O<Subscript>2</Subscript> photodesorption from a Ag<Subscript>8</Subscript>O<Stack><Subscript>2</Subscript><Superscript>-</Superscript></Stack> cluster

The decay path of an Ag₈(O₂)^- cluster photoexcited by a 3.1 eV photon is elucidated using time-resolved photoelectron spectroscopy. Photoabsorption results in the formation of an excited state giving rise to a peak in the photoelectron spectra with well-resolved vibrational finestructure. With a lifetime of about 100 fs this bound state decays into an anti-bonding state which dissociates into O₂ and Ag₈^- on a timescale of 10 ps. In the photoelectron spectra, this corresponds to a broad maximum shifting gradually towards higher binding energy while the O₂ and Ag₈^- separate. Finally, the spectrum of bare Ag₈^- appears. This process is unique to small clusters, because on metal surfaces excited state lifetimes are too short to allow for direct dissociation.     

II. Electron-impact dissociative excitation of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

Absolute cross-sections for electron-impact dissociative ionization of C₂ H₂⁺ and C₂ D₂⁺ to CH⁺, C⁺, C₂⁺ , H⁺, CH₂⁺ and C₂D⁺ fragments are determined for electron energies ranging from the corresponding threshold to 2.5 keV. Results obtained in a crossed beams experiment are analyzed to estimate the contribution of dissociative ionization to each fragment formation. The dissociative ionization cross sections are seen to decrease for more than an order of magnitude, from CH⁺ (5.37±0.10) × 10^-17 cm² over C⁺ (4.19± 0.16) × 10^-17 cm², C₂D⁺ (3.94±0.38) × 10^-17 cm², C₂⁺ (3.82±0.15) × 10^-17 cm² and H⁺ (3.37±0.21) × 10^-17 cm² to CH₂⁺ (2.66±0.14) × 10^-18 cm². Kinetic energy release distributions of fragment ions are also determined from the analysis of the product velocity distribution. Cross section values, threshold energies and kinetic energies are compared with the data available from the literature. Conforming to the scheme used in the study of the dissociative excitation of C₂H₂⁺ ( C₂ D₂⁺ )\left( {\rm C}_2 {\rm D}_2^+ \right), the cross-sections are presented in a format suitable for their implementation in plasma simulation codes.     

III. Electron-impact dissociative ionization of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

The dynamics of cold atoms in conservative optical lattices obviously depends on the geometry of the lattice. But very similar lattices may lead to deeply different dynamics. In a 2D optical lattice with a square mesh, it is expected that the coupling between the degrees of freedom leads to chaotic motions. However, in some conditions, chaos remains marginal. The aim of this paper is to understand the dynamical mechanisms inhibiting the appearance of chaos in such a case. As the quantum dynamics of a system is defined as a function of its classical dynamics – e.g. quantum chaos is defined as the quantum regime of a system whose classical dynamics is chaotic – we focus here on the dynamical regimes of classical atoms inside a well. We show that when chaos is inhibited, the motions in the two directions of space are frequency locked in most of the phase space, for most of the parameters of the lattice and atoms. This synchronization, not as strict as that of a dissipative system, is nevertheless a mechanism powerful enough to explain that chaos cannot appear in such conditions.     

I. Electron-impact ionization and dissociation of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

Absolute cross-sections for electron-impact ionization and dissociation of C₂H₂⁺ and C₂D₂⁺ have been measured for electron energies ranging from the corresponding thresholds up to 2.5 keV. The animated crossed beams experiment has been used. Light as well as heavy fragment ions that are produced from the ionization and the dissociation of the target have been detected for the first time. The maximum of the cross-section for single ionization is found to be (5.56 ± 0.03)× 10^-17 cm² around 140 eV. Cross-sections for dissociation of C₂ H₂⁺ (C₂D₂⁺) to ionic products are seen to decrease for two orders of magnitude, from C₂D⁺ (12.6 ± 0.3) × 10^-17 cm² over CH⁺(9.55 ± 0.06) × 10^-17 cm², C⁺ (6.66 ± 0.05) × 10^-17 cm², C₂⁺ (5.36 ± 0.27) × 10^-17 cm², H⁺ (4.73 ± 0.29) × 10^-17 cm² and CH₂⁺ (4.56 ± 0.27) × 10^-18 cm² to H₂⁺ (5.68 ± 0.49) × 10^-19 cm². Absolute cross-sections and threshold energies have been compared with the scarce data available in the literature.     

Description of torsional motion in ionic complexes ArH<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> and ArD<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack>

An algebraic model that describes the internal dynamics of the ionic complexes ArH₃⁺ and ArD₃⁺ in the ground electronic-vibrational state taking into account the torsional motion of the structure of identical hydrogen nuclei is constructed by symmetry-group chain methods. It is important that the correctness of this model is only limited by the correctness of the choice of geometric symmetry of the internal dynamics of the ionic complex.     

Dielectric relaxation of Cr<Superscript>3+</Superscript>-Li<Superscript>+</Superscript> pair centers in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The temperature-frequency dependences of the permittivity of crystals of lithium heptagermanate Li₂Ge₇O₁₅ doped with Cr³⁺ ions are investigated. A dielectric response to reorientation of the dipole moments of the chromium impurity centers is revealed. Anomalies of the permittivity are described within the model of a Debye relaxator.     

Investigation of the charge distribution in small cluster ions Ar<Stack><Subscript>13</Subscript><Superscript>+</Superscript></Stack> and Ar<Stack><Subscript>19</Subscript><Superscript>+</Superscript></Stack>

The results of ab initio studies of the atomic and charge structure of small clusters and cluster ions formed by 13 and 19 argon atoms are reported. It was found that the icosahedral atomic structure is energetically the most favorable for such clusters. The calculations demonstrate that when a single electron is removed from a cluster, the excess positive charge is distributed primarily over the surface of the formed cluster ion.     

Kinetics of Rb<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and Rb<Superscript>+</Superscript> formation in laser-excited rubidium vapor

We have studied the formation of the molecular ion Rb₂⁺ and the atomic ion Rb⁺. These are created in laser excited rubidium vapor at the first resonance, 5s–5p and 5p-nl transitions. A theoretical model is applied to this interaction to explain the time evolution and the laser-power dependence of the population density of Rb⁺ and Rb₂⁺. A set of rate equations which describe: the temporal variation of the population density of the excited states; the atomic ion density; and the electron density, were solved numerically under the experimental conditions of Barbier and Cheret. In their experiment the Rb concentration was 1×10¹³cm⁻³ and the laser power was taken to be 50–500 mW at vapor temperature = 450 K. The results showed that the main processes for producing Rb₂⁺ are associative ionization and Hornbeck-Molnar ionization. The calculations have also showed that, the atomic ions Rb⁺ are formed through the Penning Ionization (PI) and photoionization processes. Moreover, a reasonable agreement between the experimental results and our calculations for the ion currents of the Rb⁺ and Rb₂⁺ is obtained.      

Modification of GaAs by medium-energy N<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> ions

The modification of GaAs with a 2500-eV beam containing N ₂ ⁺ and Ar⁺ ions is examined with Auger electron spectroscopy. Most implanted nitrogen atoms are found to react with the matrix, substituting arsenic atoms to produce a several-nanometer-thick layer of the single-phase GaAs_1−x N_x (x=6%) solid solution. The GaN phase is absent. Displaced arsenic atoms and nitrogen atoms unreacted with the matrix are present in the layer and on its surface. The former segregate, whereas the latter form molecules.     

Metastable helium cluster He<Stack><Subscript>4</Subscript><Superscript>*</Superscript></Stack>

The existence of a metastable cluster He ₄ ^* with total spin S = 2 is predicted. The cluster consists of two covalently bound excited spin-polarized triplet He ₂ ^* molecules and is rectangular in shape. The electron wavefunctions, the dependence of the energy He ₄ ^* system on the distance between the He ₂ ^* triplet molecules, the atomic spacing, the frequency spectrum of natural oscillations of the cluster, and other characteristics are calculated from first principles. It is shown that the metastable state is formed if one of the excited He ₂ ^* molecules is in the ³Σ _u ⁺ state, while the other is in the ³Π_g state. The radiation lifetime τ of the metastable cluster He ₄ ^* is calculated; it is found to range from 100 to 200 s, which is much longer than the lifetime τ ≈ 20 s of the triplet molecule He ₂ ^* (³Σ _u ⁺ ). The height U ≈ 0.5 eV of the potential barrier preventing the departure from the local energy minimum is determined. The energy E_acc ≈ 9 eV/atom accumulated in the He ₄ ^* cluster is calculated; this energy considerably exceeds the energy of known chemical energy carriers. It is shown that the accumulated energy is released virtually completely during decomposition of the He ₄ ^* cluster into individual helium atoms. This means that helium clusters are a promising material with a high accumulated energy density (HEDM).     

Effect of the atomic composition of the surface on the electron surface states in topological insulators A<Stack><Subscript>2</Subscript><Superscript>V</Superscript></Stack>B<Stack><Subscript>3</Subscript><Superscript>VI</Superscript></Stack>

The results of the theoretical investigation of the surface electronic structure of A₂^VB₃^VI compounds containing topologically protected surface states are reported. The ideal Bi₂Te₃, Bi₂Se₃, and Sb₂Te₃ surfaces and surfaces with an absent external layer of chalcogen atoms, which were observed experimentally as monolayer terraces, have been considered. It has been shown that the discrepancy between the calculated Fermi level and the value measured in the photoemission experiments can be attributed to the presence of the “dangling bond” states on the surface of the terraces formed by semimetal atoms. The fraction of such terraces on the surface has been estimated.     

Geometry of the internal dynamics of C<Subscript>2</Subscript>H<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> carbocation

Using the symmetry group chain methods, the internal dynamics of the simplest carbocation, C₂H ₃ ⁺ , is analyzed under the traditional assumptions that the equilibrium structures of the carbocation are planar and that the nonrigid motion between them is in-plane. This geometry of the internal dynamics is shown to agree with the data of the microwave spectroscopy on the splittings of rotational energy levels caused by the nonrigid motion. Previously, this statement was based on the model that violated the requirement of self-adjointness of operators of physical quantities.     

A miniature tunable TEA CO<Subscript>2</Subscript> laser using isotope <Superscript>13</Superscript>C<Superscript>16</Superscript>O<Subscript>2</Subscript> and <Superscript>12</Superscript>C<Superscript>16</Superscript>O<Subscript>2</Subscript>

A miniature tunable TEA CO₂ laser using isotope ¹³C¹⁶O₂ as the active medium is developed to extend the spectral range of CO₂ lasers for further application. The optimization of the energy parameters of the tunable TEA ¹³C¹⁶O₂ laser and the same laser using ¹²C¹⁶O₂ are studied. When a gas mixture (¹³C¹⁶O₂: N₂: He = 1: 1: 3) at a total pressure of 6.4 × 10⁴ Pa is used, the TEA ¹³C¹⁶O₂ laser of a 45-cm³ active volume obtains 51 emission lines in the [00⁰1–10⁰0] and [00⁰1–02⁰0] bands. The maximum pulse energy of the TEA ¹³C¹⁶O₂ laser is about 357 mJ. The same laser using the conventional gas mixture (¹²C¹⁶O₂: N₂: He = 1: 1: 3) at a pressure of 6.66 × 10⁴ Pa is measured to obtain 69 laser emission lines and the maximum pulse energy of laser radiation is about 409 mJ.     

Influence of configuration interaction on splitting of multiplets of TmF<Stack><Subscript>6</Subscript><Superscript>3−</Superscript></Stack> and TmCl<Stack><Subscript>6</Subscript><Superscript>3−</Superscript></Stack> molecular complexes

The Stark structure of elpasolites activated by Tm³⁺ ions is analyzed with the help of standard and modified crystal field theories. Application of the modified approach reduces considerably the root-mean-square deviation between theoretical and experimental energy values. Crystal field and covalence parameters are determined, the latter being in good agreement with the covalence parameters calculated using microscopic models.     

Recognition of H<Subscript>2</Subscript>PO<Stack><Subscript>4</Subscript><Superscript>-</Superscript></Stack> and Cu<Superscript>2+</Superscript> in Water by Luminescent Terbium Silica Xerogel

Terbium silica hybrid material with imidazole ring that can be emissive in water has been designed and showed host-guest interactions with specific ions (cations and anions). In detail, we studied the sensing abilities of this material by addition of the anions H₂PO₄^-, HSO₄^-, F^-, Cl^-, Br^- and I^- to water suspension of the derived powders. Only dihydrogen phosphate resulted in the quenching of the lanthanide luminescence (detection limit 10^-5 M). The same way was found in Cu²⁺ ions which also gave rise to luminescence quenching (detection limit 10^-5 M). More interestingly, luminescent sol-gel films were successfully prepared by the same materials and exhibited emission responses to H₂PO₄^- and Cu²⁺. For the sake of improving its mechanical property, the robust and flexible inorganic/PMMA hybrid material with sensing capability was also developed for future use.     

H<Subscript>3</Subscript>O<Stack><Subscript>2</Subscript><Superscript>-</Superscript></Stack> ions with a strong quasi-symmetrical H-bond and their hydration in aqueous solutions of NaOH and KOH

Ion-molecular interactions in aqueous solutions of NaOH (0–47.8%) and KOH (0–51.95%) are studied by multiple frustrated total internal reflection IR spectroscopy. Interpretation of the spectra and analysis of the spectral data are performed based on the results of DFT calculations (B3LYP/6-31++G(d, p)) of the characteristics of the free and double hydrated H₃O ₂ ^- ion. It is established that the changes in the IR spectra of NaOH and KOH aqueous solutions caused by increasing alkali concentration are due to the formation of H₃O ₂ ^- ions with a strong quasi-symmetrical hydrogen bond and their subsequent hydration by one or two water molecules. The influence of the cation nature on the degree of hydration of H₃O ₂ ^- ions is demonstrated. The equilibrium concentrations of monohydrate (H₃O ₂ ^- ? H₂O) and dihydrate (H₃O ₂ ^- ? 2H₂O) are calculated and their IR continuous absorption spectra are isolated.     

Resonator effects of Ar<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> ionic excimer pumped by electron beam

The third continuum of argon from 200 to 270 nm is obtained by electron beam pumping argon. With the front mirror and the rear mirror, the resonator effects are observed at 240 and 220 nm. Compared with the intensity of fluorescence, the intensity near 240 nm increases more than 10 times when the resonator is formed. In order to explain the origin of the continuum, the kinetic model on Ar₂⁺ and Ar₂²⁺ ionic excimers pumped by electron beam is built and calculated. Based on the theoretical results, the Ar₂⁺ ionic excimer should be responsible for the continuum around 240 nm.     

Optical characteristics of radiation from N<Stack><Subscript>2</Subscript><Superscript>*</Superscript></Stack> dimers in a barrier discharge

The spectral and power characteristics of radiation of the second positive system of nitrogen (C ³Π _u → B ³Π _g ) in Ar-N₂ and Ar-N₂-Cl₂ mixtures excited by barrier discharge have been studied experimentally. Addition of argon to N₂ increased the radiation power by sixfold. In the triple mixture Ar-N₂-Cl₂ = 210/0.5/0.005, minor chlorine additions increased the intensity of the C ³Π _u → _B ³Π _g transition by 26% compared to Ar-N₂ mixtures. Radiation power density of 2.7 mW/cm² has been achieved. In both binary and triple mixtures, the second positive system of nitrogen was the major contributor to radiation, while the contributions of the fourth positive system of N ₂ ^* (D ³Σ _u ⁺ → B ³Π _g ), the Vegard-Kaplan transition of N ₂ ^* (A ³Σ _u ⁺ → X ¹Σ _g ⁺ ), and the D′ → A′ band of Cl ₂ ^* were negligibly small.