The compensation effect and the manifestation of lateral interactions in thermal desorption spectroscopy

If thermal desorption spectra are analysed in terms of the Polanyi-Wigner equation lateral interactions between the adsorbates may lead to coverage (θ) dependent pre-exponential factors, v, and activation energies of desorption, E. Evidence from the literature shows that E and v often satisfy the well-known compensation effect 1n v(θ) = bE(θ) + c, with constants b and c. Here we insert this compensation effect into the rate equation of desorption and simulate spectra which illustrate the influence of the compensation effect in thermal desorption spectra of adsorbate systems where pairwise lateral interactions prevail.     

Zenith-angle dependences of ρ s,600 and ρ μ,600, in giant air showers

Results of a global analysis of data from the Yakutsk array and AGASA are presented. The zenith-angle dependences ρ _s,600(θ) and ρ _μ,600(θ) of the densities of all charged particles and muons (the threshold energy is E _μ ≈ 1.0·secθ GeV) at a distance of 600 m from the axis of giant air showers (GAS) of energies E ₀≥10¹⁹ eV are considered. These dependences are compared with the results of the calculations based on the QGSJET model and performed for the case of primary protons. The results of the calculations within this model agree well with data from both arrays at E ₀≤2×10¹⁸ eV, but they are in a glaring contradiction with GAS data. The experiments indicate that the lateral structure of showers changes at E ₀≥(3–5)×10¹⁸ eV. In all probability this is due to some new processes accompanying their evolution. The neglect of this fact can lead to considerably overestimating the GAS energy (by a factor of 1.5–2.5).     

Winkelverteilung der Kernphotoreaktion3He (γ,p)2H

The angular distribution of the photonuclear reaction³He(γ,p)²H was measured with an exitation energyE _x=16 to 27 MeV using a 32.5 MeV betatron. In agreement with a theoretical calculation of Böschet al. we found for the coefficients of the distributionf(θ)=b (a/b+sin²θ(1+β·cosθ+γ·cos²θ)) the values:a/b≦0.14; β=0.77 (+0.14; ?0.07); γ≦0.30. This was the first attempt to use spark chambers as a detection device for photonuclear reactions in the low energy range. We found it to be a promising new facility.     

The effect of resonance interactions on the absorption spectra of (SF6)2 dimers in low-temperature matrices: Calculations and experiment

The IR absorption spectra of (SF₆)₂ dimers in Ar and N₂ matrices are investigated at 11 K. As a result of the resonance dipole-dipole interaction, the band of the triply degenerate vibration v ₃ is split into two components v _{X, Y} and v _Z. In comparison with the gaseous state, the splitting Δv = v _{X, Y} ? v _Z in the argon matrix decreases to 18.45 cm^?1, whereas, in the nitrogen matrix, the band v _{X, Y} is split into components v _X and v _Y, with the splitting being equal to δ ≈ 0.9 cm^?1. A model that takes into account the influence of the matrix on the spectra of dimers is developed. The model makes it possible to successively (i) calculate the resonance spectrum of an isolated dimer in terms of the model of local modes taking into account resonance interactions, (ii) determine with the help of the Monte Carlo method the structure of a matrix consisting of 512–1440 Ar (or N₂) atoms and a rigid (SF₆)₂ dimer, and (iii) take into account interactions of local dipole moments of a dimer with particles of the matrix in the approximation of dipole-induced dipole interactions. The model developed satisfactorily describes the experimental results. The calculated frequencies v _Z, v _X, and v _Y of a dimer in the matrix are shifted toward smaller frequencies as compared to the gaseous state, while the resonance splitting decreases virtually by 2 cm^?1. It is shown that, in an argon matrix with a symmetric arrangement of argon atoms nearest to a dimer, the splitting of v _{X, Y} proves to be smaller than 0.05 cm^?1. In a nitrogen matrix, this splitting increases virtually to 0.4 cm^?1.     

Adsorption isotherms of oxygen on tungsten (100), (110) and (111) surfaces in the pressure range 10−9 to 10−4 torr and temperature range 1500 to 2600 k,as measured by AES

This paper is a continuation of a previous investigation of oxygen adsorption on tungsten at high temperature using Auger electron spectroscopy. In this paper the adsorption isotherms of oxygen on (100), (110) and (111) faces of tungsten are reported. It is shown that these isotherms can be described by an equation of the form pO₂ = AF(θ) exp [?q(θ)/ kT]. The coverage depended functions F(θ) and q(θ) evaluated from the isotherms are different for all three investigated faces. The isosteric adsorption energy q has following initial values at very low oxygen coverage: q₁₀₀ = 6.1 eV, q₁₁₀ = 6.8 eV and q₁₁₁ = 6.5 eV. Increasing the oxygen coverage has only small influence on q₁₁₁; it changes from the initial value to q₁₁₁ ≈ 6.0 eV at θ ≈ 0.3 and remains constant at this value up to θ ≈ 1. q₁₁₀ shows the strongest dependence on oxygen coverage. It decreases rapidly at low coverages, slowly at moderate coverages and reaches the value q₁₁₀ = 5.0 atθ ≈ 1. The variation of q₁₁₀ with increasing oxygen coverage is monotonie from the initial value to q₁₁₁ = 4.9 eV at θ ≈ 1. Assuming that the atomic oxygen is the dominant species leaving the tungsten surface at high temperatures the functions F(θ) are used to calculate the oxygen equilibration probability ζO₂ (high temperature sticking probability) as a function of oxygen coverage θ. The main characteristic of ζO₂(θ) for all three faces is that it shows a maximum for (100) and (111) faces at θ = 0.3 and for (110) face at θ = 0.55.     

Stability of subatomic carbon films on metal surfaces against low-energy gas ion bombardment

The earlier developed original experimental technique for measuring and analyzing the parameters of low-frequency fluctuations of the field-emission current in metal film systems is used to measure the sputtering yield Y _f of carbon films (with a coverage Θ ranging from 1 to 4) applied on Fe, Nb, Ta, and U substrates. The value of Y _f is calculated by an expression derived within a theoretical model developed. The sputtering ratios were measured for the case when the carbon films are sputtered by H⁺ and He⁺ ions with an energy E _i between 2 and 10 keV. With Θ fixed, the energy dependences of Y _f are obtained for each of the ions. In addition, for each of the ions, the Θ dependences of Y _f are found for several values of E _i. In all the cases, the measured values of Y _f far exceed those for pure carbon. With another original technique that combines field-ion microscopy (FIM) and precise measurement of current and/or luminous properties of local regions in FIM images, the energy thresholds E _th of sputtering carbon films applied on the metal surfaces are found. The energy distributions of Y _f in the near-threshold energy range for various Θ are obtained.     

A molecular beam study on the interaction of NO with a Pt(111) surface

NO adsorbs on Pt(111) with a (temperature independent) initial sticking coefficient S₀=0.88. The fraction of molecules not being chemisorbed is directly inelastically scattered back due to failure of translational energy accommodation. The nonlinear variation of s with coverage can well be described by a precursor-state model, the precursor state being formed by NO molecules translationally and rotationally accommodated in a physisorbed second layer. Dissociation is essentially restricted to defect sites and is negligible on perfect (111) planes. These defect sites (present in small concentration) are first populated and are also sampled by the modulated beam technique yielding an activation energy for desorption E_d = 33.1 kcal/mole and preexponential factor v_d = 10^15.5s^?1. Isothermal desorption measurements yielded E_d and v_d as a function of coverage: E_d rapidly drops from its initial value (at defect sites) to about 27 kcal/mole — which value is considered as representing the adsorption energy on a perfect (111) plane — and then decreases continuously due to effective repulsive interactions. Simultaneously v_d is decreasing to about 10¹² s^?1 at θ = 0.25 which marks the equilibrium coverage to be reached at 300 K. If the surface is precovered with oxygen atoms the NO sticking coefficient is reduced to 0.6, and the desorption parameters are lowered to E_d = 17.1 kcal/mole and v_d= 10^12.6s^?1 (at zero NO coverage).     

Analyses of the thermal shifts of spectral lines in Nd3+-doped LiYF4 laser crystal

The red-shift of spectral line E ₁(R ₁????Y ₂) and blue-shift of line E ₂(R ₁????X ₅) with temperature in Nd³⁺-doped LiYF₄ laser crystal are studied by considering both the static contribution due to lattice thermal expansion and vibrational contribution due to electron?Cphonon interaction. The study is based on the analyses of pressure and temperature dependences of these spectral lines. It is found that for both lines, the static and vibrational contributions result in the blue- and red-shift, respectively. So, the observed red-shift of line E ₁ and blue-shift of line E ₂ are due respectively to the static contribution being smaller and larger than the vibrational one. Also, we infer that the thermal shifts of lines E ₃(R ₁????Y ₅) and E ₄(R ₂????Y ₅) are very small because both contributions may be approximately canceled. When both the contributions are contained, whether the red-shift or blue-shift of a spectral line can be fitted with the almost same theoretical expression as that by including only the vibrational contribution used in red-shift in the previous papers if we change the expression concerning electron?Cphonon interaction coefficient.     

Microwave spectrum of Trioxane in four excited degenerate vibrational states,especially in the 2ν20 (E) state and the combination state ν7 (A1) + ν20 (E)

The microwave spectra of the Trioxane molecule in the doubly excited degenerate vibrational state 2ν₂₀(E) and in the combination state ν₇(A₁) + ν₂₀(E) are reported for several J values. Molecular constants of these states have been determined. The excited states ν₂₀(E) = 1 and ν₁₉(E) = 1 previously studied by a perturbation formula have been analyzed by a direct diagonalization method of the energy matrix. Parameters which give the variation of the rotational constant B_V as a function of the quantum number v have been obtained.     

Determination of Lamb–Mössbauer factors and lattice dynamics in some nitroprusside single crystals

Mössbauer spectra have been recorded for sodium, guanidinium, barium and lithium nitroprussides single crystal cuts. The temperature dependence of the Lamb–Mössbauer fraction f, respectively, the mean-square nuclear displacement 〈x ²〉 and the mean-square (of the total) velocity of the iron nucleus 〈v ²〉 were analyzed on the basis of the Debye and Einstein lattice-vibration models. The characteristic temperatures of the two models, θ _D and θ _E, fitted to 〈x ²〉 are considerably lower than those fitted to 〈v ²〉. This effect seems to be typical for the iron complexes and was explained with the presence of low and high frequencies in the phonon vibration spectra and of low-temperature anharmonicity. The Lamb–Mössbauer fraction at 77 K in all principal crystal directions of sodium, guanidinium and barium nitroprussides has been determined. These values will be used for more precise studies of the population and the properties of the light-induced molecular states of the nitrosyl [Fe(CN)₅NO]^2? anion.     

Electronic structure characteristic of carbon nanotubules

Electronic structure near the Fermi energy of single-shell carbon nanotubules has been studied within the framework of the tight-binding approximation. The electronic density of states (DOS) of tubules shows a structure consisting of many spike-like peaks. An analytical expression in a π-electron model is derived which predicts not only the energy gap (E _g ) of semiconducting tubules, but also the energy positions of those spike-like peaks in the π-DOS near the Fermi energy in any tubule. The limitation of the π-electron model in tubules is discussed by investigating the effect of σ-πmixing. The position of σ component edge in the DOS is also investigated as a function of tubule radius (R) and chiral angle (θ). It is found that this edge energy is very sensitive to θ and largely changes with R, because the dominant contribution of θ to its change is given by g(θ)/R in contrast to f(θ)/R ² for E _g .     

Extrapolation from positive to negative energy of the Woods-Saxon parametrization of the n-208Pb mean field

The real part V(r); E) of the nucleon-nucleus mean field is assumed to have a Woods-Saxon shape, and accordingly to be fully specified by three quantities: the potential depth U_v(E), radius R_V(E) and diffuseness a_v(E). At a given nucleon energy E these parameters can be determined from three different radial moments [r^q]_v = (4π/A) ∝V(r; E)r^q dr. This is useful because a dispersion relation approach has recently been developed for extrapolating [r^q]_V(E) from positive to negative energy, using as inputs the radial moments of the real and imaginary parts of empirical optical-model potentials V(r; E) + iW(r; E). In the present work, the values of U_v(E), R_v(E) and a_v(E) are calculated in the case of neutrons in ²⁰⁸Pb in the energy domain −20 < E < 40 MeV from the values of [r^q]_V(E) for q = 0.8, 2 and 4. It is found that both U_V(E) and R_v(E) have a characteristic energy dependence. The energy dependence of the diffuseness a_a(E) is less reliably predicted by the method. The radius R_V(E) increases when E decreases from 40 to 5 MeV. This behaviour is in agreement with empirical evidence. In the energy domain −10 MeV < E < 0, R_V(E) is predicted to decrease with decreasing energy. The energy dependence of the root mean square radius is similar to that of R_V(E). The potential depth U_v slightly increases when E decreases from 40 to 15 MeV and slightly decreases between 10 and 5 MeV; it is consequently approximately constant in the energy domain 5 < E < 20 MeV, in keeping with empirical evidence. The depth U_v increases linearly with decreasing E in the domain −10 MeV < E < 0. These features are shown to persist when one modifies the detailed input of the calculation, namely the empirical values of [r^q]_v(E) for E > 0 and the parametrization [r_q]_w(E) of the energy dependence of the radial moments of the imaginary part of the empirical optical-model potentials. In the energy domain −10 MeV < E < 0, the calculated V(r; E) yields good agreement with the experimental single-particle energies; the model thus accurately predicts the shell-model potential (E < 0) from the extrapolation of the optical-model potential (E > 0). In the dispersion relation approach, the real part V(r; E) is the sum of a Hartree-Fock type contribution V_HF(r; E) and of a dispersive contribution ΔV(r; E). The latter is due to the excitation of the ²⁰⁸Pb core. The dispersion relation approach enables the calculation of the radial moment [r^q]_ΔV(E) from the parametrization [r^q]_w(E): several schematic models are considered which yield algebraic expressions for [r^q]_ΔV(E). The radial moments [r^q]_HF(E) are approximated by linear functions of E. When in addition, it is assumed that V_HF(r; E) has a Woods-Saxon radial shape, the energy dependence of its potential parameters (U_HF, R_HF, a_HF) can be calculated. Furthermore, the values of ΔV(r; E) can then be derived. It turns out that ΔV(r; E) is peaked at the nuclear surface near the Fermi energy and acquires a Woods-Saxon type shape when the energy increases, in keeping with previous qualitative estimates. It is responsible for the peculiar energy dependence of R_V(E) in the vicinity of the Fermi energy.     

Laser absorption spectroscopy of the A1Σu+-X1Σg+(v′, v″) bands of Na2

Laser absorption spectra of the band system A¹Σ_u⁺-X¹Σ_g⁺(v′,v″) of Na₂ in the range between 16835 and 16960 cm^?1 are analyzed. The spectra are free of first-order-Doppler broadening at low internal temperature. Rotational and vibrational assignments of 10 bands are reported. Rotational and vibrational levels, especially the (22, 1) band, are found to be perturbed by the b³Π_u state. The band origins and line frequencies calculated from previously reported constants do not reproduce the observed values. The disagreement between the calculated and observed values is discussed. More accurate constants are needed to predict the line locations.     

Role of U(3(A−1)) multiquantum excited states in calculating spectroscopic quantities for 4 8 Be

A set of equations for deriving the SU(3)-irreducible density matrix for U(3(A?1)) multiquantum excited states involved is given. For the ₄ ⁸ Be nucleus, the binding energies, radii, spectra, electric quadrupole moments, and probabilities B(E2) of electric quadrupole transitions are computed in the multiquantum approximation of the unitary scheme. The values obtained for the above spectroscopic quantities are investigated versus the number of basis functions involved.     

Surface state density distribution at vacuum-annealed InP(1 0 0) surface as derived from the rigorous analysis of photoluminescence efficiency

The surface state density distribution N_SS(E) and surface Fermi level E_Fs position on a sequentially ultra-high vacuum-annealed n-InP(1 0 0) sample are investigated using rigorous computer analysis of dependences of the room temperature band-edge photoluminescence efficiency (Y_PL) on the photon flux density (Φ). We have found that the minimum density of a U-shaped N_SS(E) distribution as well as the donor-like surface state density are reduced by one order of magnitude after the annealing at 250 and 300 °C. This can be assigned to the decrease in the disorder in the unintentionally formed InP native oxides. On the other hand, we demonstrate that the annealing simultaneously generates discrete surface states probably due to missing group V element (P) in the interface region which may account for electrical interface instabilities observed in InP-based devices. The results are discussed quantitatively and compared to other reports.     

The defects influence on domain wall propagation in bistable glass-coated microwires

We studied the domain wall (DW) dynamics of magnetically bistable amorphous glass-coated Fe₇₄B₁₃Si₁₁C₂ microwires. In according to our experimental results magnetic field dependences of DW velocity of studied microwires can be divided into two groups: with uniform or uniformly accelerated DW propagation along the microwire. Strong correlation between the type of the magnetic field dependence of domain wall velocity, v(H), and the distribution of the local nucleation fields has been observed.Moreover, we observed abrupt increasing of DW velocity (jump) on the magnetic field dependences of the domain wall velocity, v(H), for the both types of the v(H) dependences. At the same time usual linear increasing of the domain wall velocity with magnetic field persists below these jumps. It was found that the jump height correlates with the location of nucleation place of the new domain wall. We have measured local nucleation field distribution in all the microwires. From local nucleation field distribution we have obtained the DW nucleation locations and estimated the jump height     

On the mass and mixings of the ZE

The experimental data on the neutral current couplings are used to derive lower bounds on the mass of Z_E, the extra neutral gauge boson appearing in the minimal ‘beyond the standard model’ scenario favoured in superstring compactifications. This is based on the gauge group SU(3)_c×SU(2)_L×U(1)_Y×U(1)_E. Taking sin²θ_w=0.229, m_W=80.76 GeV and m_Z=91.59 GeV it is found that the mixing angle θ between Z and Z_E must satisfy −0.136<sin θ<−0.007 corresponding to m_ZE>152 GeV or, assuming E₆ unification m_ZE>155 GeV.     

The Raman Spectrum of The 3-Br Pyridine-Bromine Charge Transfer Complex

The formation of a charge transfer complex between pyridine and halogens or mixed halogens^(1,2) brings about perturbations in the infrared spectra of the pyridine molecule and many halogen complexes show two bands in the low frequency range, which can be attributed to the v _X-X and v _N…Y stretching vibrations. ^(3–5). However, by comparison, very little has been reported on the Raman spectra of such complexes. Klaboe⁽⁶⁾ studied the v _Br-Br Raman frequency of some bromine complexes and more recently the Raman spectra of pyridine complexed with Ni (CN)₄ ⁽⁷⁾ and TiCl₄ ⁽⁸⁾ have been reported. In this work, the Raman spectrum of the 3-Br pyridine-bromine charge transfer complex is investigated.     

核物质状态方程中动量相关作用可能的探针

研究了动量相关作用对于中子-质子比动能谱rb(Ek)的效应,发现rb(Ek)灵敏的依赖于动量相关作用而弱的依赖于介质中核子-核子碰撞截面和对称势.因此rb(Ek)是提取重离子碰撞中动量相关作用信息的可能探针.同时,对于丰中子弹核和相同质量稳定弹核在相同入射道条件下,丰中子碰撞系统明显加强了动量相关作用对rb(Ek)的效应.故两个碰撞系统rb(Ek)结果的比较为在重离子碰撞中提取动量相关作用的知识提供了另一个重要的判据。     

Mean-square displacements of metal and boron atoms in crystal lattices of rare-earth hexaborides

The intensities of the I₄₁₀ and I₄₁₁ reflections of nine rare-earth hexaborides MB₆ (M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy) are experimentally studied in the temperature range 4.2–300 K. The mean-square displacements of metal and boron atoms are calculated from the temperature dependences of the intensities I₄₁₀(T) and I₄₁₁(T). The characteristic temperatures of the metal (θ_M) and boron (θ_B) sublattices of rare-earth hexaborides are determined in the Debye approximation. It is found that the characteristic temperatures decrease with an increase in the atomic number of the metal.