Theoretical and experimental study of V–I characteristics of UV pre-ionized TEA CO2 laser for variety of laser gas mixtures

Experimental and theoretical study of V–I characteristics of UV pre-ionized TEA CO₂ laser has been carried out for a variety of gas mixtures emitting different optical pulse shapes suitable for various applications. Coupled differential equations have been solved to model the pulse excitation circuit using the numerically calculated values of ionization coefficient (α), attachment coefficient (β) and drift velocity (U_d) as functions of E/N (i.e. electric field to neutral particle density ratio) for chosen gas mixture. Calculated and experimental V–I characteristics for gas mixtures (CO₂:N₂:He::1:2:3, 1:1:4, 1:1:5 and 1:0:4.7) show a good agreement. It has been shown that gas mixture has a dominant effect on the delay between pre-ionization and main discharge; thus, determining the long-term stability of discharge. The excitation pulse duration increases with increase in molecular content of gas mixture (i.e. amount of CO₂ and N₂ in gas mixture).     

A combined ab initio, Fourier transform microwave and Fourier transform infrared spectroscopic investigation of β-propiolactone: The ν8 and ν12 bands

The pure rotational spectrum of β-propiolactone (c-C₂H₄COO) has been recorded between 7 and 21 GHz using a pulsed jet Fourier transform microwave spectrometer. The resulting ground state spectroscopic constants guided the analysis of the rotationally-resolved infrared spectra of two bands that were collected using the far infrared beamline at the Canadian Light Source synchrotron. The observed modes correspond to motions best described as ring deformation (ν₁₂) at 747.2 cm⁻¹ and CO ring stretching (ν₈) at 1095.4 cm⁻¹. A global fit of 4430 a- and b-type transitions from the microwave spectrum and the two infrared bands provided an accurate set of ground state and excited state spectroscopic parameters. To complement the experimental results, the harmonic and anharmonic vibrational frequencies of all 21 infrared active modes of β-propiolactone have been calculated using the DFT B3LYP method (6-311+G(d,p), 6-311++G(2d,3p) basis sets).     

Structure of odd–odd La at high spin

The high spin states in the N=79 odd–odd ¹³⁶La nucleus have been investigated by in-beam γ-spectroscopic techniques following the ¹³⁰Te(¹¹B, 5n)¹³⁶La reaction at E=52 MeV using an array, consisting of eight Compton-suppressed clover germanium detectors. Thirty nine new γ rays have been assigned to ¹³⁶La on the basis of γ ray singles and γγ-coincidence data. The level scheme of ¹³⁶La has been extended above the known 115 ms isomer upto an excitation energy of 4.6 MeV and spin 18. Thirty one new levels have been proposed and spin-parity assignments for most of the newly proposed levels have been made on the basis of the deduced asymmetry ratios and polarisation information for the de-exciting transitions. The observed positive parity yrast band has been compared with the theoretical calculation, done within the framework of particle rotor coupling model (PRM) where the two odd quasi-particles are coupled to an axially symmetric core. The level structure has been discussed in the light of the known systematics of the neighbouring N=79 isotonic nuclei.     

Gamma-ray spectroscopy of Cr

A study of low-lying states in the self-conjugate nucleus ⁴⁸Cr has been performed with the reaction ⁴⁰Ca(¹⁰B, pnγ)⁴⁸Cr. Several techniques of γ-ray spectroscopy have been used: activity measurements, γ-ray angular distributions, γ-γ coincidences, p-n-γ triple coincidences, and recoil-distance lifetime measurements. The following level energies, spin-parity assignments, and lifetimes have been found: 752.4±0.5 keV, 2₁⁺, τ_m = 9.7±2.6 ps; 1858.7±0.7 keV, 4₁⁺, τ_m = 1.5_−0.5^+2.0 ps; 3532.7±1.2 keV, (6₁⁺); 4064.3±1.3 keV, ( 4), τ_m < 60 ps. The 2₁⁺ state in ⁴⁸Cr is found to have the lowest excitation energy and highest E2 transition strength (33±9 W.u.) among the 2₁⁺ states of even nuclei in the 1f shell.     

Effects of the interaction between R and Fe modes of the magnetic resonance in RFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> rare-earth iron borates

Resonance modes that are due to magnetic excitations in the exchange-coupled subsystems of rare-earth ions (R = Nd³⁺, Sm³⁺, and Gd³⁺) and Fe³⁺ ions have been detected in submillimeter transmission spectra (0.1–0.6 THz) of RFe₃(BO₃)₄ iron borate-multiferroic single crystals. The strong interaction between spin oscillations of the Fe and R subsystems has been revealed, which determines the behavior of the modes depending on the anisotropy of the exchange splitting of the ground doublet of the R ion. It has been shown that the intensities of coupled modes (contributions to the magnetic permeability) depend strongly on the difference between the g factors of Fe and R ions. This dependence makes it possible to determine the sign of the latter g factor. In particular, a noticeable intensity of exchange Nd modes in NdFe₃(BO₃)₄ is due to an increase in their contribution at g _{⊥, ‖}^Nd < 0, while in GdFe₃(BO₃)₄ with g _Gd ≈ g _Fe ≈ 2, the Fe and Gd contributions compensate each other and the exchange (Gd) mode is not observed. In spite of the weak interaction of Sm ions with the magnetic field, SmFe₃(BO₃)₄ exhibits resonance modes, which are attributed to the excitation of Sm ions through the Fe subsystem.     

Isoscalar quadrupole strength in Ca from the (p,p′α0) reaction at Ep = 100 MeV

The ⁴⁰Ca(p,p′ α) reaction has been studied at an incident proton energy E_p = 99.5 MeV for proton laboratory scattering angles Θ_p^lab = 17°, 23° and 27°. Emission of α particles coincident with the scattered proton has been measured for an angular range Θ_α 0° − 180° relative to the recoil axis. A multipole decomposition for the α₀-decay channel to the ³⁶Ar ground state has been performed from the angular-correlation functions. The energy distribution of the dominating E2 strength deduced in the excitation energy range E_x = 11–21 MeV agrees reasonably well with the results from electron and α-induced α₀-decay investigations. The exhaustion of the E2 energy-weighted sum rule in this channel up to an energy of 17 MeV is 16.1(4.0)%, in accord with the study of the (α, α′ α₀) reaction. However, this value is twice what is found in the (e,e′ α₀) experiment in the same energy region. Thus, the puzzling discrepancy in the E2 strengths derived from electromagnetic and hadronic probes remains unsolved.     

Energy dependence of the quasiparticle lifetime in a 2DES

We present a novel technique for measuring the lifetime of quasiparticle excitations of a 2DES by investigating the tunnelling into a quantum dot from a 2DES over an extended range of energy from the Fermi energy to the sub-band edge. We find that the lifetime τ_qp, of a quasihole excitation, caused by removing an electron from a 2DES state with energy below the Fermi energy E_F, has the form τ_qp=α/, where α is a constant of order unity.     

Experimental investigation of the line center of Hg intercombination spectral line 253.7 nm perturbed by Kr

The collision broadening and shift of the Hg intercombination spectral line 253.7 nm (6¹S₀–6³P₁) perturbed by Kr has been investigated using a high-resolution scanning Fabry–Perot interferometer. The values of the pressure broadening and shift coefficients β and δ, respectively, for the studied line have been obtained. The obtained coefficients β and δ are compared with their corresponding published experimental values and also those calculated using Lindholm–Foley impact theory.     

Lifetimes and branching ratios of excited states of Na

The decay of ²⁴Na levels below 4.3 MeV excitation was studied by means of the ²³Na(d, pγ)²⁴Na reaction at E_d = 2.45 MeV. Gamma-ray spectra were measured at three angles, in coincidence with proton groups detected around 180°. Excitation energies, branching ratios and Doppler shifts were determined. Mean lives were obtained for the levels at 1341 keV (62±15 fs), and 1846 keV (200±50 fs). The 1347 keV level has τ >3 ps. For other levels above 1 MeV upper limits of ≈ 60 fs are set. In some cases spin restrictions follow. In particular J = 2 is assigned to the 1341 keV level.     

The Fe(α, γ)Ni reaction at excitation energies in the region of the giant dipole resonance

Excitation functions for the ⁵⁶Fe(α, γ₀)⁶⁰Ni and ⁵⁶Fe(α, γ₁)⁶⁰Ni reactions have been measured at 90° to the beam direction over the bombarding energy range 8.0–17.6 MeV. Gamma-ray angular distributions were measured at ten bombarding energies. Excitation functions for the ⁵⁹Co(p, γ₀)⁶⁰Ni and ⁵⁹Co(p, γ₁)⁶⁰Ni reactions were measured over the range E_x = 16.58–16.92 MeV and compared with the (α, γ) data. The angular distribution data indicate that the (α, γ₀) and (α,γ₁) reactions proceed through 1⁻, and 1⁻ and 3⁻ states, respectively. The (α, γ) excitation functions are discussed with respect to isospin splitting of the ⁶⁰Ni giant dipole resonance. The fine structure observed in the excitation functions is shown to be most probably due to Ericson fluctuations. The gross (α, γ) cross sections are shown to be in reasonable agreement with the results of calculations made using the theory of Hauser and Feshbach assuming excitation of the giant dipole resonance.     

Photoluminescence properties of thenardite activated with Eu

Na₂SO₄:Eu phosphors were prepared by heating pure natural thenardite with EuF₃ at 900 °C for 20 min in air. The photoluminescence (PL) and excitation spectra of as-prepared and γ-ray-irradiated phosphors were observed at 300 K. The PL spectrum under 394 nm excitation consisted of strong narrow bands with peaks at 579, 592, 616, 652, 697 and 741 nm, assigned to the ⁵D₀→⁷F_J (J=0, 1, 2, …, 5) transitions, respectively, within Eu³⁺. The PL spectrum under 340 nm excitation consisted of a broad Eu²⁺ band with a peak at 435 nm. The excitation spectrum obtained by monitoring the violet luminescence consisted of a weak band with a peak at approximately 261 nm and a broad Eu²⁺ band with a peak at approximately 338 nm. The relative efficiency of the violet luminescence of the γ-ray-irradiated phosphor at the exposure of 46 kGy increased up to 3.0 times that of the unirradiated phosphor. The enhancement of violet luminescence by γ-ray irradiation was ascribed to the conversion of Eu³⁺ to Eu²⁺ in Na₂SO₄.     

Isotope and Hyperfine Structure in the "Orange" System of FeO: Evidence for Two Δi Excited States

The orange system of FeO has been reinvestigated using low-temperature molecular beam laser-induced fluorescence spectra, obtained by supersonic jet cooling. Two new weak bands have been found, and analyses of some of the previously known bands extended. Measurements of the ⁵⁴Fe-⁵⁶Fe isotope shifts have been made for most of the bands, and the hyperfine structure of the low-J lines has been recorded for two of the strongest bands of ⁵⁷FeO. The isotope shifts are consistent with the presence of two ⁵Δ_i-⁵Δ_i transitions lying within 1000 cm⁻¹; the origins of the Ω = 4 spin components lie at 5583 and 6110 Å, respectively. The hyperfine patterns and the spin-orbit structure indicate that the upper state electron configurations are (3dδ)³ (3dπ)² (3dσ)¹, (D⁵Δ_i, 5583 Å) and O(2pπ)³ (4sσ)¹ (3dδ)³(3dπ)³, (D′⁵Δ_i, 6110 Å). The bond length in the D′ state (r₀ = 1.654 Å) has been obtained from a deperturbation of the 6110 Å band; it is only 0.035 Å longer than in the ground state, which indicates that electron promotion between the two π orbitals, nominally O(2pπ) and Fe(3dπ), has only a small effect on the strength of the bonding. The new isotope data still do not clarify the vibrational assignments of the higher levels, which are disorganized by extensive electronic perturbations.     

Oxalyl halides : Analysis of the 3480 Å absorption system of oxalyl chloridefluoride

Oxalyl chloridefluoride (COCl)(COF) exhibits moderately strong discrete absorption in the 3050–3540Å region. The band spectrum has been analyzed as an allowed electronic transition of the planar trans molecule. The most active vibrations are the carbonyl stretching modes ν₁′ and ν₂′ and the in-plane bending mode ν₉. Various other fundamental frequencies in the combining electronic states have been identified. The 0₀⁰ band is at 28 724.5 cm⁻¹; partial rotational analysis confirms that this band is type C. The appearance of “line” structure in the wings of the band is discussed and an explanation offered. The vibrational and rotational analyses confirm that the transition is under the C_s point group, as expected for a singlet-singlet n → π^* type of excitation.     

Status of the CHORUS neutrino oscillation experiment

The CHORUS experiment is designed to search for ν_μ → ν_τ oscillation with a hybrid detector system containing 800 kg nuclear emulsions as target and vertex detector. Run I (320 000 recorded ν_μCC in 1994/5) and more than half of the run II (460 000 ν_μCC in 1996/7) data taking have been successfully completed. Approximately 80 000 events have been analyzed so far, searching for and τ⁻ → h⁻ (nπ⁰) ν_τ decays. No candidate has been found, leading to a limit sin² 2θ_μτ ≤ 4.5 10⁻³ for large Δm².     

Conductance studies of NaCl, KCl, NaBr, NaI, NaBPh4, Bu4NI and NaClO4 in water + 2-methoxyethanol mixtures at 298.15 K

The electrical conductances of the solutions of sodium chloride (NaCl), potassium chloride (KCl), sodium bromide (NaBr), sodium iodide (NaI), sodium tetraphenylborate (NaBPh₄), tetrabutylammonium iodide (Bu₄NI) and sodium perchlorate (NaClO₄) in water (1) + 2-methoxyethanol (2) mixtures containing 0.01, 0.025, 0.05, 0.075, 0.10, 0.15 and 0.20 mol fractions of 2-methoxyethanol have been reported at 298.15 K. The conductance data have been analyzed by the Fuoss–Justice equation. The individual limiting ionic conductivities of Na⁺, K⁺, Bu₄N⁺, BPh₄⁻, I⁻, Cl⁻, and Br⁻ ions have been determined using the Fuoss–Hirsch assumption. The dependencies of the limiting molar conductances, Λ_o, and Walden products, Λ_oη, versus mixed solvent composition have been discussed.     

High-Resolution Laser Spectroscopy of YbCl: The AΠ–XΣ Transition

The A²Π–X²Σ⁺ transition of ¹⁷⁴Yb³⁵Cl and ¹⁷²Yb³⁵Cl has been rotationally analyzed for the first time. Doppler-limited laser excitation spectroscopy with selective detection of fluorescence was used to obtain spectra of the 0–0 and 1–0 bands with a measurement accuracy of approximately 0.0035 cm⁻¹. Resolved fluorescence was used to record the 0–1, 0–2, and 0–3 bands and to unequivocally assign the rotational numbering, N, to the laser excitation spectra. In total, over 1300 line positions have been measured and assigned for each of the two isotopomers and employed in least-squares fits of molecular parameters. The principal results for the A²Π state are A_e = 1491.494(2) cm⁻¹ and R_e = 2.4433(1) Å, and for the X²Σ⁺ state, R_e = 2.4883(2) Å and γ_e = 4.59(2) × 10⁻³ cm⁻¹. The interaction between the X²Σ⁺ and A²Π states has been investigated and is shown to be the main contributor to the spin–rotation splitting in the ground state.     

Theory of chemical bonds in metalloenzymes XX: magneto-structural correlations in the CaMn4O5 cluster in oxygen-evolving complex of photosystem II

ABSTRACT

Magneto-structural correlations in oxygen-evolving complex (OEC) of photosystem II (PSII) have been elucidated on the basis of theoretical and computational results in combination with available electron paramagnetic resonance (EPR) experimental results, and extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) results. To this end, the computational methods based on broken-symmetry (BS) UB3LYP solutions have been developed to elucidate magnetic interactions in the active manganese catalyst for water oxidation by sunlight. The effective exchange interactions J for the CaMn(III)Mn(IV)₃O₅(H₂O)₃Y(Y = H₂O or OH^?) cluster (1) model of OEC of PSII have been calculated by the generalised approximate spin projection (GAP) method that eliminates the spin contamination errors of the BS UB3LYP solution. Full geometry optimisations followed by the zero-point energy (ZPE) correction have been performed for all the spin configurations of 1 to improve the J values that are compared with accumulated EPR in the S₂ state of Kok cycle and magnetic susceptibility results of Christou model complex Ca₂Mn(IV)₃O₄ (2). Using the calculated J values, exact diagonalisation of the spin Hamiltonian matrix has been carried out to obtain excitation energies and spin densities of the ground and lower excited states of 1. The calculated excitation energies are consistent with the available experimental results. The calculated spin densities (projection factors) are also compatible with those of the EPR results. The calculated spin densities have been used to calculate the isotropic hyperfine (A_iso) constants of ⁵⁵Mn ions revealed by the EPR experiments. Implications of the computational results are discussed in relation to the structural symmetry breaking (SSB) in the S₁, S₂ and S₃ states, spin crossover phenomenon induced by the near-infrared excitation and the right- and left-handed scenarios for the O–O bond formation for water oxidation.     

Magnetische untersuchungen an mischkristallen der quasibinären legierungssysteme (Cd Te)1-x (Mn Te)x und (Sn Te)1-x (Mn Te)x

The magnetic susceptibility and magnetization are presented for polycrystalline samples of the alloy systems Cd_1-xMn_xTe 0 < x 0.1 and Sn_1-xMn_xTe 0 < 4 0.4. The magnetic measurements were performed between 2.3 K and 300 K in external magnetic fields up to 11 kOe. At sufficiently high temperatures the susceptibility can be described by a Curie-Weiss law. In the system Sn_1-xMn_xTe θ_p is positive. A linear dependence θ_p ∞ x was found with θ_p(0.4) = 49 K. In the series Cd_1-xMn_xTe θ_p changes sign. For θ < x < 0.04 θ_p is positive with a maximum θ_p ≈ 10 K at x = 0.02. In the region x #62; 0.04 θ_p becomes negative with θ_p = -35 K at x = 0.1. The effective spin value of manganese is S_eff #62; 5/2 for all the samples. The investigation was done to check the assumption that ferromagnetic coupling may exist in tellurides of manganese if the shortest distance d_MnMn is greater than 3.4 Å. This hypothesis has been stated. In the case θ_p #62; 0 the results are partly explained by the RKKY exchange coupling.     

Al nuclear magnetic resonance study of synthetic and natural corundum (α-Al2O3) Some experimental aspects of quantitative Al nuclear magnetic resonance spectroscopy

The ²⁷Al nuclear magnetic resonance (NMR) response of a series of natural and synthetic corundum (α-Al₂O₃) samples is studied quantitatively by short-pulse excitation and frequency-stepped adiabatic half-passage (FSAHP). Using on- and off-resonance nutation NMR, it was established that the quadrupole coupling parameters of visible Al is identical in all samples. Remarkably, the relaxation behavior for the aluminum is very different in the various samples and has a marked effect on the quantitative response. In natural corundum samples the ²⁷Al spin-lattice relaxation is very efficient as these samples contain paramagnetic impurities. As a result, however, the full signal could not be recovered, which is attributed to relaxation broadening of spins in the vicinity of these impurities. In synthetic samples, containing no impurities, the full signal could be recovered, although the relaxation behaviour appeared to depend strongly on the preparation method. We observed differences in the spin-lattice relaxation by a factor 20; the longest T₁ was observed in a crushed single crystal. This implies that α-Al₂O₃ can only be used as a standard in quantitative analyses if it has been characterized thoroughly. Furthermore, the effective relaxation behaviour for different types of excitation is studied. Finally, a method to measure the spin-lattice relaxation of half-integer quadrupole nuclei is introduced, using a frequency-stepped adiabatic passage (FSAP) to invert the spin system.