysis of the Li+ and F− secondary ions shows that, at low incident energies (below 0.50 MeV), these ions are sputtered from the surface by a collision cascade (nuclear sputtering). Such indication comes from the E z 2m−2 (where 0 < m< 1) asymptotic behavior of the axial kinetic energy distribution. The observed values for m are m ∼ 0 for Li+ emission and m ∼ 0.4 for F− emission. The mean axial kinetic energies are, respectively, σ 6eV and ∼ 9eV. At higher projectile energy (above 1 MeV), there is a change in the Li+ emission behavior: the high energy tail of the axial energy distribution becomes exponential, the desorption yield increases presenting a maximum near 2.0 MeV and the mean energy of emission decreases to ∼ 3 eV. No such change has been found for F− desorption process. Its yield decreases proportionally to the nuclear stopping power of N projectiles in LiF.

Lie algebraic method for vibrational and rotational transitions in inelastic collisions of a molecule with a solid surface

The vibrational and rotational transitions of a diatomic molecule in inelastic gas-surface scattering have been studied using the Lie algebraic approach of Alhassid and Levine [Phys. Rev. A 18 (1978) 89]. We have build a Hamiltonian describing the molecule-surface scattering and used it to construct a dynamical Lie algebra h 6. The expressions for the wavefunction of the system near the surface ( z 2~ 0) are explicitly obtained. All the internal rotational and vibrational modes of a diatomic molecule and their coupling have been taken into account in the theoretical model. This wavefunction is applied to compute analytically the transition probabilities from an initial-vibrational state to another final one, for a model problem of H 2 scattering from the LiF(001) surface. The computed probabilities are in good agreement with those obtained using an accurate coupled-channel method.

Li enrichment of LiF surfaces bombarded by MeV nitrogen ion beams

The PDMS (Plasma Desorption Mass Spectrometry) technique was employed for monitoring surface modifications in a LiF sample under ion bombardment at different doses. The flux of the 2.5-MeV N + beam was kept at 10 10 particles/s (∼ 1 n nA) to induce radiation effects in the LiF sample and was decreased to ∼ 500 particles/s during the PDMS analysis. The resulting time-of-flight spectra exhibit peaks characteristic of the sample, Li + and Li 2F +, and others such as H + and C 2H n +, originated by the ionization of the adsorbed layer. It was observed that only the desorption yields corresponding to the LiF species present a dose dependence. This result is interpreted in terms of Li enrichment of the surface due to F,H center defect production.

Electron emission in slow collisions of protons with a LiF-surface

Electron spectra have been measured for collisions of protons with surfaces of LiF grown on a tungsten substrate. The measurements were carried out for 5° grazing incidence collisions at energies from 50 eV up to 1 keV. The experimental spectra are compared with theoretical spectra obtained from model calculations. The model is based on the assumption that electron emission is dominated by the mechanism of electron promotion via the 3dσ molecular orbital of the HF − quasi-molecular system. Using theoretically obtained functions for the scattering potential and for the 3dσ orbital energy, and calculating the spectra in semiclassical approximation for a distribution of impact parameters obtained from Monte Carlo simulations for the relevant scattering geometry, good agreement between experimental and theoretical spectra is obtained. This agreement is taken as strong evidence for the assumed ionization mechanism.

Quantitative cleaning characterization of a lithium-fluoride ion diode

An ion source cleaning testbed was created to test plasma-cleaning techniques, and to provide quantitative data on plasma-cleaning protocols prior to implementation on the SABRE accelerator. The testbed was designed to resolve issues regarding the quantity of contaminants absorbed by the anode source (LiF), and the best cleaning methodology. A test chamber was devised containing a duplicate of the SABRE diode. Radio-frequency (RF) power was fed to the anode, which was isolated from ground and thus served as the plasma discharge electrode. RF plasma discharges in 1-3 mtorr of Ar with 10% O/sub 2/ were found to provide the best cleaning of the LiF surface. X-ray photoelectron spectroscopy (XPS) showed that the LiF could accrue dozens of monolayers of carbon just by sitting in a 2/spl times/10/sup -5/ vacuum for 24 h. Tests of various discharge cleaning protocols indicated that 15 min of an Ar/O/sub 2/ discharge was sufficient to reduce this initial 13-45 monolayers of carbon impurities to 2-4 monolayers. Rapid recontamination of the LiF was also observed. Up to ten monolayers of carbon returned in 2 min after termination of the plasma discharge and subsequent pumping back to the 10/sup -5/ torr range. Heating of the LiF also was found to provide anode cleaning. Application of heating combined with plasma cleaning provided the highest cleaning rates.

Energy gain of highly charged ions in front of LiF

We present estimates of the energy gain of highly charged ions approaching a LiF surface, based on a modified classical-over-barrier model for insulators. The analysis includes the energy gain by image acceleration as well as the deceleration due to charge-up of the surface in a staircase sequence. The resulting velocity-dependent total energy gain is studied in detail and the results are compared with experimental data.

Lattice Parameter and Thermal Expansion Measurements of a LiF(001) Surface by He-Atom Beam Diffraction Method

A new method for determining the lattice parameter using atomic beam diffraction is described. Using this method, we have measured the temperature dependence of the lattice parameter of LiF(001). It was found that the value of the surface thermal expansion coefficient is higher by a factor of 3.6 than the bulk value for temperatures up to 180°C.

State-to-state Scattering of Metastable CO Molecules from a LiF(100) Surface

Survival of electronically excited, state-selected $\mathrm{CO}({a}^{3}\ensuremath{\Pi})$ molecules scattering from a cleaved LiF(100) surface is studied. Absolute survival probabilities are determined for various impact energies. Time-of-flight profiles, angular distributions, and internal state distributions of the surviving metastable CO molecules are measured. The dependence of their translational and rotational temperatures on the incoming velocity unambiguously indicates a direct inelastic scattering process even though the angular distributions are broad, both in plane and out of plane.

Scattering of alkali atoms and ions from alkali-halide surfaces: No evidence found for electronic surface states within the band gap of the insulator

Fast alkali atoms and ions are scattered with keV energies under grazing incidence from the surface of the alkali halides LiF(100), KCl(100), KI(100), and the scattered beams are analyzed with respect to their charge fractions. From our experiments we find no evidence for occupied or unoccupied electronic surface states within the band gap of the insulator. {copyright} {ital 1997} {ital The American Physical Society}

Water Adsorption on Alkali Halide Surfaces.

We have investigated the behavior of water adsorption on alkali halide surfaces in air at room temperature using the noncontact monitoring mode of a scanning force microscope (SFM). We have found a formation of water droplet pattern on LiF (001) surface and subsequent formation of a uniform film, as reported on BaF2(111) surface. However, no water droplet pattern was observed on NaCl (001) surface. The SFM image from NaF (001) surface has revealed complicated features with water droplet pattern. The different H2O adsorption patterns for LiF, NaF and NaCl seem to be induced by the competition between water-water and water-substrate interactions.

Role of tip contamination in scanning force microscopy imaging of ionic surfaces

We present a theoretical model of the scanning force microscope (SFM) using a new quantum-mechanical embedding technique and a molecular dynamics method for calculation of the interaction between a crystalline sample and a tip nanoasperity. These are combined with a semi-empirical 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 modelled, including force vs. distance curves at various tip positions on the surface and scanning of a perfect LiF surface in contact regime. 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-organisation’ 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.

Investigation of condensation and evaporation of alkali halide crystals by molecular beam methods. XIV. Development of two-dimensional islands on the (100) surface of LiF

The development of two-dimensional islands on the (100) surface of LiF is studied for temperatures T = 706−771 K and saturation ratios S = 1.9−13. From the dependence of the island size on the growth time the velocity of monatomic steps and with it the mean diffusion distance x s = a exp( E s 2kT ) = 3.7( 12 11 ) × 10 −8 exp[ (1.04 ± 0.15) eV 2kT ] cm is obtained. This result confirms the model of surface diffusion with jumps from one lattice site to a neighboring one. With earlier measured desorption times a surface diffusion coefficient D = 8.6 × 10 −3 exp( −0.32 kT ) cm 2 s −1 is obtained. The comparison of the experimental shape development of the islands with computer simulations of the island growth gives the relative strength of step to surface diffusion β ≈ 5.5(± 50%). Because of a strong scatter of the measurement points due to impediments of step movements caused by divalent cation impurities and adjacent steps, a temperature dependence of β cannot be determined.

Formation of negative ions in grazing scattering from a LiF(100) surface

We have studied charge exchange between fast atoms/ions and the surface of an insulator. Based on investigations on the formation of negative hydrogen, oxygen and fluorine ions in the grazing scattering from a clean and flat LiF(100) surface, we discuss specific features of the interaction of fast atomic projectiles with the surface of an insulator. Under specific kinematic conditions large fractions of negative ions are observed.

On the formation of hollow atoms in front of an insulating LiF surface

KLL Auger spectra of hydrogenic (1s) N, O and Ne ions impinging on an insulating LiF(100) single crystal are presented. Beam energy and incident angle have been varied such that the lowest possible velocity towards the target is achieved, at the same time varying the velocity parallel to the target by a factor of 10. Similarities and differences between the LiF spectra and spectra measured on (semi) conducting Si(100) and Al(110) surfaces are discussed.

Scattering of neutral NH3 clusters off LiF(100): angular distributions of NH3 and small clusters

The scattering behavior of neutral ammonia clusters off a LiF(100) surface is studied. Ammonia clusters are produced by a coexpansion of NH3 and Kr with an average kinetic energy of 48 meV per monomer molecule. Using single photon VUV laser ionization at λ = 118 nm (hv = 10.49 eV) the mass distribution of scattered particles is obtained in a reflecting time-of-flight mass spectrometer. Compared with the incoming cluster beam the average cluster size of the scattered particles is drastically decreased. The angular distribution of NH 3 + and NH 4 + after scattering reveals a strong inelastic interaction between the clusters and the LiF(100) surface which is described in the context of a thermokinetic model and a phonon excitation along the (001) azimuth of the LiF(100) surface.