Tuning the optical properties of phosphorene by adsorption of alkali metals and halogens

Optical properties of phosphorene are tuned by adsorption of alkali metals (Li and Na) and halogens (Br and Cl). It has been found that on increasing the size of alkali metals and halogen adsorbed phosphorene layer the absorption coefficient reduces and shifts towards visible region. The refractive index in alkali metal adsorbed phosphorene increases with size of phosphorene layer. For halogen adsorbed structure it decreases with increase in size of phosphorene layer. Optical absorption is observed to depend on both dielectric constant and refractive index. Since adsorption of alkali and halogen materials modifies the refractive index of phosphorene, absorption is seen to reduce in all cases where refractive index increases due to adsorption even when the dielectric constant was high.     

Delay times in the sputtering of atoms from alkali-halide crystals during low-energy electron bombardment

Delay times in the sputtering of atoms from RbI, RbBr and NaCl by 540 eV electrons were investigated. This was done by simultaneously using the correlation technique and a slotted disc velocity selector. We observed delays in thermally ejected alkali and halogen atoms. These times were found to be independent of target temperature and are ascribed to lifetimes of excitons. Non-thermal halogen atoms are always sputtered without any delay.     

On the mechanism of carbon nanotube formation in electrochemical processes

Quantum-chemical methods are used to analyze the mechanism of carbon nanotube formation in the electrochemical bath, where tiny fragments of graphene planes are in the environment of atoms and ions of alkali metals and halogens. In the optimal configuration, alkali metal atoms move toward the edge of a graphene fragment, whereas halogen atoms remain at the sites of their initial attachment. When the graphene fragments “burdened” by alkali metal and halogen atoms interact with each other, the overall graphene configuration twists in a natural way into a nanotube-like open-end structure.     

Semiempirical pseudopotential surfaces for chemical reactions: The (AB)+ X - dialkali halide systems

A generalization of the Roach-Child semiempirical pseudopotential calculation for K + NaCl to several analogous dialkali halide systems has been used to elucidate the chemical interactions governing the reaction dynamics. The Li + LiF ground-state potential surface, which exhibits a ～ 20 kcal/mole basin for isosceles Li₂F, is qualitatively similar to one obtained in a recent configurational interaction calculation. It is shown that regions of the Na₂Cl ground-state surface corresponding to Na₂ ⁺ interacting with Cl^- can be described in terms of an ion-pair Rittner potential model similar to that employed for the alkali halides. Chemical trends in the triangular complex well depths satisfactorily account for the experimentally observed transition between the collision complex mechanism (Rb + KCl) and the osculating complex model (Li + KBr) for the alkali-alkali halide exchange reactions at thermal energies. For collinear configurations with the alkalis on opposite ends, avoided intersections between the lowest two potential surfaces are characterized in terms of diabatic surfaces computed from truncated basis sets. Crossings of these surfaces account for the vibrational-electronic energy transfer between alkali atoms and vibrationally excited alkali halides. The ionic X ^- + A ₂ ⁺ potential surfaces are used to predict the product electronic excitation and partitioning of exoergicity in reactions of halogen atoms with alkali dimer molecules.     

Thermally stimulated depolarization of Eu2+-cation vacancy dipoles in alkali halide single crystals

Ionic thermocurrent (ITC) measurements have been performed on eight alkali halide single crystals doped with divalent europium. In all cases, the observed ITC peaks were fitted with a mono-energetic model without to appeal to any dipole-dipole interaction. Values for the reorientation parameters have been calculated. The relationship T_M1nτ^?1 α E previously found for I–V complexes in alkali halides has been found to be very well obeyed for the experimental data obtained in this investigation. It is also reported that the logarithm of the experimentally determined energies for free dipole reorientation shows a linear dependence on the interaction distance between the Eu²⁺ ion and the surrounding halogen ions in the distorted cubic site occupied by this impurity in the alkali halides.     

Observation of negative alkali ions from alkali halides during 248-nm laser irradiation

Below laser fluences where a plasma is formed (the so-called plasma or plume formation threshold) a number of fundamental phenomena can occur where particles such as atomic and molecular ions, atoms and molecular neutrals, and electrons can be emitted. An understanding of such processes is necessary to develop predictive models for material removal from laser irradiated surfaces—at the foundation of laser etching, machining, and pulsed laser deposition. We have reported on a number of the mechanisms for such emission processes. Here, due to space limitations, we present a summary of our studies on the formation of negative alkali ions from single crystal KCl during exposure to pulsed 248-nm radiation at fluences well below the threshold for plasma formation. Despite the high electron affinities of the corresponding halogen atoms, negative halogen ions were not detected. Significantly, the positive and negative alkali ion distributions overlap strongly in time and space, consistent with K formation by the sequential attachment of two electrons to K⁺. Negative alkali ions are also observed under comparable conditions from LiF, NaCl, and KBr. In each material, the strong overlap between the positive and negative alkali ion distributions, and the lack of detected negative halogen ions, suggest that negative ion formation involves a similar mechanism.     

目标自辐射与干扰目标反射光谱的氧气吸收特性分析

为分析不同目标反射太阳光谱的特性,研究被动测距中抑制干扰目标的方法,利用高光谱成像光谱仪作为测量设备,200 W卤钨灯作为模拟探测目标,玻璃板、光滑铝板、塑料板作为干扰目标,对模拟目标发射光谱和干扰目标反射太阳光谱进行了实验采集与分析。实验采集了晴朗天气条件下干扰目标反射光谱及卤钨灯目标辐射光谱的信息,并对其氧气吸收率进行了计算,综合分析了镜面反射干扰目标、漫反射目标及背景反射光与卤钨灯发射光光谱的氧气吸收率差异,结果表明:在455m处,镜面反射目标和漫反射目标反射光的氧气吸收率是卤钨灯的2~3倍。因此,在一定距离下可以利用氧气吸收率的差异设置阈值来对目标进行判别,提高探测概率。     

Electron stimulated desorption of neutral species from (100) KCl surfaces

Composition changes of a (100) KCl surface bombarded by 1 keV electrons have been studied by Auger electron spectroscopy. Intensity ratios of characteristic alkali and halogen Auger lines were monitored as a function of target temperature and beam current density. In addition, for the first time angle-resolved energy distributions of electron desorbed K and Cl atoms were measured using mass-analyzed time of flight techniques. For temperatures higher than about 100°C, a near-stoichiometric surface composition was obtained and a significant non-thermal component was observed in the kinetic energy distributions of Cl atoms emitted normal to the (100) surface. These results can be interpreted in terms of new concepts regarding the excitonic mechanism of electron stimulated desorption (ESD).     

内嵌复合物M@C60（M=Li,Na,K,Rb,Cs）的分子力学研究

利用分子力学方法计算了碱金属内嵌复合物Ｍ＠Ｃ６０中Ｍ与Ｃ６０之间的相互作用，考察了Ｍ在Ｃ６０笼内的平衡位置。研究表明Ｌｉ和Ｎａ的平衡位置偏离Ｃ６０分子的中心，Ｋ、Ｒｂ和ＣＳ的平衡位置在Ｃ６０分子的中心。平衡位置的确定取决于色散作用和排斥作用的大小。最后，讨论了碱金属原子进入笼内的可能机制。     

Donor Properties of Ligands and the Structure and Spectra of Uranyl Halogenides in the Solid State

Complexes of uranyl halogenides with neutral organic ligands in the solid state have been obtained, the luminescence spectra at T = 77 K have been recorded, and the dependences of them on the composition of the complex have been investigated. The linear dependence of the change in the frequencies of electron and vibronic transitions on the donor number of the neutral ligands has been revealed, thus allowing us to propose a spectral method for determining the structure of the system studied. It is shown that the increased strength of the bond of some halogen ligands (in particular, fluorine) with uranyl favors the nonsaturation of their inner coordination sphere. At the same time, the most stable complexes of the substances considered are formed when two halogen ions are supplemented with three molecules of organic ligand.     

BeXY、MgXY（X、Y=F、Cl、Br）与ClF3和ClOF3形成复合物的理论研究

与卤素原子相比,超卤素表现出电负性更大、结构更丰富、性质更新颖等更加丰富的特征。本文以碱土金属Be、Mg的卤化物BeXY、MgXY（X、Y=F、Cl、Br）为母体,分别与卤素氟化物ClF3、ClOF3形成的复合物为研究对象,设计了12种配合物。采用密度泛函理论研究了这些化合物的结构、垂直电离能(VDE)、绝热电离能(ADE)等性质。研究发现,配合物中存在超卤素阴离子,所表现出来的性质远远优于常见的超卤素阴离子,因此,此类化合物具有很好的应用前景。     

Effect of X-irradiation on impurity-vacancy dipoles in lead and calcium-doped NaCl

In the present paper the effects of the room temperature X-irradiation in calcium and lead-doped NaCl have been reinvestigated in order to get a deeper insight into the nature of the radiation induced halogen interstitial traps. Special attention has been paid to determine whether a correlation exists between the change in the concentration of Ca²⁺ (Pb²⁺-cation vacancy dipoles and the color center formation. Our results indicate that for both systems, the dipoles are the fundamental traps for the interstitial defects in agreement with the theoretical models recently developed for F-center formation in alkali halides doped with divalent impurities. This conclusion is, however, in disagreement with that reported by Ikeya et al in a previous work on calcium-doped NaCl in which it was concluded that the isolated positive ion vacancies were the dominant traps for the halogen interstitial defects. Some comments about this discrepancy are given.     

On the compressed-ion model of cohesion in alkali halides

First-principle local-density calculations of the total energy of ions confined within a sphere were performed The results for positive alkali and negative halogen ions are used to test the validity of the compressed-ion model for the cohesion of alkali halides recently proposed by Narayan and Ramaseshan It was found that the model is not realistic since the calculated ionic compression energies are by far too large to account for the observed equilibrium lattice constants and bulk moduli of alkali halides.     

A new theory of compressible ions — structures of the alkali halides

Ions in ionic crystals are considered to exist in compressible space-filling polyhedral cells analogous to the Wigner-Seitz cell in metals. Repulsion arises from the compression energy of the ions written as a surface integral over the ionic cells. Two adjustable parameters are introduced per ion with the provision that the same parameters can be used in any crystal of any structure in which the ion occurs. The 18 parameters for the 5 alkali and 4 halogen ions have been determined from PV data on the 20 alkali halides. The important successes of the theory are: (i) All the twenty alkali halides are correctly predicted to occur in their observed structures (ii) The thermal transition in CsCl is explained (iii) The pressure transitions in the alkali halides are predicted well (iv) The calculated values of the variation of transition pressures with temperature agree well with experiment. These results are much better than those obtained by earlier theories.     

Charge-Transfer Processes in Doped Alkali Halides

Defects formation under UV-irradiation in the impurity-induced absorption bands at 4.2 K has been studied for crystals with and without traps for electrons (CsI:Pb and Eu^2?-doped alkali halides, respectively). In both cases the results have been explained by an electron transfer from the impurity-perturbed halogen ion states, resulting in the appearance of electrons and holes in the crystal. In CsI:Pb, the electrons are trapped by lead ions and the holes are self-trapped. In Eu^2?-doped crystals, the electrons and the holes recombine with the formation of excitons, whose decay results in the creation of Frenkel defects.     

Effect of substitution and cooperativity on the Cl–F blue shift in single-electron halogen-bonded H3C ··· ClF complex

The effect of substitution and cooperativity on the blue shift of Cl–F stretch vibration in H₃C ··· ClF complex has been studied with quantum chemical calculations at the UMP2(Full)/aug-cc-pVTZ level. The electron-withdrawing group (F atom) in the electron donor decreases the blue shift, whereas the electron-donating group (methyl group) in the electron donor cause it to increase. The cooperativity between two different types of halogen bonds in H₃C ··· ClF ··· ClF complex enhances the strength of single-electron halogen bond and the blue shift. The natural bond orbital (NBO) and atoms in molecules (AIM) analyses have been performed for the halogen-bonded complexes.     

Off-center substitutional ions in alkali-halide crystals

The potential wells of alkali and halogen ions as impurities in alkali halides are investigated by means of a deformation-dipole model with single-ion parameters. The calculations indicate that Li⁺ is off center in seven and F^? in six of the sixteen rocksalt-type crystals considered. In about half of the off-center systems the configuration has D_4h symmetry if the impurity is kept at its lattice position. The minimum-energy configuration of the off-center system KCl:Li⁺ is calculated in detail, and the influence of hydrostatic pressure is investigated. Our results are in qualitative agreement with experiment.     

Computational study of the cooperative effects between tetrel bond and halogen bond in XCN⋅⋅⋅F2CO⋅⋅⋅YCN complexes (X = H,F, Cl,Br; Y = F,Cl, Br)

ABSTRACT

Ab initio calculations have been accomplished to study the cooperativity between the halogen bond and tetrel bond in the XCN???F2CO???YCN (X = H, F, Cl, Br; Y = F, Cl, Br) complexes. F₂CO at the same time plays the role of Lewis acid with the π-hole on the C atom and Lewis base with the O atom to participate in the tetrel bond and in halogen bond, respectively. According to the geometry survey, the effect of a tetrel bond on a halogen bond is more pronounced than that of a halogen bond on a tetrel bond and the intermolecular distances in the triads are always smaller than the corresponding values in the dyads. In all cases, the halogen bond and tetrel bond in the termolecular complexes are stronger compared with those in the bimolecular complexes. So, from the intermolecular distances, interaction energies and many-body interactions demonstrate that there is positive cooperativity between the halogen bond and tetrel bond. The molecular electrostatic potential, atoms in molecules and natural bond orbital methodologies are used to analyse the nature of interactions of the complexes.     

Acoustodesorption of alkali metals and halogens from single-layer graphene: Simple estimates

Physics of the Solid State - An increase in the thermodesorption probability under deformation wave, i.e., acoustodesorption, was estimated for alkali metal and halogen atom adsorption on...     

Comparison of hydrogen and halogen bonds between dimethyl sulfoxide and hypohalous acid: competition and cooperativity

A theoretical study of the complexes formed between dimethyl sulfoxide (DMSO) and hypohalous acid (HOX, X = Cl, Br, and I) has been carried out at the MP2/aug-cc-pVTZ level. For each HOX, four minima binary complexes were found, two mainly with an OH???O hydrogen bond and the other two with an OX???O halogen bond. The hydrogen-bonded complexes are more stable than the halogen-bonded analogues for HOCl and HOBr, while both types of complexes have similar stability in the iodine case. A red shift was found for the associated H–O and X–O bond stretch vibrations and a small blue shift for the distant bonds. As the oxygen of DMSO simultaneously binds with two HOCl molecules, the corresponding interactions are weakened with diminutive effect. This diminutive effect is the largest in the complexes with two OH???O hydrogen bonds but the smallest in those with two OCl???O halogen bonds.