Rapid exchange of Ca2+ between the bulk and the coordination site of ascorbic acid radical anion as observed by ESR spectroscopy

The rate constant for dissociation of [Ca(AA⁻)]⁺, where (AA⁻) is the radical anion of ascorbic acid, has been measured by observation of specific line broadening in the ESR spectrum. Its value in DMSO at 298 K is 4.68 × 10⁵ s⁻¹. The mechanism for the line broadening is a combination of a g and a hyperfine shift upon coordination of the metal ion.     

Collisional deactivation Of Ca(4p3PJ) by barium atoms

Collisional deactivation of Ca(4p³P_J by barium atoms proceeds with a thermal cross section of 0.26±0.02 nm² at 850 K. No evidence for the corresponding deactivation of electronically excited Ca(4p³P_J) by Ca(4s¹S_o) was obtained. The optical lifetime of Ca(4p³P_J) was measured to be 0.33 ± 0.03 ms, in good agreement with previous studies.     

Polarization and velocity dependence of associative ionization in Na(3p)+Na(3p) collisions

We have excited Na atoms of two counterrunning thermal beams by means of linearly polarized laser light and have investigated associative ionization processes. To this end we measured the total ionization signal as a function of the angle ? between light polarization and the relative collision velocity. Velocities of the excited atoms were selected by exploiting the Doppler effect. We found an increasing polarization dependence at increasing collision velocities. At all velocities the preparation of the collision partners in the |M _j |=1/2 sublevel of the Na² P _3/2 state is most efficient in producing ionization.     

Collisions at thermal energy between metastable hydrogen atoms and hydrogen molecules: total and differential cross sections

A metastable hydrogen (deuterium) atom source in which groundstate atoms produced by a RF discharge dissociator are bombarded by electrons, provides a relatively large amount of slow metastable atoms (velocity 3–5 km/s). Total integral cross sections for H*(D*)(2s) + H₂(X ¹Σ _g ⁺ ,v=0) collisions have been measured in a wide range of relative velocity (2,5–30 km/s), by using the attenuation method. A significant improvement of accuracy is obtained, with respect to previous measurements, at low relative velocities. Total cross sections for H* and D*, as functions of the relative velocity, are different, especially in the low velocity range. H* + H₂ total differential cross sections have also been measured, with an angular spread of 3.6°, for two different collision energy distributions, centered respectively at 100 meV and 390 meV. A first attempt of theoretical analysis of the cross sections, by means of an optical potential, is presented.     

Contribution de la spectrometrie X à l'étude du manganèse dans les oxydes mixtes de structure spinelle: Analyse du spectre MnKβ et du seuil d'absorption K

Chemical shifts of Kβ_1,3 and Kβ₅ emission bands and X-ray absorption spectra near the K edge have been measured in several manganese spinel oxides with the metal in the formal oxidation states +2, +3, and +4. The position of line MnKβ_1,3 is determined mainly by the valence of manganese. The relative intensity of Kβ′ satellite with respect to the Kβ_1,3 line gives qualitative information about the presence of Mn(II) in mixed oxides. Mn(IV) oxides are characterized by a small chemical shift of the Kβ₅ band unlike Mn(II) and Mn(III) compounds. The first high resolution XANES spectra for these materials were performed at the DCI storage ring at LURE (Orsay, France). The chemical shifts ΔE (K absorption discontinuty) and ΔE_max (main peak) are correlated with the oxidation state of metal. Spectra of Mn³⁺ and Mn⁴⁺ ions in the octahedral environment are characterized by the splitting of 1s → 3d transitions (2 eV). In mixed oxides, the first Mn(II) 1s → 4s-4p transition is observed as a peak (or shoulder) located at 7 eV above the 1s → 3d transition. The study of the X-ray absorption fine structure in the near edge region can be used for qualitative solid-state analysis of mixed oxides such as NiMn₂O₄ or CuMn₂O₄.     

Burning velocity measurement of HFC-41, HFC-152, and HFC-161 by the spherical-vessel method

The burning velocities of fluoromethane (HFC-41), 1,2-difluoroethane (HFC-152), fluoroethane (HFC-161) and ethane were measured by the spherical-vessel (SV) method at room temperature and at initial pressures of 80-107 kPa over a wide range of HFC/air equivalence ratios (?). The burning velocities were determined from the measured pressure increases by application of a spherical flame model. Schlieren photography was used to directly observe flame propagation behavior in a cylindrical vessel equipped with optical windows. The time evolution of the flame radii derived from the pressure increases agreed with the time evolution observed with the Schlieren technique. The maximum burning velocities of HFC-41, HFC-152, HFC-161 and ethane were 28.3 cm s⁻¹ at ? = 1.01, 30.1 cm s⁻¹ at ? = 1.07, 38.3 cm s⁻¹ at ? = 1.07 and 40.9 cm s⁻¹ at ? = 1.05, respectively. The maximum burning velocities for the HFCs, including previously reported C₁ and C₂ fluoroalkanes, decreased with increasing F-substitution rate (the ratio of the number of F atoms to the sum of the number of H and F atoms). The concentrations of chemical species in the flames were investigated by means of an equilibrium calculation, and the results suggested that the burning velocity was correlated with the concentrations of H and OH radicals that were not deactivated by F radicals in the flame. The results also suggested that the burning velocities were linearly related to the heats of combustion of the C₁ and C₂ fluoroalkanes.     

Narrowing of spectral lines by energy pooling collisions in laser enhanced ionization spectroscopy

A remarkable narrowing of the Doppler profile of the calcium 4s ^{2 1} S ₀–4s4p ³ P ₁ line have been measured by laser enhanced ionization spectroscopy in a thermionic diode. By variation of the laser intensity and the calcium number density, and by analysis of the line profiles, there is evidence that energy pooling in higher states due to the collision of two and also three excited 4s4p ³ P ₁ atoms is responsible for the effect.     

Absolute detection of metastable rare gas atoms by a cw laser photoionization method

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔI _S in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔI _P (γ = ΔI _S/ΔI _P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s ³ P ₂), Ar* (4s ³ P ₂), and Kr* (5s ³ P ₂) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (2³ S, 2¹ S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.     

Collisions of metastable Ne*, He* atoms with ground-state He,Ne atoms studied by atomic beam and laser techniques

A crossed nozzle-beam experiment is used to investigate thermal energy collisions: Ne*(2p ⁵3s,³ P _{0, 2})+He(1s ²,¹ S ₀), almost purely elastic, and He*(1s2s,^{1, 3} S)+Ne(2p ⁶,¹ S ₀), in which inelastic excitation transfers occur. State and velocity selection of the scattered Ne* atoms is performed using a tunablecw dye laser frequency locked on a definite Zeeman component of the transition 1s ₅→2p ₆ (λ=614.3 nm) of²⁰Ne or²²Ne. In the purely elastic case, this technique allows the selection of one of the two final velocities, and then an unambiguous LAB-CM transformation. The differential cross section at 62 meV tallies on accords with a calculation using a single effective potential. In He* on Ne collisions, the main inelastic processes are endothermic excitation transfers from He*(2¹ S). Experimental results obtained at different energies (62, 95, 109, 124 meV) show that the transfers essentially result in levels 3s and 4d of Ne.     

Velocity dependence of collision broadening cross sections in NH3

The velocity dependence of collision broadening cross sections for both self-broadening and Xe broadening of an infrared transition in NH₃ has been studied by measuring laser saturation resonance linewidths for molecules with specified velocities along the laser propagation direction. For self-broadening, the velocity dependence is in accord with inelastic collisions due to a predominantly dipole-dipole interaction potential, with smaller contributions due to shorter range forces, whereas for Xe broadening, the magnitude and velocity dependence of the collision broadening cross section is in better agreement with velocity changing collisions in the pressure range studied.     

Lifetimes of the 4s4p 3 P 1 and 4s3d 1 D 2 states of Ca I

The lifetimes of the 4s4p ³ P ₁ and 4s3d ¹ D ₂ metastable states of Ca have been studied using the time-of-flight technique. Two kinds of observations were performed. First, the exponential decay of the fluorescence, using a (continuous) dc discharge for excitation and then the velocity distribution of the radiating atoms, using a pulsed discharge, were measured. From the combined results of these measurements the lifetimes were derived. The lifetimes of the 4s4p ³ P ₁ and 4s3d ¹ D ₂ states of Ca are determined to be 0.57±0.03 ms and 1.5±0.4 ms, respectively.     

Proton NMR studies of lanthanum nickel hydride: Structure and diffusion

The proton magnetic resonance spectrum of lanthanum nickel hydride LaNi_5.3H₆ was measured over the temperature range 118°K < T < 300°K. The second moment of the absorption at 118°K is M₂ = 13.4 ± 0.3 G². Several possible arrangements of the hydrogen atoms are discussed. Narrowing of the line above 140°K is analyzed in terms of proton diffusion and gives an activation enthalpy E = 21 ± 1 kJ mol^?1, NMR correlation time pre-exponential 0.2 ps < τ_c⁰ < 1.6 ps and a self diffusion coefficient at 300°K of 2 × 10^?12 m² s^?1 < D < 2 × 10^?11 m² s^?1.     

Fine structure of the 1s5f and 1s5g levels of He I

The fine structure of the 1s5f and 1s5g levels of He I was measured using microwave spectroscopy. The helium atoms were excited by ion impact, and the eleven allowed 1s5f ^2S+1 F _J –1s5g ^2S′+1 G _J , transitions near ν≈15 GHz were induced and detected by measuring the 1s4d–1s2p or 1s3d–1s2p spectral-line intensities of the impact radiation as a function of the microwave frequency. The measured transition frequencies are in accord with theoretical values and, except for one transition frequency, with earlier experimental data. The existing discrepancy between these earlier data and theory could be solved.     

Line broadening mechanisms in the low pressure laser-induced plasma

The spectral profiles of Ca and Rb lines have been studied in a laser induced plasma as a function of pressure (1–10 torr) and delay time with respect to the plasma initiation (1–10 μs). Measurements were made in a plasma induced by the 1064-nm output of a Nd:YAG laser on a calcium carbonate matrix, doped with Rb. Spectral profiles were measured in absorption using a narrow-band cw Ti:Sapphire laser. It was shown that in the case of a trace element (Rb in a CaCO₃ matrix), the broadening mechanism was Doppler-dominant, whereas for a major matrix component (Ca), resonance broadening was the main contributor to the line shape. The plasma was shown to be non-equilibrium provided by the difference between the kinetic (3000 K) and the excitation (8000 K) temperatures. The electron number density at delay times of 5–10 μs and pressures of 1–10 torr was estimated not to exceed 10¹⁵ cm⁻³. The number densities of Ca atoms in the ground and the excited (23 652 cm⁻¹) states were evaluated by measuring line width and peak absorption at 732.6 nm. They were found to be in the range of (1.5–2.2)×10¹⁷ cm⁻³ for the ground state and (1.5–33)×10¹¹ cm⁻³ for the excited state.     

Thermally induced incomplete high-spin (5T2) ⇌ low-spin (1A1) transition in solid bis[2-(2-pyridylamino)-4-(2-pyridyl) thiazolato]iron (II)

The ⁵⁷Fe Mössbauer effect in two samples (A and B) of [Fe(papt)₂] and in its solvates with CHCl₃ and C₆H₆ has been studied between 4.2 and 343 K and clearly indicates a temperature induced high-spin (⁵T₂) ? low-spin (¹A₁) transition in these compounds [paptH = 2-(2-pyridylamino)-4-(2-pyridyl) thiazole]. At 343 K, sample B shows a doublet with ΔE_Q = 2.03 mm s^?1 and δIS = +0.87 mm s^?1, characteristic of a ⁵T₂ ground state. At 257 K, a second doublet, typical for a ¹A₁ ground state, is observed and its intensity increases as the transition progresses but levels off below ～ 100 K. At 4.2 K, 83% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.56 mm s^?1 and δ^IS(¹A₁ = +0.32 mm s^?1. In an applied magnetic field, V_zz(¹A₁) < 0 and η ≈ 0.7 have been determined, whereas for the ^sT₂ ground state, V_zz(^sT₂) > 0, η ≈ 0.75, and an internal hyperfine field H_n ≈ ?13 kG have been observed. Similar results have been obtained with the other samples.Debye-Waller factors f5_T2 and f1_A1 were determined from the saturation corrected areas in the Mössbauer spectra, assuming Curie-Weiss dependence of the magnetic susceptibility for the ⁵T₂ and constant υ_cff for the ¹A₁ ground state. The temperature dependence of ?In f1_A1 closely follows the Debye model with Θ1_A1 = 165 K, whereas the same applies to ?ln f5_T2 only above ～ 210 K and Θ5_T2 = 134 K. The nature of the observed transition is discussed and the data presented are shown to be incompatible with a model based on a Boltzmann distribution between the two states.     

Symmetry of the dissociation of hydrogen: angular dependence of the doppler profile of the excited hydrogen atom (n = 4) produced in e-H2 collisions

The angular dependence of the Doppler profile of the Balmer-β line indicates that the asymmetry parameter, b, is positive and the polarization of the electric vector, J_p, is 0.7 ± 0.1 for the formation of H¹(n=4) from H₂. Thus, H¹(n=4) is produced in a parallel transition, and the transition moment of H¹ lies along the dissociation axis. This result suggests that the intermediate states for the fast and anisotropic H¹(n=4) atoms should be of the type ¹Σ⁺_u(2pσ_u)(n/σ_g).     

57Fe Mössbauer effect study of a presumed singlet-triplet transition in [Fe(P4)X]BPh4 complexes,X = Br,I

The ⁵⁷Fe Mössbauer effect in the solid compounds [Fe(P₄)X]BPh₄ (X = Br, I; P₄ = hexaphenyl-1,4,7,10-tetraphosphadecane) has been studied between 4.2 and 298 K. A single temperature dependent doublet is observed. For the iodo complex, ΔE_Q = 2.25 mm s^?1, δ^IS = +0.13 mm s^?1, V_tzz 0, η ≈ 0.8 at 4.2 K, whereas ΔE_Q = 1.81 mm s^?1, δ^IS = +0.20 mm s^?1 at 298 K. The results are consistent with a singlet-triplet transition with k > 10⁹ s^?1.     

Precision measurement of the 1s 4f fine structure of HeI

Helium atoms were excited by hydrogen-ion impact, and electric dipole transitions between Stark substates of the 1s 4f multiplet of HeI were induced. Resonance signals were investigated at various frequencies 200 MHz<ν<800 MHz by scanning a static electric fieldF ₀?1 kV/cm and observing the intensity of the impact radiation for the singlet or triplet 1s 3d?1s 2p spectral line. From these measurements the following zero-field transition frequencies of the 1s 4f fine structure were deduced: ν(¹ F ₃?³ F ₂)=232.2(1.0) MHz; ν(³ F ₃?³ F ₄)=214.0(1.3) MHz; ν(¹ F ₃?³ F ₄)=490.6(0.4) MHz. Using calculated parameters of the magnetic fine-structure coupling, the exchange integral 2K=158.0(0.6) MHz of the 1s 4f configuration was evaluated.     

Densities, viscosities, and ultrasonic velocity studies of binary mixtures of trichloromethane with methanol, ethanol, propan-1-ol, and butan-1-ol at T = (298.15 and 308.15) K

Densities, viscosities, and ultrasonic velocities of binary mixtures of trichloromethane with methanol, ethanol, propan-1-ol, and butan-1-ol have been measured over the entire range of composition, at (298.15 and 308.15) K and at atmospheric pressure. From the experimental values of density, viscosity, and ultrasonic velocity, the excess molar volumes (V^E), deviations in viscosity (Δη), and deviations in isentropic compressibility (Δκ_s) have been calculated. The excess molar volumes, deviations in viscosity and deviations in isentropic compressibility have been fitted to the Redlich-Kister polynomial equation. The Jouyban-Acree model is used to correlate the experimental values of density, viscosity, and ultrasonic velocity.     

Polarization and collision velocity dependence of associative ionization in cold Na(3p)-Na(3p) collisions

We studied the polarization dependence of the associative ionization (AI) process Na(3p) + Na(3p) → Na ₂ ⁺ at collision velocities between 100 and 700 m/s (5 and 200 K), using linearly and circularly polarized light for the excitation. We found that the polarization dependence varies strongly in the collision velocity range under study. At the high end of our scale preparation of both collision partners in the |m _j |=1/2 substates yields the highest AI-rate, but at low velocities this ceases being so: the polarization dependence becomes less pronounced and at the low end of our scale the preparation |m _j 2|, |m _j 2|=3/2, 1/2 is most effective. All cross sections increase strongly at lower velocities. Combined with previous work this maps the detailed polarization dependence from 100–2400 m/s (5-2500 K). From these total results it is concluded that at collision velocities ≧500 m/s mainly one molecular potential curve leads to AI, but that this changes considerably at lower collision velocities. The findings are compared with recent theoretical results by Geltman, and we find large discrepancies.