LiF Surface Research Articles

Observation of Atomic Defects on LiF(100) Surface with Ultrahigh Vacuum Atomic Force Microscope (UHV AFM)

We have constructed an atomic force microscope (AFM) operating under an ultrahigh vacuum (UHV). We have imaged the cleaved (100) surface of LiF ionic crystal to clarify the AFM's performance. As a result, for the first time, we have obtained atomically resolved AFM images of atomic defects penetrating the surface. This result seems to suggest that the cleaved LiF (100) surface was imaged under the condition of monoatomic or small cluster tip-sample interaction.

Hydrogen exposure-enhanced CN ∗ emission from alkali-halide surfaces

We report recent measurements of the enhancement of electron-stimulated desorption (ESD) of excited CN from KCl, NaCl, NaF, and LiF surfaces due to the exposure of the surfaces to gaseous H 2. The CN ∗ desorption yield varies linearly with the ESD yield of atomic hydrogen. The measured rotational distribution of CN ∗ desorbed from a H 2-dosed LiF surface is different from that for an un-dosed LiF surface. We propose an HCN-mediated process to explain the hydrogen-enhanced CN ∗ desorption.

Hydrogen exposure-enhanced CN ∗ emission from alkali-halide surfaces

We report recent measurements of the enhancement of electron-stimulated desorption (ESD) of excited CN from KCl, NaCl, NaF, and LiF surfaces due to the exposure of the surfaces to gaseous H 2. The CN ∗ desorption yield varies linearly with the ESD yield of atomic hydrogen. The measured rotational distribution of CN ∗ desorbed from a H 2-dosed LiF surface is different from that for an un-dosed LiF surface. We propose an HCN-mediated process to explain the hydrogen-enhanced CN ∗ desorption.

The orientation of adsorbed methyl halides on a LiF(001) surface: A Monte Carlo study

Monte Carlo simulations have been carried out to study the adsorption structure of methyl halide molecules (CH3I and CH3Br) on a LiF(001) surface. The ionic substrate is treated as a three-dimensional stationary mesh of point charges and the adsorbed molecules are taken as rigid bodies. The adsorption at different coverages in the monolayer and submonolayer regimes is investigated. The interaction potential between the surface and adsorbate includes electrostatic contributions due to ion–dipole (both permanent and induced) interactions and Lennard-Jones terms that are introduced to account for the remaining interactions. The adsorbate–adsorbate interaction is assumed to be the sum of the permanent dipole–dipole and Lennard-Jones potentials. All the interaction potentials are assumed to be pairwise. The adsorbate–adsorbate interaction is dominated by the Lennard-Jones component, largely due to the halogen–halogen interaction, while Coulomb forces account for a few percent of the total adsorption energy. As a result, the adsorbates tend to form close-packed structures on the surface. The optimal adsorbate structure on the surface is determined by the relative strength of the adsorbate–adsorbate vs adsorbate–substrate interactions. For high coverages close to monolayer, the system has the lowest energy when the molecular axes of the adsorbates are parallel to the surface normal with alternating methyl-up and methyl-down configurations. For an isolated molecule, however, the preferred orientation is perpendicular to the surface normal.

Theoretical modeling of photodissociation dynamics of CH3I on LiF(001)

A new method is developed for the simulation of atoms and molecules interacting with ionic surfaces. This approach, based on a quasi-two-dimensional Ewald sum and a two-dimensional Fourier transformation, is capable of evaluating the long-range Coulomb interactions for a semi-infinite ionic solid. We have applied this method to investigate the photodissociation dynamics of CH3I on a LiF(001) surface. All the degrees of freedom of the adsorbed molecule are considered and the excited state dissociation potentials of CH3I are described by analytical functions derived from a recent ab initio calculation. The substrate (LiF) is represented by 6×6×3 movable atoms surrounded by static ions at their equilibrium positions. The adsorbate/substrate interaction is modeled as a sum of Coulomb and Lennard-Jones pairwise potentials. A phenomenological term is introduced to account for the adsorbate/adsorbate interaction. The equilibrium configurations of the system at a given temperature are obtained by a Monte Carlo method, which shows that there exist two stable configurations with the CH3I molecular axis perpendicular to the surface, either methyl up or down. The dissociation dynamics of the adsorbate is studied with a classical molecular dynamics method and the angular, kinetic energy, and rovibrational distributions of the fragments are calculated. When the molecule is adsorbed with the methyl up, the methyl fragment dissociates into the vacuum promptly with kinetic energy and internal state distributions similar to those in the gas phase. If the molecule is adsorbed with the methyl down, however, the methyl fragment could collide with iodine after rebounding from the surface, transferring a significant amount of kinetic energy to the iodine. A much broader and more energetic kinetic energy distribution of the iodine fragments is observed under such circumstances. The energy transfer is most effective when the parent molecule orients parallel to the surface normal and decreases as the angle deviates from this direction. We also observed a substantial increase in the rotational angular momentum of the methyl fragment and a cooler vibrational distribution for the umbrella mode as a result of the collision.

Ab-initio perturbed-cluster study of carbon monoxide adsorption at a stepped LiF(001) surface

The perturbed-cluster ab-initio Hartree-Fock approach to the study of local defects in crystals [J. Chem. Phys. 92(1990)7448] is applied to the study of CO adsorption at a stepped LiF(001) surface. The step is simulated by a tablet of four ions superimposed on an infinite LiF(001) monolayer. The geometry of the step is first optimized, and corresponds to an important relaxation of cations and anions of the tablet inwards and outwards, respectively. The equilibrium configuration, adsorption energy and vibrational frequency of CO at a corner of the tablet occupied by a lithium cation are calculated. With respect to adsorption at a perfect (100) face, there is a large increase in interaction energy, especially when adsorption occurs via the oxygen atom. This difference is essentially related to modifications of the electrostatic field experienced by the adsorbed molecule.

The desorption of excited atoms by photon bombardment of alkali halide crystals

We report measurements of photon-stimulated desorption of excited atoms from LiF and NaCl crystal surfaces. Based on these measurements, we propose a new model of photon-initiated, defect-mediated desorption involving exoergic surface reactions between alkali dimers and halogens. This model consists of two steps: (a) reactants are formed at the surface, directly or by the migration to the surface of holes and defects created in the near surface bulk, due to uv photon bombardment, and (b) excited alkali atoms are emitted following a reaction between the alkali and halogen surface species.

The desorption of excited atoms by photon bombardment of alkali halide crystals

We report measurements of photon-stimulated desorption of excited atoms from LiF and NaCl crystal surfaces. Based on these measurements, we propose a new model of photon-initiated, defect-mediated desorption involving exoergic surface reactions between alkali dimers and halogens. This model consists of two steps: (a) reactants are formed at the surface, directly or by the migration to the surface of holes and defects created in the near surface bulk, due to uv photon bombardment, and (b) excited alkali atoms are emitted following a reaction between the alkali and halogen surface species.

Dynamics of surface-aligned photochemistry (theory). II. Localized H-atom scattering in the HBr(ad)/LiF(001)+hν system

The angular and energy distributions of 1.1 and 2.6 eV H-atom scattering from a LiF(001) surface are compared for two different sources of hot H-atoms (1) localized H-atoms produced by the photolysis of HBr molecules adsorbed on the LiF surface; and (2) H-atoms from a beam at the same energy and angle of approach to the LiF surface as in (1). These distributions are the results of classical stochastic trajectories carried out on a 7×7 slab of LiF(001). The calculated angular distributions are compared with experimental distributions [E. B. D. Bourdon et al., J. Chem. Phys. 95, 1361 (1991)]. The computed effects of changing H-atom energy and surface temperature are also reported; localized scattering becomes increasingly nonspecular at lower photon energies, and broader at higher surface temperatures.

Electrostatic energy in the adsorption of monatomics on MgO(001) and LiF(001)

The electrostatic polarization energy is evaluated for atomic hydrogen and helium in the very nonuniform fields near the LiF(001) and MgO(001) surfaces. The variational trial functions, which for helium are at the Hartree–Fock level, include configurations corresponding to many orders of multipole polarizability. The results for helium follow the trends expected from linear nonuniform response theory. The multipole series of second order perturbation theory for the polarization energy diverges for atomic hydrogen on these substrates; the calculations show comparable contributions from the first four multipole terms. The point–ion–substrate model has a charge-transfer pathology for H/MgO(001). The effective polarizability of the adatoms is calculated and the electrostatic contribution to the corrugation energy amplitude is estimated. Consequences for more polarizable inert gases are discussed.

Dynamical simulations of the photodissociation of CH3Br on a LiF (001) surface

The photodissociation dynamics of CH3Br adsorbed on LiF(001) is investigated using a classical molecular dynamics method. The adsorbate is modeled as a pseudodiatomic molecule and the LiF surface is represented by a cluster surrounded by static walls. All the particles involved are allowed to move in all three dimensions. It is found from Monte Carlo simulations that CH3Br binds to the LiF surface perpendicularly with a binding energy of approximately 0.3 eV. Both methyl up and Br up orientations of the adsorbate are found to be stable. The results from the dynamical simulations of the photodissociation process indicate that the experimental data for the CH3Br/LiF(001) photodissociation can only be explained on the basis of one dissociation channel open for the CH3Br photodissociation, in contrast to dual channels in the gas phase. When the methyl is up, its translational energy distribution resembles the corresponding gas phase result. In the case of methyl down and Br up, however, the dissociating methyl rebounds from the surface and collides with Br. With only a single collision, the kinetic energy of Br can be increased up to 1 eV, in agreement with experiment.

Low energy helium diffraction studies of CH3Br overlayers physisorbed on C(0001), NaCl(001), and LiF(001) surfaces

Low energy helium diffraction has been used to determine the unit mesh parameters of overlayers of CH3Br physisorbed on C(0001), NaCl(001), and LiF(001) at ≊35 K. CH3Br forms a uniaxially commensurate overlayer on C(0001) with unit mesh parameters 4.26 Å×6.75 Å. On NaCl(001), CH3Br forms a high coverage and a low coverage phase. The high coverage phase is incommensurate and has unit mesh parameters 4.54 Å×6.73 Å, whereas the low coverage phase is commensurate with a ((2)1/2×3(2)1/2)R45° unit mesh. The structure of CH3Br/LiF(001) is essentially the same as that of the high coverage phase of CH3Br/NaCl(001) with unit mesh parameters 4.52 Å×6.71 Å. The unit mesh parameters (with the exception of low coverage CH3Br/NaCl ) are very similar to the lattice parameters of the a-b [or (001)] plane of bulk crystalline CH3Br at ≊153 K. By analogy with the bulk crystal, it is likely that there are two molecules per unit mesh and that the CH3Br dipoles are nearly perpendicular to the surface and antiferroelectrically ordered. Our results suggest that the unit mesh for the low coverage phase of CH3Br/NaCl contains four molecules and that the molecular axes are parallel to the substrate surface. The implications of these results for photodissociation studies of physisorbed CH3Br are briefly discussed.

Simulation of photodissociation dynamics of HBr adsorbed on a LiF (001) surface

Computer simulations are performed to study the dynamics of the photodissociation of a single HBr adsorbed on a LiF (001) surface. A cluster model, including 50 moving atoms in two layers surrounded by five static walls consisting of 274 fixed atoms, is used to represent the surface. Since only one HBr molecule is allowed to sit on the surface, interactions between coadsorbates and possible collisions between adsorbed species are not considered in this model. The equilibrium properties of the HBr molecule on the surface prior to the photon absorption are obtained through Monte Carlo simulations, while the subsequent dissociation dynamics is described by molecular dynamics. Two dissociation channels corresponding to the ground and excited spin–orbit states of bromine are explicitly considered and transitions from one excited-state surface to another are treated with the Landau–Zener model. The kinetic energy and angular distributions of the dissociated H atoms as well as the Br/Br* ratio are determined from 600 trajectories. At 100 K and 193 nm, the calculated kinetic-energy distribution of the H fragment agrees with experimental data very well, as does the Br/Br* ratio. Neither the H kinetic energy nor the Br/Br* ratio is significantly affected by the presence of the surface. The energy exchange between the H atom and the surface is minimal, but increases when H is replaced by deuterium. The calculated H fragment angular distribution deviates from experimental data by approximately 15°. The deviation can be attributed to the scattering of the dissociating H fragment by a coadsorbate. We have also investigated the influence of the bulk temperature and the incident photon wavelength on the dynamics.

Classical trajectory simulations of photodissociation of CH3Br at surfaces

We have simulated the photodissociation of CH3Br adsorbed at a variety of surfaces. We have considered photodissociation at a smooth LiF (001) substrate and at three rough LiF surfaces which were constructed by removing atoms from the smooth surface. We have also considered photodissociation from several surfaces which have the same structure as the β phase of solid CH3Br to simulate dissociation from high coverages of the adsorbate where CH3Br ice is formed. The simulations were performed using the stochastic classical trajectory method. The asymptotic photofragment kinetic energy and angular distributions were determined and compared with the experimental results of Harrison et al. [J. Chem Phys. 89, 1475 (1988)] and Tabares et al. [J. Chem. Phys. 86, 738 (1987)]. When CH3Br is oriented with CH3 toward a surface, the CH3 kinetic energy distributions are shifted to much lower energies due to energy loss from multiple collisions with Br and the surface; the angular distributions are also significantly broadened. Much of the energy loss in these collisions goes into the translational mode of the Br fragments, causing the Br kinetic energy distributions to have a high-energy tail. When the molecule is in this orientation in a restricted geometry, collisions from the CH3 fragment lead to more effective energy transfer causing the peak of the Br kinetic energy distributions to be shifted to much higher energies and the corresponding angular distributions to become narrower. The main features of the experimental results from photodissociation of CH3Br adsorbed on LiF can be qualitatively explained using the results of the classical trajectory simulations.

Sputtering of LiF(100) with low energetic Ne+ and Ne2+ ions

Sputtering of a LiF(100) surface by singly and doubly charged Ne ions with impact energies between 10 and 500 eV has been performed. The emission of Li+, Li0, F+ F0 and LiF+ was measured by means of a quadrupole mass spectrometer. The yield of sputtered Li+ ions and Li0 atoms decreases with decreasing impact energy and is slightly higher for Ne2+ compared to Ne+ at impact energies below 100 eV. The situation changes very drastically regarding the emitted F particles. In the whole energy range investigated, the F+ and F0 yield was about one order of magnitude larger if Ne2+ projectiles were used instead of Ne+. According to the experimental data we propose the following model for the emission of F+ [1,2]: (1) The F ion of the solid surface is changed to F+ by Auger neutralization of the Ne2+ into Ne+ or by double resonance transition into doubly excited states of Ne0 with subsequent autoionization into Ne+. These processes take place several Ångströms in front of the surface. (2) The initial repulsive forces between the F+ ion and the surface are changed into attractive ones within a very short time by rearrangement of the alkali halide lattice [2]. (3) The neutralized projectile sputters the F+ as in a normal sputtering event on a surface with low surface binding energy, and F+ can he emitted as long as the kinetic energy of the projectile is sufficiently high.

IF(A,B) electronic excitation in F+I2F induced by the surface reaction F(ad)+I2(ad)

This paper reports on a dynamical study of the reaction of an effusive F atom beam with iodine molecules condensed on a temperature-controlled LiF(001) surface. The observation of IF ground state products by laser-induced fluorescence and of IF(A,B) excited species by visible chemiluminescence can be explained by means of a reaction scheme consisting of two channels. In the first channel IF (X 1Σ+) ground state molecules are formed on the surface and desorb into the gas phase above a threshold temperature of about 200 K, where they are probed by laser-induced fluorescence. The vibrational population ratio [N′(v=0)/N′(v=1)] shows equilibrium of the desorbing molecules with the surface. The population distribution of the IF(v=0) rotational manifold can be well-characterized by a rotational temperature which follows the surface temperature. Surface equilibration of products implies that, unlike in the gas phase reaction F+I2, the products have no memory of their exoergic formation, indicating a Langmuir–Hinshelwood mechanism. In a second reaction channel I2F, as the intermediate in the reaction of fluorine atoms with adsorbed iodine molecules, stabilizes at the surface and desorbs into the gas phase above a surface temperature of 300 K. There it reacts with F atoms according to the chemiluminescence process F+I2F→IF*+IF. This gas phase reaction is identified by the exoergicity limit of the IF (B 3Π) state vibrational population distribution, obtained from analysis of the B→X chemiluminescence. In comparison with Kahler and Lee’s molecular beam synthesis of I2F from I2 and F2 with subsequent secondary encounter with the product F atom, the above reaction scheme represents an alternative induction of the F+I2F reaction. It can explain visible IF chemiluminescence from fluorine–iodine systems under experimental conditions in which F atoms are present and I2 wall adsorption is possible. In this work, evidence is given that I2F is a common precursor in the two reaction channels. The overall branching ratio strongly favors the formation of IF ground state molecules in these experiments.

Electron micrography and x-ray study of dip-lacquered LiF (220)

It has been proposed to use the 220 reflection of LiF with a multilayer deposited upon the top for simultaneous spectroscopy near Fe-k and O-k and below the C-k absorption edge (284 eV) in x-ray astronomy. We demonstrate that a substantial reduction of surface roughness is obtained by dip lacquering state-of-the-art polished LiF(220) surfaces. Using a microdensitometer analysis of electron micrographs of surface replicas and x-ray reflection, we have measured approximately 10-A rms roughness of Au-coated dip-lacquered LiF(220) crystals, as opposed to approximately 60 A measured on the bare LiF(220) crystal surface.

Preparation of lithium tetrafluoroborate of high purity

The reaction of LiBF 4 formation in the system LiF-BF 3 was studied. It was found out that in the temperature range T 1-T 2 the final product LiBF 4 forms on the surface of LiF the solid film, through which the reacting gas BF 3 should diffuse. The quantity of the material, reacting per unit of free bed surface, will be described by the equation ▪ where Ω is a porosity of LiF, B is a constant proportional to the product of a diffusion coefficient and a constant of the reaction rate, h means LiF pores diameter, n is reaction order. The influence of interaction temperature pressure, time, bed depth of LiBF 4, structure of the porous surface were examined, T. and T 2 were found out. Using the optimal conditions we obtained LiBF 4 with the degree of purity not less than 99,9 (wt) %.

Desorption kinetics of Li atoms from LiF under electron bombardment

Measurements on electron stimulated desorption (ESD) of Li atoms in the electronical ground state (Li 0) from air-cleaved LiF(100) surfaces have been performed. The electron energy was varied between 50 and 450 eV. After electron bombardment was stopped, a prompt decrease of the Li 0 signal followed by a delayed emission of Li 0 was observed. In the temperature range from 620 to 720 K the delayed emission of Li 0 shows a behavior, which could not be explained by diffusion of F centers to the surface only. Delayed emission of Li 0 up to minutes was observed and we therefore suggest, that energy must be stored in the bulk through the formation of F center aggregates in order to explain the experimentally observed behavior of the delayed emission. A system of rate equations has been developed, which describes the kinetics of formation and decay of F center aggregates.

Electronic effects in low-energy ion sputtering of LiF

Sputtering of a LiF(100) surface by singly (He and Ar) and doubly charged rare gas ions (Ar) with impact energies between 20 and 500 eV has been performed. The emission of Li +, Li 0, F + and F 0 was measured by means of a quadrupole mass spectrometer.The yield of sputtered Li + ions and Li 0 atoms decreases with decreasing impact energy and becomes slightly higher for Ar ++ compared to Ar + at impact energies below 100 eV. The situation changes very drastically when looking at the emitted F particles. In the whole energy range investigated, the F + and F 0 yield was more than one magnitude larger for Ar ++ as projectiles than for Ar +. Furthermore, for He + bombardment a higher yield was observed than for Ar +. Therefore an electronic effect has to be assumed which causes enhancement of the sputtering of F + if projectiles with larger potential energy are used. However, for He + below 100 eV impact energy, a drastic decrease for the F + yield was observe-which indicates that F + emission cannot be due to a pure electronic effect. Our proposed model includes formation of F + by Auger neutralization (if energetically possible) of projectiles in front of the target, and emission of the F + ion due to momentum transfer in the collision with the already neutralized projectile.