Laser control of desorption through selective surface excitation

We review recent developments in controlling photoinduced desorption processes of alkali halides. We focus primarily on hyperthermal desorption of halogen atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. We then extend the exciton mechanism of desorption, developed for alkali halides, to metal oxide surfaces, in particular magnesium oxide. In addition, these results demonstrate that laser desorption can serve as a solid-state source of halogen and oxygen atoms, in well-defined electronic and velocity states, for studying chemical processes in the gas phase and at surfaces.     

Alkali-halide association and dissociation rates in an ambient gas

Calculations are carried out on the rate coefficients for the association of alkali and halogen atoms in an ambient gas. Because of curve crossing the rate coefficients can be quite high. Curve crossing will facilitate other association processes. Reciprocal lifetimes of alkali halides towards collisional dissociation into atoms and into ions are also evaluated.     

Interpolation determination of the lattice energy of ionic crystals within the framework of stereoatomic model

A new method based on graph theory was suggested for interpolation calculation of the lattice energy U of ionic crystals. The method is based on revealing matrix correlation between the ionic radii and U values for MX compounds, where M is a metal and X is halogen, hydrogen, or chalcogen. A new formula was obtained for calculating the lattice energy solely from the ionic radii, without introduction of abitrary factors. The mean error of determining U for alkali metal halides is 0.49%. The lattice energies were calculated for a large group of inorganic substances. The accuracy of the interpolation calculation of the lattice energy of ionic crystals depends on the degree of ionicity of the bond: With an increase in the covalent contribution, the error increases.     

Relation between alkali ion affinity and ionic radius ratio by means of fast atom bombardment tandem mass spectrometry

Cationization of organic molecules has been studied using fast atom bombardment mass spectrometry and tandem mass spectrometric methods. The order of the A⁺ affinity to G (G ? H + A) and AX was determined by this tandem mass spectrometric approach, where G, A and X represent glycerol, alkali metal element and halogen element, respectively. The affinity orders of most alkali halides are divided into three classes. By analogy with the ionic radius ratio rule in solid ionic crystals, this particular parameter, ionic radius ratio, was introduced to analyse the experimental results. This classification is thought to be caused by the ionic radius ratio (X^?/A⁺). These ratios of the I, II and III classes exist between 0.657 and 1.026, 1.096 and 1.776, and 1.856 and 2.289, respectively. The ionic radius ratio plays an important role for determination of the order of alkali ion affinity.     

Laser control of product electronic state: desorption from alkali halides

We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin-orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl(2P(1/2)), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.     

Semiempirical approach to the problem of the radiative lifetime of the excited F-color center

It is emphasized that for the theoretical consideration of many problems of defect centers in crystals (especially of problems which need the correct asymptotic behavior of the wave functions) the semiempirical approach can be effective. As an example the spontaneous radiative time decay of the excited F-center in alkali halides is calculated by using the experimental energies of absorption and emission bands.     

Effect of crystal potential on dynamic polarizability of negative ions

Summary We combine a time-dependent approach with a crystal potential model to study environment-specific optical linear response of closed-shell ions within crystals under the influence of an external time-varying field. It is shown how the dynamic dipole polarizability of free halogen anions within the normal dispersion region is altered due to the crystal potentials felt by the anions in environments appropriate for different binary cubic ionic lattices. The pole-positions of the in-crystal anion polarizability are found to be consistent with the vacuum ultraviolet absorption edges of the corresponding alkali halides and to vary linearly with the lattice potentials at the anion sites in these salts. It is observed that the crystal potential induces quite a large enhancement in the anionic absorption oscillator strengths of these dipole transitions, thereby making these values conform well with those of the first excitonic absorptions in such crystals.     

Nano-scale modification of ionic surfaces induced by electronic transitions

This paper presents a review of recent experimental and theoretical work on surface modification of alkali halides due to electronic excitations caused by electron and photon irradiation. In particular, several examples of free exciton and hot electron–hole pair formation in ionic materials are given. It is demonstrated that evolution of these primary excitations with subsequent defect formation and diffusion from the bulk to the surface, leads to dynamic surface modification and sputtering, often periodically varying with the irradiation dose. In turn, modification of the surface topography could affect and modulate periodically, the diffusion processes driving the defects from the bulk of the material towards its surface. Spectacular examples of such oscillatory yield dependencies, and corresponding surface nano-structure formation and evolution, are shown for electron- and photon-irradiated NaCl, KBr and KCl(0 0 1) surfaces of bulk crystals and thin epitaxial films. Important applications of these findings for quantitative characterisation of insulator surfaces and mass spectrometry are presented.     

Metal-halide molecular clusters: from fundamentals to model interactions

Computer simulation studies of molten salts and disordered ionic solids employing phenomenological ionic models have proved useful in helping to understand the structure of these systems and their structure-dependent properties. A minimal requirement on such models is that they should give a reasonable account of cohesion in crystals and in molecules and small clusters of these compounds, and in turn the analysis of cohesive and vibrational properties of ionic systems in solid and molecular states has helped to determine useful model interactions. For an introduction to these topics the author first reports from the work carried out in the 1980s with W. Andreoni and G. Galli on what is learnt in Hartree-Fock and configuration–interaction calculations on the neutral and ionized monomer and dimer of NaCl. Then the author recalls how a deformation-dipole model was built for the cohesion of the neutral alkali halide monomers by combining a classical multipolar expansion with a quantum overlap expansion, and how this model relates to the theory of the cohesion and the vibrational spectrum of the alkali halide crystals. Finally, the author illustrates some applications to molecules and microclusters of polyvalent metal halides, from work carried out with Z. Akdeniz and coworkers. Transferability of model parameters for the halogen ions between different compounds and different aggregation states is crucial for these applications, and is achieved in the deformation-dipole model.     

A halogen-bonding-based heteroditopic receptor for alkali metal halides

A new heteroditopic receptor for alkali metal halides has been designed and synthesized. It is comprised of a well-established motif for cation binding and a motif for halogen-bonding-based anion recognition processes. The single-crystal X-ray structure of the complex between the heteroditopic receptor and sodium iodide is reported. Thanks to the cooperativity of metal coordination and the strong I-...I halogen bonding, the ion pair is fully separated. The boosting effect of the binding of the anion through halogen bonding on the coordination of the cation by the receptor has been proved also in solution by NMR experiments. The selectivity of the new heterotopic receptor toward different alkali metal halides has been tested by ESI mass experiments.     

Radiation resistance of ionic and ionic-molecular crystals

The factors that determine the stability of ionic and ionic-molecular crystals in ionizing radiation fields are analyzed. The concepts of the fundamental reactivity (stability) of a solid (the ability of the ideal crystal lattice of the material, in which only intrinsic point defects—interstitial ions and vacancies—are present, to undergo radiation-chemical degradation) and the structure-sensitive reactivity determined by the presence of structural and extrinsic defects are introduced. Data on the radiation resistance of crystalline solids (alkali halides, silver halides, and silver azide) are summarized. It is pointed out that the AgHal crystalline matrix should be resistant toward radiation and alkali halide and AgN₃ crystals should not.     

Mew method for estimating polarity of chemical bonds

For alkali-metal and halogen atoms, a relationship has been established between their electronegativity (EN) and the charge on the atoms. Bond polarities have been determined in molecules and crystals of alkali-metal halides by the method of equalization of ENs of the bond partners. The reasons for differing dimensional behavior of metal and nonmetal atoms upon change in coordination are discussed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2701–2704, December, 1989.     

Highly charged ion induced nanostructures at surfaces by strong electronic excitations

Nanostructure formation by single slow highly charged ion impacts can be associated with high density of electronic excitations at the impact points of the ions. Experimental results show that depending on the target material these electronic excitations may lead to very large desorption yields in the order of a few 1000 atoms per ion or the formation of nanohillocks at the impact site. Even in ultra-thin insulating membranes the formation of nanometer sized pores is observed after ion impact. In this paper, we show recent results on nanostructure formation by highly charged ions and compare them to structures and defects observed after intense electron and light ion irradiation of ionic crystals and graphene. Additional data on energy loss, charge exchange and secondary electron emission of highly charged ions clearly show that the ion charge dominates the defect formation at the surface.     

A new method of calculation of the melting temperatures of crystals of Group 1A metal halides and francium metal

A new method of calculation of melting temperatures of binary ionic crystals has been suggested. The method is based on finding a matrix relation between the ionic radii (the lattice energy U) and melting temperature of ionic crystals of the MX type, where M is a Group 1A metal, and X is a halogen. From the equation for the lattice energy U, a new equation has been derived for calculation of the melting temperature of ionic crystals with the use of only the ionic radii and the degree of bond ionicity ?: T _m = f(U, ?). The average error of determination of T _m for alkali-metal halides is 2.80%. The melting temperatures of francium halides and alkali-metal astatides (including FrAt) have been calculated. It has been shown that the accuracy of calculation of the melting temperature of ionic crystals depends on the degree of bond ionicity: the error increases with an increase in the covalent contribution. On the basis of the melting temperatures of metal halide crystals, a method has been developed for the calculation of the melting temperatures of corresponding metals. The melting temperature of francium has been calculated to be 24.861 ± 0.517°C.     

Evidence for the direct ejection of clusters from alkali-halides during laser vaporization

We have studied the formation of clusters of alkali halides during laser vaporization. Measurements of the abundances of cluster ions produced by several different source configurations indicate that clusters are ejected directly from the source sample and do not necessarily grow from an atomic or molecular vapor. Using samples consisting of mixed alkali halide powders, we have found that unalloyed clusters are easily produced in a source that prevents growth from occurring after the clusters leave the sample surface. Melting the sample or encouraging growth after vaporization lead to the production of alloyed cluster species. We also obtain information about the relative binding energies for substituted halogen ions bound to alkali halide clusters.     

Application of symmetry-adapted perturbation theory to small ionic systems

The application of symmetry-adapted perturbation theory (SAPT) to small ionic systems was investigated in the context of the accuracy of calculated interaction energies for alkali halides. Two forms of alkali halides were considered: ion pairs M(+)X(-) (M = Li, Na, K, Rb, and X = F, Cl, Br, I) and dimers (MX)(2). The influence of the order of energy correction terms included in SAPT and the effect of the so-called hybrid approach to SAPT on the accuracy of the calculated energies (such as the interaction energies in the ion pairs and the binding energies in the dimers with respect to two free monomers) were studied. The effects of the size of basis sets, combined with SAPT, on the accuracy were also established.     

Immobilization of HX：[Hmim]X as Halogenating Agent, Recyclable Catalyst and Medium for Conversion of Alcohols to Alkyl Halides

HX is immobilized by reaction of halogen acid with methylimidazole, and the formed ionic liquid [Hmim]X was used as halogenating agent, catalyst as well as medium for conversion of alcohols to alkyl halides. Excellent yields were obtained. The halides produced could be easily separated from the reaction mixture via simple decantation, and the ionic liquid [Hmim]X could be regenerated conveniently by adding equivalent of halogen acids followed by removal of water.     

Thermal desorption of quasimolecular ions

The thermal desorption of [M + Alkali]⁺ quasimolecular ions from a heated metal surface is reported for some alkali salts of carboxylic acids and mixtures of alkali halides with a crown ether, glucose and adenosine. No quasimolecular ion could be detected from sucrose. With benzo[15]crown-5 the desorption of [M + Na]⁺ ions takes place even below the threshold temperature for thermionic emission of alkali ions. In addition, the desorption of intact [B(C₆H₅)₄]^? ions from a layer of NaB(C₆H₅)₄ is reported.     

Impurity-vacancy interactions in the alkali halides

The interaction energies of vacancies with divalent impurities are calculated using recently developed efficient computer simulation methods. The results show that nearest neighbour and next nearest neighbour complexes are roughly equally bound, thus emphasising the importance of including both types of defect pair in analyses of spin resonance and other experimental data on doped alkali halides. We also find that a simple Coulomb expression for defect interactions gives energies close to those obtained by the full calculation, for all but the nearest neighbour complex. This result encourages the use of Debye-Hückel treatments of defect activities in alkali halide crystals.     

Cluster-model study of the interaction of halogen atoms with Ag clusters

Analysis of ab initio wave functions shows that the interaction between halogen atoms (F, Cl, and Br) and Ag clusters is ionic, and that the halogen ionicity is essentially ?1. The interaction and the bonding arise, almost entirely, from the Coulomb attraction between the charged halogen and the metal and from polarization of the two subunits. Large shifts in equilibrium bond distances and vibrational energies are caused by an external electric field. These changes arise from a dominant Stark effect.