Reconstruction dependence of the etching and passivation of the GaAs(001) surface

The microscopic nature of the selective interaction of iodine with an As- and Ga-stabilized GaAs(001) surface has been investigated by the photoelectron emission and ab initio calculations. The adsorption of iodine on the Ga-stabilized (4 × 2)/c(8 × 2) surface leads to the formation of the prevailing chemical bond with gallium atoms; to a significant redistribution of the electron density between the surface Ga and As atoms; and, as a result, to a decrease in their binding energy. Iodine on the As-stabilized (2 × 4)/c(2 × 8) surface forms a bond predominantly with surface arsenic atoms. Such a selective interaction of iodine with the reconstructed surfaces gives rise to the etching of the Ga-stabilized surface and the passivation of the As-stabilized surface; this explains the layer-by-layer (“digital”) etching of GaAs(001) controlled by the reconstruction transitions on this surface.     

Chemical oxygen-iodine laser with external production of iodine atoms in CH3I/Ar dc glow discharge

A cw chemical oxygen-iodine laser (COIL) operating in a subsonic mode with a high water content of ～15% and external production of iodine atoms in CH₃I/Ar dc glow discharge has been demonstrated. A straightforward comparison of COIL performance for two cases—conventional, when I₂ was injected in the singlet oxygen flow, and when iodine atoms produced externally together with other discharge products were injected—was made. In the latter case nearly four times increase in output power was observed, suggesting that the relaxation of the energy stored in the singlet oxygen slowed down substantially, when the laser operated using CH₃I/Ar discharge products instead of iodine molecules.     

X-ray standing wave study of iodine on Ge(111)

X-ray standing wave studies for an iodine monolayer adsorbed on a (2 × 8) reconstructed Ge(111) surface indicate that the germanium surface atoms revert back to bulk-like positions with the iodine atoms saturating the dangling bonds and occupying the atop site positions.     

Reversible superstructural transitions on the GaAs(001) surface under the selective effect of iodine and cesium

The selective interaction of the iodine and cesium atoms with the GaAs(001) surface, which leads to a decrease in the bond energy of the Ga and As surface atoms, respectively, owing to the redistribution of the electron density in the near-surface region under the effect of electronegative and electropositive adsorbates, has been experimentally investigated. This selective interaction makes it possible to remove alternately the Ga and As monolayers in the iodine and cesium adsorption followed by heating at T ≤ 450°C and, thus, to implement reversible low-temperature transitions between the Ga-and As-stabilized superstructures, as well as the atomic layer etching of the semiconductor with the physically ultimate monolayer accuracy.     

On the determination of the stored optical energy of an atomic iodine laser amplifier from the energy measured in the free running mode

An exact determination of the stored optical energy of an amplifier from the energy measured in the free running mode has been difficult for the iodine laser because of the influence of the recombination of ground state iodine atoms with radicals to the parent molecule on the population of the lower laser level. A formula is derived relating the stored optical energy to the one measured in the free running mode with due consideration of this effect.     

Simulations of iodine adsorbed Ge(0 0 1) surface and its STM images

A layer of iodine at Ge(0 0 1) surface develops an ordered structure of iodine atoms bound to Ge dimers. Here are discussed atomic structures of Ge(0 0 1) surface covered by 0.25 monolayer of iodine. The p(2×4), p(2×2), c(2×4) and p(1×4) surface structures are found in calculations. The structure with two iodine atoms of the dissociated I₂ molecule adsorbed at both ends of the same germanium dimer is found to be energetically favourable over iodine adsorption at neighbouring dimers. Simulated STM images of the obtained surface structures are presented and compared with experimental data.     

碘原子共振多光子电离光谱研究

用可调谐的染料激光(4600～5000(?)),共振(3+2)和(4+1)多光子电离探测了I(5p~2P_(3/2)~0)和1~#(5p~2P_(1/2)~0).碘原子是由碘分子(I_2)经激光解离而产生的.在上述激光波段中共观察到12个原子跃迁,其中,(4+1)多光子电离跃迁是首次观察到的.     

Kinetics of I2 and HI photodissociation with implications in flame diagnostics

This paper investigates the application of iodine (I₂) tracer in reactive flows to image the concentration filed by using photodissociation spectroscopy (PDS). The PDS technique first uses a short laser pulse to completely photodissociate iodine-containing species into iodine atoms, and then uses a second laser pulse to image the iodine atoms. Due to the completeness and rapidity of the photodissociation (PD), the concentration of I atoms after PD represents that of the iodine element seeded into the flow, which forms a conserved scalar and can be used to image the concentration field. The feasibility of the PDS technique is evaluated in three steps. First, a multi-level kinetic model was developed to capture the major physics relevant to the PDS technique. Second, controlled experiments were conducted to validate the multi-level kinetic model. Third, the validated model was applied to evaluate the applicable range and accuracy of the PDS technique in representative hydrocarbon flames. The results suggest a simple approach to implement the PDS technique using I₂ seeding in flames, which can provide good accuracy across a wide range of mixture fractions and strain rates.     

GGA+U study of the incorporation of iodine in uranium dioxide

Ab initio calculations based on the Density Functional Theory are carried out in order to investigate the incorporation of iodine in uranium dioxide. The GGA+U approximation is used to describe the strong correlations of uranium 5f electrons. We studied several defects that are likely to accommodate the incorporation of iodine in the material, such as uranium and oxygen vacancies, divacancy and Schottky defects. We find the iodine atoms to be stable in a neutral Schottky defects, with an incorporation energy of -1.3 eV. This result may account for the solubility of iodine in uranium dioxide observed experimentally. We also notice that the incorporation of iodine involves steric and electronic contributions. The larger the defect iodine is incorporated in, the lower is its incorporation energy. Besides, we find iodine to be charged -1, thus getting the stable electronic configuration of rare gases. We also highlight the fact that the use of GGA+U increases the number of metastable states (non global energy minima), compared to the LDA/GGA approximations. Consequently, special care has to be taken on the 5f electronic occupancies in order to ensure that the absolute energy minimum has been reached.     

Development of the Chemical Oxygen-Iodine Laser (COIL) with chemical generation of atomic iodine

This article addresses the development of a Chemical Oxygen-Iodine Laser (COIL) with alternative chemical ways of generating atomic iodine. Injection of atomic iodine as opposed to molecular iodine has the potential to improve the COIL efficiency. This paper describes two chemical methods for generating iodine atoms based on the gas phase reactions of hydrogen/deuterium iodide with fluorine or chlorine atoms, which are also produced chemically. Simplified one-dimensional gas dynamic modeling that describes the stream-wise profiles of species concentrations within both reaction systems is used to gain a theoretical understanding of both reaction systems under COIL conditions. The modeling results are used for the design of an experimental device and the interpretation of experimental data. The first experimental investigation studies the production of iodine atoms produced from reactions of Cl with HI. Atomic iodine yields of 70–100% in nitrogen are obtained, and the gain on the I(² P _1/2)–I(² P _3/2) transition in a flow of singlet oxygen is measured. Received: 7 October 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +420-2/8689-0527, E-mail: kodym@fzu.cz     

A re-interpretation of the LEED structures formed by iodine on W(l10)

The early work done by Avery on the adsorption of I₂ on W(110) has been re-interpreted using adatom models originally developed for the Cl₂/Br₂/I₂/Fe (100) systems. The experimental coverages and LEED patterns are described precisely using variable, non-coincident nets of halogen atoms. It is shown that the movement of spots within the diffraction pattern arises from the movements of iodine atoms along simple crystallographic directions. The model assumes repulsive lateral interactions between iodine adatoms which is consistent with the desorption behaviour. The reasons for structural changes within the adlayer are discussed using the model, and the internuclear spacings and geometry of the adlayer are shown to be consistent with previous work on Fe(100) and W(100).     

Evolution of the atomic order and valence state of rare-earth atoms and uranium in a new carbon-metal composite—diphthalocyanine pyrolysate C64H32N16 Me (Me = Y,La, Ce,Eu, and U)

The structure of a metal-carbon composite formed by the pyrolysis of diphthalocyanine of some rare-earth elements (Y, La, Ce, Eu) and uranium in the temperature range T _ann = 800–1700°C has been investigated for the first time by the methods of X-ray diffraction analysis and X-ray line shift. It has been shown that, in the general case, the studied pyrolysates consist of three phases. One phase corresponds to the structure of graphite. The second phase corresponds to nitrides, carbides, and oxides of basic metal elements with a crystallite size ranging from 5 to 100 nm. The third phase is amorphous or consisting of crystallites with a size of ～1 nm. It has been found that all the basic elements (Y, La, Ce, Eu, U) and incorporated iodine atoms in the third phase are in a chemically bound state. The previously unobserved electronic configurations have been revealed for europium. The possibility of including not only atoms of elements forming diphthalocyanine but also other elements (for example, iodine) in the composite structure is of interest, in particular, for the creation of a thermally, chemically, and radiation resistant metal-carbon matrix for the radioactive waste storage.     

The formation of a surface iodide on Ni{100} and adsorption of I2 at low temperatures

Iodine adsorption on clean Ni[100] has been investigated using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). At temperatures below 340 K. a saturated surface of adsorbed iodine atoms in a c(2 × 2) structure is observed. Adsorption of iodine on clean Ni{100} at temperatures in exces of 370 K forms a structure identified as a single layer of the layered compound NiI₂ on the metal substrate. Solid iodine is shown to grow epitaxially on both the c(2 × 2) chemisorbed surface and the surface iodide at temperatures less than 185 K. Heating to 185 < T < 226 K leaves a physisorbed molecular iodine layer, while on returning to room temperature the original c(2 × 2) or iodide is restored.     

Chemical structure of iodine-doped polyisoprene

A nonconjugated conducting polymer (1,4-rans-polyisoprene) at different iodine doping levels has been studied with the 27.7-keV Mössbauer resonance of¹²⁹I. At the light doping level, almost all iodines are covalently bonding to the carbon atoms, breaking the double bonds of the main chain. At the medium doping level, other iodine species are found, which are idendified as anion species, l^?, l ₃ ^? and l ₃ ^? .At the heavy doping level, polyiodide anions, l ₃ ^? and l ₅ ^? , increase as compared to the covalently bonding iodine atoms and addtionally molecular iodine l₂ exists.     

Multiphoton ionization of iodine atoms and CF<Subscript> 3</Subscript>I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF₃I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF₃I molecules is the I⁺ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(²P_3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I^*(²P_1/2). The ionization of the CF₃I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.     

Possibility of using recombination reactions and alkali metals to obtain inversion on atomic iodine

The possibility of producing a continuous-flow laser operating on an atomic iodine transition is considered. Population inversion is the result of depletion of the lower laser level on account of predominant recombination into iodine molecules in the ground state. Alkali-metal atoms are used to obtain atomic iodine and free radicals without consumption of electric energy. The use of an electric discharge is proposed for the excitation of atomic iodine. The kinetics of the basic chemical reactions is considered and the possibility is demonstrated of obtaining inversion at reasonable working-mixture parameters. The schematic diagram of the laser setup is discussed.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 125, pp. 202–209, 1980.     

Atomic structure of GaAs(001)-c(8×2) and adsorption sites of iodine atoms at low coverage

Atomically resolved scanning tunneling microscopy images observed by us correspond to the ζ model of GaAs(001)-c(8×2) atomic structure. At low coverage (θ < 0.1), iodine atoms occupy sites above vacation rows between arsenic atoms located in the upper layer.     

CuI晶体缺陷态电子结构及其施主-受主对发光机理的研究

运用相对论密度泛函理论和嵌入分子团簇方法，模拟计算了具有γ相CuI晶体的本征缺陷态电子结构.结果表明，四面体间隙Cu和Cu空位最有可能在禁带中引入浅施主和受主能级，从而形成施主-受主对(DAP)，产生420—430 nm的DAP复合发光. 关键词： γ-CuI晶体 密度泛函理论 电子结构 发光机理     

A diode laser system for spectroscopy of the ultranarrow transition in ytterbium atoms

A narrow-line diode laser system has been developed for high-resolution spectroscopy of the magnetic quadrupole transition in ytterbium (Yb) atoms at 507 nm. The system consists of an extended-cavity laser diode at 1014 nm, a tapered amplifier and a periodically poled lithium niobate nonlinear crystal which converts the wavelength from 1014 nm to 507 nm. We have stabilized the laser frequency and reduced the linewidth below 1 kHz by tightly locking the laser to a high-finesse optical cavity. By using the developed laser system, we have successfully observed the ultranarrow transition in Yb atoms. Furthermore, our simple and compact laser system could be a high performance and portable frequency reference using iodine spectra whose linewidth becomes less than 50 kHz around 507 nm. We have also demonstrated spectroscopy of iodine molecules. PACS 32.30.-r; 07.60.-j     

Study of chemical bonding in the interhalogen complexes based on density functional theory

The density functional theory analysis was used for a number XYL complexes (XY is a dihalogen molecule and L is a Lewis base), formed between molecules I₂, ICl, IBr and pyridine. The calculated geometrical parameters, IR spectra and nuclear quadrupole interaction constants of iodine are consistent with the data of microwave spectroscopy and nuclear quadrupole resonance. The good correlation between the experimental and calculated binding energies of the inner electrons of iodine, chlorine and nitrogen atoms were found with the calculation using both Gaussian and Slater functions. The comparison of experimental and calculated changes in the electron density on the atoms upon complex formation suggested the choice of scheme for calculating the effective charge on the atoms, which allow us to interpret the experimental spectra. It is shown that the use of both calculated schemes allows us to predict the enthalpy of complex formation in close agreement with the experimental values. The energy analysis shows that in the complexes the electrostatic binding energy dominates that of covalent binding.