LiF Surface Research Articles

Role of corrugation in molecule—surface interaction: flow experiments using the LiF(001) surface in two orientations

The role of surface corrugation in molecule—surface interaction is studied in Knudsen flow of H 2, HD and N 2 along LiF(001) surfaces under different azimuth angles. This is achieved by measuring, for each angle, the production of rotational polarization, using the effect of a magnetic field on the flow. The corrugated surface structure is found to manifest itself most strongly for H 2, where the results are in qualitative agreement with quantum-mechanical model calculations by Levi and Tarditi. Corrugation is found to be much less important for HD and N 2. This is explained in terms of a different effective molecule—phonon interaction range for different magnitudes of the rotational level splitting.

Energy distribution in aniline scattered from various low energy surfaces

Aniline has been scattered from three organic substrates and a LiF single crystal surface. Applying multiphoton ionization and time of flight measurements the vibrational, rotational, and translational energy distributions were measured simultaneously. The NH2 ‘‘umbrella’’ like mode was found to be a very efficient accepting mode in the energy transfer process. The less rigid the surface, the greater the efficiency with which this mode is populated. The mode specificity does not exist for the rigid LiF surface. A model is presented, which explains all observations based on the collision time and kinematics.

Thermal-annealing studies of defects in lithium fluoride after electron-beam irradiation.

We have bombarded single-crystal LiF surfaces with 400-eV electrons at fixed target temperatures between 330 and 470 K and varied the irradiation time between 30 and 4200 sec. After irradiation we heated the crystals up to 750 K and simultaneously recorded the Li desorption. Interpreting the amount of desorbed Li atoms as a measure for frozen electron-center defects and Li agglomerates in the crystal, we were able to obtain an activation energy for the thermal migration of the H centers of about 0.13 eV. Investigations, where the bombarding time has been varied, indicate efficient H-center trapping processes and different lifetimes for H-center clusters of different sizes in the crystal. In addition, we found evidence for a substantial formation of F-center agglomerates under the surface and Li islands on the surface.

Atomic resolution on the surface of LiF(100) by atomic force microscopy

Atomic force microscopy (AFM) has already demonstrated its usefulness on a nanometer scale surpassing the resolution of conventional profilometers and probing different interactions such as repulsive contact, electrostatic, and magnetic forces. On an atomic scale the fundamental contrast mechanisms are still under investigation. The atomic resolution on layered materials such as graphite, boron nitride, transition metal dichalcogenides, and MnPS3 shows the great potential of this technique. Several experimental and theoretical investigations on these layered materials have shown that the presence of the AFM tip can lead to a significant distortion of the electronic and atomic structure. For small loadings (≤10−8 N) the tip causes long-range elastic deformations while for higher loadings a sharp tip can puncture the sample. To explain the atomic scale features being measured with higher loadings, different mechanisms such as the dragging of flakes or shearing of layers have been suggested. The application of AFM to nonlayered structures gives the opportunity to exclude several of these influences. Here we present atomically resolved images of LiF(100). The measurements are compared to results from different surface sensitive techniques such as helium scattering and low energy electron diffraction (LEED). The contrast mechanisms of AFM are discussed in relation to these experiments.

Photon-stimulated desorption of excited alkali atoms from alkali halides following core-level excitation

We report excitation-functions for excited alkali atoms from NaCl, KCl and LiF surfaces irradiated with ultraviolet light in the range 15 to 70 eV, an energy range including several alkali core levels. The excitation function of the excited Li yield is shown to be similar to the total electron yield obtained simultaneously. The results are consistent with desorption initiated by a localized two-hole valence excitation following the decay of the core excitation. The excited potassium yield from KCl is seen to decrease with temperature, suggesting a dependence on surface conditions such as the presence of excess metal.

Model calculations of electronic excitation of Li adsorbed on LiF(001)

An efficient method to solve the three-dimensional Schrödinger equation of an adsorbed atom on a rigid corrugated surface is presented. Formulas for calculating the Franck–Condon coefficients on two potential surfaces are given. Numerical results are obtained for a model of Li adsorbed on the LiF(001) surface.

Exploration of surfaces by atomic scattering in the almost classical regime

Inelastic atom-surface scattering at hyperthermal ( ∼ 1 eV) energies is discussed. In this regime, the scattering is almost classical. However, for a surface at low temperature, peaks in the final energy distribution are broadened by quantum effects. The widths of these peaks provide a direct measure of the zero point motion of the surface atoms. For impulsive collisions, we derive a simple expression for these widths. As an example, we present a semi-quantitative analysis for He striking a LiF surface, using Bom-Mayer forces between the atoms to estimate the amplitude of the zero point fluctuations. We discuss, in detail, how single peaks may be isolated in an experiment. Measurements of this effect could provide an important probe of surface dynamics.

Application of the reduced-equations-of-motion formalism to the scattering of molecular NO from a LiF (001) surface at nonzero temperature

The reduced-equations-of-motion (REOM) model for gas–solid energy transfer in molecule–surface collisions [D. J. Diestler and M. E. Riley, J. Chem. Phys. 89, 4137 (1989)] is extended to include effects of nonzero surface temperature by combining the REOM damping theory with a stochastic partial velocity reset (PVR) algorithm [M. E. Riley, M. E. Coltrin, and D. J. Diestler, J. Chem. Phys. 88, 5934 (1988)]. The REOM/PVR procedure, which involves integration of the gas–molecule EOM above the frozen solid with stochastic resetting of the molecule’s velocities, is tested by comparing results for NO+LiF(001) scattering with those of a previous stochastic-trajectory study of this system [R. R. Lucchese and J. C. Tully, J. Chem. Phys. 80, 3451 (1984)]. The REOM/PVR results reproduce the trends in the stochastic-trajectory results very well for translational and rotational energy transfer as a function of the various system parameters. However, it is found that the coupling of the vibrational mode, whose frequency is greater than that of the Debye frequency of the solid, is not treated accurately by the REOM theory, which is based on adiabatic approximation to the solid’s response function.

Modification of the INDO Calculation Scheme and Parametrization for Ionic Crystals

AbstractA new parametrization of the semiempirical quantum‐chemical INDO method is proposed for the study of the electronic structure of a wide class of perfect and defective ionic insulating crystals. The applicability of the “cut‐off function” concept for the calculations of exchange interactions in the large unit cell (LUC) model is also briefly discussed. Both, the proposed calculation scheme and the parametrization are tested on a number of molecules, ionic crystals (LiH, LiF, LiCl, KCl, MgO, K2SO4), and (100) crystal surfaces of LiF, KCl, MgO.

Energy thresholds and delayed emission for electron-stimulated desorption of neutral ground- and excited-state Li atoms from lithium fluoride

We have bombarded single-crystal surfaces of LiF with 5–500 eV electrons at different target temperatures (300–800 K) and the temperature dependence of the yield of ground- and excited-state Li atoms was studied. Measuring the electron energy threshold for desorption of Li∗ we found an onset for desorption of Li∗ around 60 eV, which correlates well with the Li+ core-exciton levels of Li+ in LiF. Based on these findings we propose a Knotek-Feibelman-like mechanism for the desorption of excited Li atoms from LiF initiated by Li core electron excitations (core excitons). On the other hand, we found the threshold for desorption of Li° atoms to be much lower, around 12 eV. This is explained with a process involving valence band excitation and subsequent F- and H-center formation. In related experiments we have studied the delayed emission of ground-state atoms after turning off the electron beam, which can be explained in terms of F-center production by the primary electron beam, Li metal colloid formation and the subsequent evaporation and diffusion of Li to the surface, resulting in metal desorption.

Atomic resolution on LiF (001) by atomic force microscopy

The first atomically resolved atomic force microscopy images of the LiF (001) surface are presented. A square lattice with a spacing of 2.8±0.1 A is resolved, which can be attributed to the F− ions. Li+ is not resolved due to its small size. The origin of the image contrast is discussed briefly and the results are compared with those from helium scattering.

Non-dissociative laser induced desorption of molecules from dielectric surfaces

We have been concerned with the non-dissociative desorption of 1,3-cyclohexadiene, pyridine, 2-piccoline, N-methyl-aniline and methyl-pyrazine adsorbed on surfaces of LiF (100), MgF 2, quartz and surfaces coated with Ar, Xe and N 2. As an example we present our results on the non-dissociative desorption of methyl-pyrazine from quartz. We used tunable UV laser light (248–280 nm) to excite the π→π ∗ electronic transition centered at 266 nm. The dependence of the desorption cross sections on layer thickness, laser fluence and laser wavelength has been obtained by quantitative analysis of the TOP (time of flight) mass spectra of the desorbing molecules. Two different desorption channels have been found with quantum yields two orders of magnitude different from each others. A non-thermal channel has a quantum yield of 10 −4–10 −5 at 266 nm. The second channel occurs at thicker layers and higher laser fluences with quantum yields⩾10 −3. The desorption signal is proportional to the absorption coefficient.

Thermal and intrinsic stresses in single-crystal nickel films

The thermal and intrinsic stresses in single-crystal nickel films, in the thickness range 500-3000 AA, prepared by epitaxial growth onto the (100) cleaved surfaces of NaF, LiF, NaCl and MgO have been obtained. Calculated results from ferromagnetic resonance experiments show that the magnitudes and directions of the film stresses depend on the nature of the substrate conditions and on the sequential ordering of the deposited layers. The experimental results revealed that the thermal stresses, which are elastic and isotropic in character, are larger than the intrinsic stresses and can be of either compressive or tensile type. This type of stress disappears after the film is removed from the substrate. The present results also show that there is no single model which sufficiently accounts for the magnitudes of the intrinsic stresses in the strained films.

Surface enrichment of Li on LiF single crystal after cleaving or under electron bombardment

Abstract The build-up of an excess metal layer on the surface of LiF(100), after cleaving or due to electron irradiation, is examined by monitoring the neutral atom and molecule emission during sputter depth profiling. At room temperature the Li enrichment on the surface saturates at an electron fluence of 2.5 × 10 15 electrons cm 2 at 500 eV energy. In addition, lesser enrichment of the crystal surface by Li after cleavage without electron irradiation was observed, which also reappears after removal by moderate sputtering by a 3 keV ion beam. Enrichment is attributed to the difference in the formation energies of cation and anion vacancies. The time constants for the build up of the metal layer at room temperature for the unirradiated but cleaved crystal and the cleaved crystal exposed to the saturation dose of electrons are 2 and 40 min, respectively.

Surface enrichment of Li on LiF single crystal after cleaving or under electron bombardment

The build-up of an excess metal layer on the surface of LiF(100), after cleaving or due to electron irradiation, is examined by monitoring the neutral atom and molecule emission during sputter depth profiling. At room temperature the Li enrichment on the surface saturates at an electron fluence of 2.5 × 10 15 electrons cm 2 at 500 eV energy. In addition, lesser enrichment of the crystal surface by Li after cleavage without electron irradiation was observed, which also reappears after removal by moderate sputtering by a 3 keV ion beam. Enrichment is attributed to the difference in the formation energies of cation and anion vacancies. The time constants for the build up of the metal layer at room temperature for the unirradiated but cleaved crystal and the cleaved crystal exposed to the saturation dose of electrons are 2 and 40 min, respectively.

Temperature resistant and vacuum tight LiF connections

A method is described for making LiF constructions involving LiF-LiF and LiF-metal seals which are vacuum tight over a wide temperature range. As an application, rectangular LiF (001) surfaces have been assembled to form a flat channel which is connected to a copper vacuum flange. LiF machining was done using an ultrasonic technique. Mixtures of LiF and PbF2 were used as sealing agents throughout. The entire construction has been proved vacuum tight in the temperature range 78 K to 700 K.

Evidence for threshold effects in positron diffraction from NaF and LiF.

We have measured the energy dependence of the intensity of a positron beam specularly reflected from the (100) surfaces of NaF and LiF. A 1-eV-wide peak located at an energy that decreases with increasing angle of incidence \ensuremath{\theta} is qualitatively identified as a beam threshold effect. Given the known small positron affinity for the solids, the narrowness of the peak is consistent with its being due to a true surface resonance. However, the dispersion appears to depart from the expected energy versus \ensuremath{\theta} trajectory parallel to the kinematic threshold. Additionally, a search in 50-meV steps did not reveal any fine structure in the peaks, possibly due to our 0.3-eV effective instrumental resolution. Further experiments to map the dispersion with greater precision, to measure the binding energies of the resonances, and to search for possible fine structure are proposed.

Real-time observation of the surface alteration in LiF during electron irradiation

The electron beam induced structures on a LiF(100) surface were investigated by low-energy electron energy loss spectroscopy. During continued electron irradiation, we observed a loss peak induced at 2.8 eV in the band gap region of LiF, in addition to the other three electron induced peaks which had been formerly reported. The peak was assigned to an F 2 center. The temperature dependence of the peak formation by incident electrons was observed from 300 K to 500 K. The surface diffusion or evaporation seemed to affect the F 2 center formation and the restoring reaction on the surface.

Comparison of the interactions of H2 and rare-gas atoms with surfaces of insulators.

Calculations are presented for the interactions of ${\mathrm{H}}_{2}$, He, Ne, and Ar with the (001) surfaces of LiF, NaCl, and MgO. We have used a model potential that we developed previously and here applied to new systems. The proposed interaction potential has only one free parameter. This is chosen to be a coefficient describing the long-range interaction, since such a coefficient is the least well known. We then discuss the applicability of this model by comparing the results for different systems. In particular we discuss the He/MgO(001) system for which contradictory experimental determinations of the well depth exist.

Interaction of H and D with the (001) surfaces of LiF and NaCl.

Calculations are presented for the interactions of H and D atoms with the (001) surfaces of LiF and NaCl. The attractive part of the potential is constructed using a damped-dipole, damped-quadrupole, and the induced-dipole terms. The repulsive part is computed using the ``effective-medium theory.'' The potential so constructed has too shallow a well depth. By adjusting ${c}_{6}^{\mathrm{\ensuremath{-}}}$, the least-known parameter, by 50--60 %, it is possible to fit most of the experimental bound states of the atom-surface potential within the uncertainty of the experimental data. Similar corrections have been done in the past for other atom--alkali-halide systems. The quality of the potential was also checked by comparing computed probabilities of the scattered beam, ${P}_{T}$(m,n), with those from atom-beam-scattering experiments, ${P}_{E}$(m,n). This potential yields better agreement with experimental data than achieved before.