On the electronic structure of N,N′-ethylenebis(acetylacetonatiminato)Co(II), Co(II)acacen

Single crystal E.P.R. and cobalt ENDOR measurements on N,N′-ethylenebis(acetylacetonatiminato)Co(II), Coacacen, diluted in Niacacen·1/2 H₂O are reported. Forbidden Δm _Co = 1,2 transitions in the E.P.R. spectra have been observed. The g-tensor (E.P.R.) and the cobalt hyperfine and quadrupole tensors (ENDOR) have been determined. The g- and cobalt hyperfine tensors are discussed. They support the |² A ₂,yz? groundstate proposed for four coordinated low-spin Schiff base complexes of cobalt(II). The measured quadrupole coupling is also compatible with a |² A ₂,yz? groundstate, if anisotropic contraction of the cobalt 3d orbitals is taken into consideration.     

Single-crystal EPR study and DFT structure of the [Mo(CO)5PPh3]+· radical cation

A radical species characterized by a large g-anisotropy and a clearly resolved hyperfine structure with ^95/97Mo and ³¹P nuclei is formed, at 77 K, by radiolysis of a single crystal of Mo(CO)₅PPh₃. The corresponding EPR signals disappear irreversibly with increasing temperature and the angular dependence of the various coupling constants imply a spin delocalization of ～60% and ～4% on the molybdenum and the phosphorus atoms, respectively and are, a priori, consistent with the trapping of a one-electron deficient centre. The ability of DFT to predict the EPR tensors for a 17-electron Mo^(I) species is verified by calculating the g-tensor and the various ¹⁴N and ¹³C coupling tensors previously reported by Hayes for [Mo(CN)₅NO]^3?. Calculations at the B3LYP/ZORA/SOMF level of theory show that, in contrast to Mo(CO)₅PH₃, one-electron oxidation of Mo(CO)₅PPh₃ causes an appreciable change in the geometry of the complex. The g-tensor and the ^95/97Mo and ³¹P isotropic and anisotropic coupling constants calculated for [Mo(CO)₅PPh₃]^+· confirm the trapping of this species in the irradiated crystal of Mo(CO)₅PPh₃; they also show that the conformational modifications induced by the electron release are probably hindered by the nearby complexes.     

A study of high spin states in the transitional nucleus90Mo

The reaction⁵⁸Ni(³⁶Ar, 4p)⁹⁰Mo has been studied at a beam energy of 149 MeV. A detector array consisting of the OSIRIS spectrometer (12 Compton-suppressed Ge detectors), four charged-particleδE detectors and seven NE-213 neutron detectors at the VICKSI accelerator in Berlin has been used to measure the gamma radiation inγγ- and particle-γγ-coincidence mode. An additional Ge detector was placed at 162? to the beam direction to provide information on DCO ratios. The level scheme of⁹⁰Mo has been extended up to an excitation energy of about 12 MeV and probable spin 23?. Some 70 transitions and 40 levels have been newly identified. Spin assignments have been proposed on the basis of measured DCO ratios. Shell model calculations in a model space consisting of the proton 1f _5/2, 2p _3/2, 2p _1/2, and 1g _9/2 orbits and the neutron 2p _1/2, 1g _9/2, 1g _7/2, 2d _5/2, 2d _3/2, and 3s _1/2 orbits with some truncation were made for states above 9? and the predicted structure of these states is discussed.     

Electronic Absorption Spectrum of Fe3+ Doped in Ammonium Perchlorate Single Crystal

Abstract

Single crystals of Iron - doped ammonium perchlorate were grown at room temperature. The electronic absorption bands observed at room and liquid air temperatures have been assigned transitions from the ground ⁶A_1g state to the excited ⁴A_1g, ⁴E_g, ⁴T_1g and ⁴T_2g states. The crystal field parameters Dq = 870 cm^?1, B = 615 cm^?1 and C = 4.2 B are found to give a good fit to the observed band positions.     

Magnetic moments of the 2 1 + states in102, 104Mo

Theg factors of the first excited 2⁺ levels in the neutron-rich nuclei¹⁰²Mo and¹⁰⁴Mo have been studied through the measurement of the perturbed angular correlations for theγ-γ cascades between the 0 ₂ ⁺ -2 ₁ ⁺ -0 ₁ ⁺ level sequences. The results of g=0.42±0.07 for¹⁰²Mo and ofg= _?0.11 ^+0.12 for¹⁰⁴Mo agree with the prediction of the vibrational-rotational model. In terms of IBA, with the assumption ofN _π=3 for the Mo isotopes which takes into consideration a two particle, two-hole excitation across the Z=40 subshell, the proton-bosong factor is deduced to beg _π=1.00±0.23. It is shown that this value provides evidence for subshell effects in¹⁰⁰Zr.     

The momentum constraints of general relativity and spatial conformal isometries

Transverse-tracefree (TT-) tensors on (R ³,g _ab), withg _ab an asymptotically flat metric of fast decay at infinity, are studied. When the source tensor from which these TT tensors are constructed has fast fall-off at infinity, TT tensors allow a multipole-type expansion. Wheng _ab has no conformal Killing vectors (CKV's) it is proven that any finite but otherwise arbitrary set of moments can be realized by a suitable TT tensor. When CKV's exist there are obstructions — certain (combinations of) moments have to vanish — which we study.Supported by Fonds zur Förderung der wissenschaftlichen Forschung in Österreich, Project No. P9376-PHY.Partially supported by Forbairt Grant SC/94/225.     

Critical consideration of the statistical model analysis of photonuclear isomeric cross-section ratios

We have measured isomeric cross-section ratios for the photonuclear reactions ¹⁸¹Ta(γ, 3n) ^178m,gTa, ¹⁴²Nd(γ, n)^141m,gNd, ¹¹⁰Pd(γ, n)^{109m₁,m₂,g}Pd, ¹⁰⁸Pd(γ, n)^107m₁,m₂Pd, ¹⁰⁰Mo(γ, n) ^99m,gMo, ⁹²Mo(γ, n)^91m,gMo and ⁸⁹Y(γ, n)^88m₁,m₂Y. The experimental results were compared with statistical model calculations according to the method of Huizenga and Vandenbosch. The applicability and limits of this method, especially the simple decision model for the population of the isomeric state and the ground state of the residual nucleus, were tested and discussed. Additionally, it is shown that for photonuclear reactions on magic nuclei, contributions of a direct reaction mechanism are not negligible.     

Spectroscopic properties of alexandrite crystals II

Several aspects of the optical spectroscopic properties of alexandrite crystals not previously investigated are reported here. For Cr³⁺ ions occupying the Al³⁺ lattice sites with mirror symmetry in BeAl₂O₄, the positions of the zero-phonon lines for absorption transitions to the ²T_1g and ⁴T_2g levels are identified, and vibronic transition peaks in the fluorescence spectrum are compared to transitions appearing in the Raman spectrum and Stokes excitation spectrum. In addition, the effects of radiation trapping are shown to lengthen the fluorescence lifetime of the ²E_g - ⁴A_2g transition for ions in these sites at low temperatures. For Cr³⁺ ions occupying the Al³⁺ lattice with inversion symmetry, the ground state splitting of the ²E_g - ⁴A_2g transition is reported and the decrease of the fluorescence lifetime with temperature is shown to be due to the increase in vibronic emission probability as well as increased probability of direct radiationless decay.     

Excitation of the 5p<Superscript>5</Superscript>7p levels of xenon by electron impact

We have used our relativistic distorted-wave method to calculate cross sections for the electron-impact excitation of the ground state of xenon to all the 5p ⁵7p fine-structure levels. The results are compared with the recent experimental measurements of Jung et al. [Phys. Rev. A 80, 062708 (2009)]. Analytic fits to our cross sections are also provided for use in plasma modeling studies.     

EPR Study of Single Crystals of PROXYLs

Electron paramagnetic resonance study of the single crystals of two PROXYLs, trans-2,5-bis(4-methylphenyl)-2,5-dimethylpyrrolidine-1-oxy [(±)-1] and trans-2-(4-methylphenyl)-2,5-dimethyl-5-phenylpyrrolidine-1-oxy [(±)-2] was conducted. The obtained spectra were always fitted well by a single Lorentzian. The principal g-values (g _x , g _y , g _z ) of (±)-1 and (±)-2 were determined as (2.00990, 2.00639, 2.00266) and (2.01106, 2.00742, 2.00389), respectively. The profiles of half width at half maximum (HWHM) depending on two polar angles, θ and ϕ, were measured in all directions. The angular profiles of the exchange interaction (|J ₀|) were attempted to be estimated by dividing the experimental HWHM by the calculated total second moment (M ₂ ^tot) with the crystallographic data. For (±)-1, |J ₀| along the c-axis was estimated to be much larger than that along the a- or b-axis. This suggests that (±)-1 is a one-dimensional magnet along the c-axis. Authors' address: Yohei Noda, Electron Spin Chemistry Laboratory, Department of Chemistry, Graduate School of Science, Kyoto University, 606-8502 Kyoto, Japan     

Two-photon transitions of Ag− centers in KBr

Abstract

Two-photon excitation spectra of Ag^? centers in KBr have been measured. The main excitation peak appears only with linearly polarized excitation and has been assigned to the ¹A_1g → ¹A_1g transition. Weaker structures have a polarization dependence corresponding to ¹A_1g → ¹E _g and ¹A_1g → ¹T_2g transitions.     

ESR study of exchange coupled pairs in copper diethyldithiocarbamate

ESR investigations on exchange coupled pairs of Cu ions in single crystals of Cu(dtc)₂, isomorphously diluted with the corresponding diamagnetic zinc salt, are reported. The spin Hamiltonian parameters for the coupled species (S=1) are:g _‖=2.1025,g ₊=2.031,A=75.1×10⁻⁴ cm⁻¹,B=14.8×10⁻⁴,D=276.0×10⁻⁴ cm⁻¹ andE=46.7×10⁻⁴ cm⁻¹. While theg andA tensors show tetragonal symmetry, the zeor-field splitting tensor is rhombic and has principal axes different from those of theg andA tensors. Intensity measurements made down to 4.2 K indicate that the exchange is ferromagnetic with |FFF| ∼ 10 cm⁻¹. Direct dipole-dipole interaction appears to be the major contribution to the zero-field splitting. A calculation on the distributed point dipole model shows that dipolar interaction is considerably modified by the high covalency of the Cu-S bond and accounts for the rhombic nature of the tensor. The possible exchange mechanisms in Cu(dtc)₂—direct exchange and superexchange through the bridging sulphurs—are discussed.     

The excitation functions and Isomer ratios for neutron-induced reactions on Mo and Zr

Absolute cross sections of the reactions ⁹²Mo(n, 2n)^91m,91gMo, ⁹²Mo(n, p)⁹²Nb and ⁹²Mo(n, α)^{89m, 89g}Zr, relative cross sections of the reaction ⁹⁰Zr(n, 2n) ^89mZr and isomer ratios of the ⁹⁰Zr(n, 2n) reaction have been measured in the neutron energy range 13–15 MeV. The results for the (n, 2n) reactions are in good agreement with those of the previous studies. The present results for the (n, p) and (n, α) reactions are in disagreement with the previous works. The experimental data are analysed by the statistical model to determine the level-density parameter a, the moment of inertia ? and the strength of the γ-ray transition S_l in order to simultaneously reproduce the experimental data on the excitation function and the isomer ratio in the (n, 2n) reaction. The γ-ray competition, the yrast level and the experimental information on the excited levels of the residual nucleus in the (n, 2n) reaction are taken into account. The obtained values of a, ? and S_l are consistent with those deduced from other types of nuclear data.     

The X2Π and A2Σ+ states of FH+, ClH+ and BrH+: theoretical study of their g -factors and fine/hyperfine structures

Theoretical calculations on the fine, hyperfine and Zeeman (g-factor) parameters are reported for the X²Π and A²Σ⁺ states of FH⁺, ClH⁺ and BrH⁺. The fine-structure constants [spin–orbit (A), Λ-doubling (p, q) and spin–rotation constants (γ _Π, γ _Σ)] are evaluated up to second order (via SO/L couplings with several excited states) using a multireference configuration interaction (MRCI) method, a Breit–Pauli Hamiltonian and 6-311++(2d,2pd) basis sets. Hyperfine constants of magnetic and electric type [Frosch–Foley (a, b, c, d) and nuclear quadrupole (eQq ₀, eQq ₂)] are studied with density functional methods and various basis sets. Magnetic dipole moments (parameterized via g-factors) are calculated in second order like the fine structure constants. The situation is somewhat complex for X²Π since no less than five different g’s have to be evaluated in second order. In general, our results are in good agreement with those reported in the literature, mostly limited to the ground state. Our calculations confirm that, at equilibrium, all second-order properties are dominated by the couplings between the electronic states X and A.     

Spin relaxation of triplet pyrene in a crystal and in a glass

We have investigated the EPR spectral lines of the photo-excited triplet state of pyrene-d-10 both in a fluorene single crystal and an ethanol glass matrix. In the crystal we have measured the width, shape and saturation parameters, λ′, a and B _{1/2 ^u} , of the ΔM = 1 lines from both sites in the cleavage plane, Y-Z, at 193 K and along the principal directions, X, Y, Z, between 143 K and 300 K. In the glass the same parameters have been measured for the six resonances of the absorption derivative at 77 K. We have used a general deconvolution procedure to extract the unresolved inhomogeneous and homogeneous broadenings, ΔB _G and ΔB _L ≡ 1/γT ₂, and the spin-lattice relaxation rate, T ₁ ^-1, from λ′, a and B _{1/2 ^u} for the different types of resonances. The use of Fourier-series expansions permits optimal utilization of the data and resolution of the terms of different symmetries for these quantities. The application of the method of moments, using assumed spin densities, permits the demonstration that the main contribution to the constant term of ΔB _G, but only a small part of its anisotropy, originates from the intramolecular hyperfine interactions. As T ₂ and T ₁ are of the same order of magnitude, we have assumed the same mechanism, i.e. modulation of the fine-structure tensor D, for both relaxation effects. A first-order semi-classical treatment leads to partial agreement between the main features of the calculated and observed orientation dependences if one assumes strongly anisotropic librations coupled to large modulations of D. This is consistent with the maximal libration amplitudes estimated from the differences between the D tensors measured in the crystal and in the glass. Variations with temperature are attributed to a competition between the increase of modulation amplitudes and decrease of correlation time with heating.     

Multi-magnon luminescence sidebands in antiferromagnets

Using pulsed excitation and detection, we have observed multi-magnon (? 7) luminescence sidebands of the ⁴T_1g (⁴G) → ⁶A_1g (⁶S) excitonic transition in MnF₂, KMnF₃, and RbMnF₃. A simple model is proposed to explain the results qualitatively.     

Flux enhancement model for cold cesium fine-structure changing collisions

The measurements of fine-structure changing collisions in a cesium magneto-optical trap, reported in a previous work [A. Fioretti et al., Phys. Rev. A 55, R3999 (1997)], are reanalyzed within a model based on the flux enhancement effect, which takes place in cold atomic collisions. In the present analysis, we consider the cooperative effect of the long-range and the shorter-range excitation by the strong trap laser. We evidence also the important role of the hyperfine structure of the Cs₂ molecular levels asymptotically connected to the ground-state and excited-state dissociation limits. Received 22 July 1999 and Received in final form 4 May 2000     

Direct optical excitation of singlet oxygen in organic solvents

Efficient excitation of singlet oxygen is demonstrated for several organic solvents (CS₂, CCl₄, and C₆F₁₄) that are irradiated using LED in the visible spectral range in the absorption bands of the O₂-O₂ collision complexes at the corresponding cooperative transitions. It is shown that the two-photon interaction of the pumping radiation in the Herzberg I band of molecular oxygen with its excitation to the ³Σ _u ⁺ state and the subsequent collisional relaxation to the ¹Σ _g and ¹Δ _g singlet states contributes to the excitation of singlet oxygen.     

Kinetics of processes in the middle atmosphere during the laser excitation of O2 molecules

Kinetic processes taking place in the atomic-molecular system O-O₂-O₃ in the middle atmosphere with the participation of oxygen molecules in the excited electronic states O₂(a ¹Δ_g) and O₂(b ¹Σ _g ⁺ ) are analyzed in detail. The possibility of increased ozone production under the influence of solar radiation during the laser excitation of O₂ molecules in the a ¹Δ_g state is demonstrated on the basis of numerical modeling. Upper and lower bounds are determined for the densities of O₂(a ¹Δ_g) molecules at which the ozone concentration increases in the irradiated zone. Zh. Tekh. Fiz. 68, 15–23 (August 1998)     

Electron impact excitation collision strengths for neon-like Ni XIX calculated using the relativistic R-matrix method

In a recent paper [Pramana — J. Phys. 64, 129 (2005)] results have been presented for electron impact excitation collision strengths for transitions among the fine-structure levels of the 2s²2p⁶ and 2s²2p⁵3s configurations of Ni XIX. In this paper we demonstrate through an independent calculation with the relativistic R-matrix code that those results are unreliable and the conclusions drawn are invalid.