Calculation of the pressure shift of the positronium hyperfine interval

We calculate the fractional positronium hyperfine pressure shift Δ(T) using a quantum mechanical thermal averaging and obtain for Δ(293 K)(±30%) in He, Ne, Ar, Kr, Xe, N₂ and SF₆ respectively; (-0.15, -0.34, -0.96, -1.41, -1.85, -1.12, -1.1) × 10^-7/torr (0°C).     

Molekularstrahlmessungen von Stoßquerschnitten für Übergänge zwischen definierten Rotationszuständen zwei-atomiger Moleküle

Inelastic collision cross sections for transitions between specified rotational states designated by (J, M) have been measured in a molecular beam apparatus. With an electrostatic four pole field molecules in a specified rotational state are separated out of a molecular beam and focussed into a gas filled scattering chamber. Molecules which have been scattered by less than 1/2° are then collected in a second four pole field, located directly behind the scattering chamber, and are analyzed for their rotational state. From a comparison of the measured pressure dependence with calculated curves a determination of inelastic collision cross sections for specified quantum jumps is possible. Measured inelastic scattering cross sections for the transitions (2,0→3,0) are reported for the gases He, Ne, Ar, Kr, CH₄, SF₆, H₂, O₂, Air, N₂O, H₂O, CF₂Cl₂. The values range between about 5 and 100 Ų in the order indicated. The scattering gases NH₃ und ND₃ yielded larger cross sections of about 600 Ų and, in addition, the transitions (3,0)→(2,0),(1,0)→(2,0), (2,0)→(1,0) and (3,0)→(1,0) were observed. Total cross sections for the same gases were also measured with the apparatus.     

Is the critical Reynolds number universal?

This paper is devoted to checking whether the critical Reynolds number is universal in identical conditions for the flow of different fluids. The laminar-turbulent transition in a circular pipe flow has been tested experimentally. The flows of inert gases (He, Ne, Ar, Kr, Xe), molecular gases (N₂, CO, CO₂, SF₆), and two similar liquids (H₂O, D₂O) have been tested. A considerable, up to 40%, difference in critical Reynolds numbers was observed. The possible reasons for nonuniversality of the critical Reynolds number are discussed.     

From quantum chemistry to dissociation kinetics: what we need to know †

The relationship between rate constants for dissociation and the reverse association reactions and their potential energy surfaces is illustrated. The reaction systems e^? + SF₆ ? SF₆ ^? →SF₅ ^? + F, H + CH₃ ?CH₄, 2 CF₂ ? C₂F₄, H + O₂ →HO₂, HO + O ?HO₂ ? H + O₂, and C + HO →CHO are chosen as representative examples. The necessity to know precise thermochemical data is emphasised. The interplay between attractive and anisotropic components of the potentials influences the rate constants. Spin–orbit and electronic–rotational coupling in reactions between electronic open-shell radicals so far generally has been neglected, but is shown to have a marked influence on low temperature rate constants.     

The viscosity of normal butane,isobutane and their mixtures

The paper reports measurements of the viscosity of normal butane and isobutane at atmospheric pressure in the temperature range 25–195°C. In addition, the viscosity of mixtures of the two gases has been determined in the more limited temperature range 25–100°C. The absolute accuracy of the viscosity data is estimated as ±0.2%, whereas the ratio of the viscosity of the two pure components is determined with an uncertainty of ±0.1%. The viscosity data allow the evaluation of the diffusion coefficients for the n-butane-i-butane system.The experimental data demonstrate that there are discernible differences between the intermolecular potentials for the pair-interactions n-C₆H₁₀-n-C₄H₁₀; and i-C₄H₁₀-i-C₄H₁₀. All of the experimental data can be correlated within their experimental uncertainty with the aid of the extended law of corresponding states developed by Kestin, Ro and Wakeham. The optimum values of the two scaling parameters, σ_ij and ?_ij, for each interaction which secure this agreement are determined.     

Content and Isotope Ratios of Noble Gases in Congelation Ice of Lake Vostok

Isotope ratios of noble gases (He, Ne, Ar) were studied in samples collected by degassing of cores of water frozen over a glacier of Lake Vostok. The gases were collected into glass retorts during three days of degassing of cores, which have just been extracted from the borehole. Within the error, the isotope ³He/⁴He ratios of 0.28 ± 0.08 R_A (R_A = 1.38 × 10^–6 is the ratio for air) correspond to those from [1]. The ⁴He/²⁰Ne and ⁴⁰Ar/³⁶Ar ratios (12.4 ± 4.6 R_A and 1.0074 ± 0.0023 R_A, respectively) exceed their contents in air (⁴He/²⁰Ne_A = 0.29; ⁴⁰Ar/³⁶Ar_A = 298.6) and may indicate some contribution of terrigenous gas to the gaseous balance of the lake, as well as the high content of ancient ground waters in the lake. The ³He/⁴He ratio of 0.28 R_A means low mantle ³He flux typical of continental platforms far from active rift zones.     

Expanded orbitals in SCF-LCAO-MO computations and reliability of virtual orbitals

An experiment to measure the two-body collision-induced light scattering spectrum of Ar, CH₄, CD₄, CF₄ and SF₆ is described. Contrary to previous studies, the argon spectral intensity is shown to exceed that of the predictions of the first order dipole-induced-dipole model for the pair polarizability anisotropy. It is demonstrated empirically that this intensity can be accounted for by second-order quantum mechanical effects. The spectral profile is analysed in terms of various modifications to the pair polarizability. The main object of this study is to provide a basis for comparison with the molecular spectra described in paper II of this series.     

IR multiphoton absorption of SF6 in flow with Ar at moderate energy fluences

6 in flow with Ar (SF₆: Ar=1:100) in conditions of a large vibrational/rotational temperature difference (T_V≃230 K, T_R≃60 K) was studied at moderate energy fluences from ≃0.1 to ≃100 mJ/cm², which are of interest for isotope selective two-step dissociation of molecules. A 50 cm Laval-type slit nozzle for the flow cooling, and a TEA CO₂-laser for excitation of molecules were used in the experiments. The laser energy fluence dependences of the SF₆ MPA were studied for several CO₂-laser lines which are in a good resonance with the linear absorption spectrum of the ν₃ vibration of SF₆ at low temperature. The effect of the laser pulse duration (intensity) on MPA of flow cooled SF₆ with Ar was also studied. The results are compared with those obtained in earlier studies. Received: 4 September 1995/Revised version: 15 February 1996     

Vibrational relaxation in CD4 and CD4-rare gas mixtures

Using the optic-acoustic effect, measurements of the relaxation time were made after exciting the v₄ mode of CD₄. This was done for pure CD₄ and for mixtures of CD₄ with He, Ne, Ar and Kr, the results being 3.9±0.2, 1.8±0.2, 18±2, 70±15 and ? 100 μs at 1 atm, respectively. From the relaxation time dependence on mass as well as from a comparison between results for CH₄ and CD₄ it is concluded that energy transfer takes place via a vibration-rotation-translation (V-R-T) mechanism.Experiments were also conducted to study the most probable deexcitation path from the higher energy mode, v₃, to translation. This was done for CD₄-CD₄ as well as for CD₄-atom collisions. For pure CD₄ deexcitation occurs via the lower energy group (v₂, v₄), while for CD₄ mixtures it may also occur via the higher energy group (v₁, 2v₂, v₂ + v₄, 2v₄).     

Die Winkelabhängigkeit der Resonanzstreuung niederenergetischer Elektronen an He,Ne, Ar und N2

The energy dependence of the elastic resonance scattering of low energy electrons from He, Ne, Ar, and N₂ has been measured in the angular range from 8° to 110°. From the measurements are derived the total angular momenta and hence the configurations of the quasistationary negative ion states. The line shapes depend very strongly on the phase shifts for the potential scattering and on the interference of different partial waves. In the cases of He and Ne phase shifts are determined. The resonance structure in the elastic scattering from N₂ in the energy range from 1.75 eV to about 3,5 eV seems to be due to the formation of the N ₂ ^? -ion in its electronic ground state with a²Π configuration. The separations of its first few vibrational states are 237±10 meV, the natural half-width of these states being approximately 150 meV.     

Pressure-broadened iodine-laser-amplifier kinetics and a comparison of diluent effectiveness

An iodine laser kinetic model is used to investigate inversion parameters in highly pressure-broadened amplifier systems using i-C₃F₇I as the parent compound and CO₂, N₂, He, Ne, Ar, Kr, and Xe as the diluent gas. These data are used to calculate a diluent merit function for diluent pressures P_d = 1–100 atm. The most effective choices for buffer gases are as follows: P_d ? 1 atm, CO₂; 1 atm ? P_d ? 10 atm, Ar; and P_d ? 10 atm, Ne.     

The viscosity and diffusion coefficients of the binary mixtures of xenon with the other noble gases

This paper presents new experimental data for the viscosity of binary mixtures of xenon with the remaining monatomic gases, helium, neon, argon, and krypton. The measurements have been performed in a high precision oscillating-disk viscometer at atmospheric pressure and within the temperature range 25–500°C. The data have an estimated uncertainty of ±0.1% at 25°C increasing to ±0.3% at 500°C. The collision integrals for the interactions of xenon with the other monatomic species conform to the extended law of corresponding states formulated by Kestin, Ro and Wakeham. For each binary interaction the scaling parameters σ_ij and ∈_ij have been obtained. The ensemble of experimental results can be correlated by means of the appropriate kinetic theory expressions reinforced by the extended law of corresponding states. The deviations do not exceed 0.5%. The binary diffusion coefficients were calculated from the measured mixture viscosity and compared with the available experimental results. The standard deviation was estimated as ±2% which is within the mutual uncertainty of the two sets.     

克里奇中间体CH2OO与CF3CF=CF2反应的动力学研究

本文使用OH激光诱导荧光方法研究了结构最简单的克里奇中间体CH₂OO和CF₃CF=CF₂的反应动力学. 在压强为10 Torr条件下，测量了温度在283，298，308和318 K的反应速率常数，分别为(1.45±0.14)×10^-13，(1.18±0.11)×10^-13，(1.11±0.08)×10^-13和(1.04±0.08)×10^-13 cm³·molecule^-1·s^-1. 根据阿伦尼乌斯方程，获得该反应的活化能为(-1.66±0.21) kcal/mol. 在6.3∽70 torr压力范围内，未观察到该反应的速率常数存在压力相关.     

About Some Structural Differences of Certain Inorganic Salt Solutions in Natural and Heavy Water

Based on viscosity measurements of seven electrolyte soultions (KCl, K Br, KI, LiCl, Na₂CO₃, Li₂SO₄, and MnSO₄) in heavy water and the viscosity data of four electrolyte solutions (KCl, KBr, Kl, and Na₂CO₃) in ordinary water the activation energies of viscosity have been determined from the Pantchenkov equation. The dtermination were made at 25°, 35°, 45°, 60°, 75°, and 90°, at various concentrations up to near saturation. The activation energies of viscosity for pure heavy and ordinary water have been determined, too. The results allowed conclusions on the various influences of the investigated salts on the structure of heavy and ordinary water solutions.     

The excitation temperature functions of some spectrum lines of He,Ne, Ar,Kr and Xe

The excitation temperature functions of 19 spectrum lines of 5 rare gases He, Ne, Ar, Kr, Xe have been calculated according to the method given by Fowler and Milne. The results make it possible to determine the radial plasma temperature distribution in the range of temperatures of 1.4 × 10⁴°K to 14 × 10⁴°K. Some examples are given for the determination of the plasma temperature distribution in space and time by using the Ar ecxitation temperature functions calculated.     

Cross sections for production of vacuum ultraviolet radiation by 5–150-keV protons in gases

Ultraviolet light produced by collisions of 5–150 keV protons with He, Ne, Ar, H₂, N₂, O₂, and CO₂ was detected by allowing the radiation to fall directly on the first stage of a 17-stage electron multiplier with Cu-Be dynodes. The product of photon production cross sections and photoelectric yield of the first dynode integrated over wavelength was determined. Measurements were made at low enough pressures that corrections could be made for resonance absorption effects for the targets (He and Ne) for which they were important. The angular distribution of the emission for hydrogen gas was measured at 20, 50, and 100 keV and found to be isotropic.     

Near threshold effects in positron-atom (molecule) scattering

The energy dependence of the total cross-sections for ionization by positron impact has been measured for He, Ne, Ar, CO₂ and O₂. The interplay between this and other scattering channels has been investigated. The data for He, Ne and Ar has been interpreted using a combination ofR-matrix and threshold theories in the neighbourhood of the Ps formation threshold. In CO₂, an enhancement of the Ps formation probability is found when the remnant ion is left in an excited state. In O₂, structure has been observed which indicates channel-coupling with excitation to the Schumann-Runge continuum.     

Messung und Deutung des Leitwertabklingens zylindrischer Bögen

The decay of the electrical conductance of the column of a cascaded arc (100 A) after current interruption is measured for the gases N₂, SF₆, CO₂ and Ar. Numerical computations of temperature and conductance decay are in good agreement with experiment for Ar but for N₂ and SF₆ there are drastic deviations. An interpretation of the decay processes is given by the aid of a simple model for the energy transfer including nonequilibrium processes with regard to recombination in the wall near arc regions. Introducing the model by a formal manner in the computer program leads to good agreement with experiment.     

Multi-species laser absorption sensors for in situ monitoring of syngas composition

Tunable diode laser absorption spectroscopy sensors for detection of CO, CO₂, CH₄ and H₂O at elevated pressures in mixtures of synthesis gas (syngas: products of coal and/or biomass gasification) were developed and tested. Wavelength modulation spectroscopy (WMS) with 1f-normalized 2f detection was employed. Fiber-coupled DFB diode lasers operating at 2325, 2017, 2290 and 1352 nm were used for simultaneously measuring CO, CO₂, CH₄ and H₂O, respectively. Criteria for the selection of transitions were developed, and transitions were selected to optimize the signal and minimize interference from other species. For quantitative WMS measurements, the collision-broadening coefficients of the selected transitions were determined for collisions with possible syngas components, namely CO, CO₂, CH₄, H₂O, N₂ and H₂. Sample measurements were performed for each species in gas cells at a temperature of 25 °C up to pressures of 20 atm. To validate the sensor performance, the composition of synthetic syngas was determined by the absorption sensor and compared with the known values. A method of estimating the lower heating value and Wobbe index of the syngas mixture from these measurements was also demonstrated.     

Effect of buffer gas on the generation of eye-safe 1.54 μm radiation using the stimulated Raman scattering technique in CH4

Experimental results on the generation of 1.54 μm eye-safe radiation in pure CH₄, CH₄:He and CH₄:Ar mixtures pumped by the fundamental of an Nd:YAG laser, using the stimulated Raman scattering (SRS) technique, are described. A decrease in the energy conversion efficiency and degradation in the beam quality of S₁ was observed in pure CH₄ at high pump energies. This problem was overcome in CH₄:He and CH₄:Ar mixtures. Compared with the first Stokes (1.54 μm) energy conversion efficiency in pure CH₄, at a pump energy of 126 mJ, an enhancement of 50% in energy conversion efficiency was observed in the CH₄:Ar mixture (60% argon concentration) and as much as 100% in the CH₄:He mixture (60% helium concentration). The use of these buffer gas mixtures improved the spatial beam quality of the Stokes radiation considerably and also resulted in raising the pump threshold for optical breakdown of the Raman gain medium.