Showing papers on "Spin states published in 1988"

Branching ratio in x-ray absorption-spectroscopy

Abstract: The origin of nonstatistical branching ratios in spin-orbit-split x-ray absorption spectra is explained. Atomic calculations for transition metals show a systematic change which is due to initial-state spin-orbit splitting and electrostatic interactions between core hole and valence electrons. We have formulated the results of these atomic calculations in general rules, which are also applicable to solids. In the free atom the branching ratio reaches a maximum for the Hund's-rule ground state and its value decreases gradually for S, L, and J levels of higher energy. The presence of a crystal field results in a lower branching ratio when it produces a low-spin ground state. The rules can be used to assess the spin state and the spin-orbit splitting from the experimental branching ratio in transition-metal and rare-earth compounds. A specific example is given for the influence of second-order spin-orbit interactions in high-spin Ni compounds.

Molecular/Organic Ferromagnets

Abstract: Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.

Multiplet structure in the L 2 , 3 x-ray-absorption spectra: A fingerprint for high- and low-spin Ni 2 + compounds

Abstract: High-resolution x-ray absorption spectra of high- and low-spin compounds have been analyzed using a newly developed general crystal-field computer program. The calculations show clearly that the gross features of the multiplet structure at the ${L}_{2}$,3 absorption edge can be used to determine the spin state of the 3${d}^{8}$ configuration in Ni and Cu compounds.

Dynamics of Spin Equilibria in Metal Complexes

Abstract: Publisher Summary This chapter examines the dynamics of spin-equilibrium processes, principally from studies in solutions. The properties of the complexes that are relevant to the dynamics studies are reviewed. The techniques used to observe these rapid processes are also described in the chapter. The chapter presents some aspects of solid-state dynamics and describes some implications for the description of intersystem crossing processes in excited states and for spin equilibria in heme proteins. Many of the complexes that occur in spin equilibrium possess ligands with complicated structures. Trivial abbreviations are used, with structural formulas presented in the table. Generally the complexes are of low symmetry, but in the description of their electronic structure idealized symmetries are assumed and the appropriate term symbols are used accordingly. Some of the concepts, which are used to describe intramolecular spin equilibria, can be extended to the description of these coordination-spin equilibria. Examples include equilibria among four-, five-, and six-coordinate nickel (II) complexes and equilibria involving coordination number changes in iron porphyrin complexes and in heme proteins. Anomalous magnetic susceptibility is the characteristic feature of spin-equilibrium complexes. The Evans nuclear magnetic resonance (NMR) method gives excellent results provided adequate care is taken. There have been very few reports of the Raman spectra of spin-equilibrium complexes.

Coordination of Pyridine-Substituted Pyrazoles and Their Influence on the Spin State of Iron(II)

Abstract: Iron(II) and nickel(II) [MN6]X2 type complexes have been prepared from 2-(pyrazol-1-yl]pyridine (1pp), 2-(pyrazol-1-yl) imidazoline (pi), 2- (pyrazol-3-yl)pyridine (3pp) and 2,6-bis(pyrazol-3-yl)pyridine ( bpp ). Variable-temperature magnetic and Mossbauer spectral studies establish that [Fe(1pp)3]X2 is low spin and [Fe(pi)3]X2 is high spin over an extended temperature range, while both [Fe(3pp)3]X2 and [Fe( bpp )2]X2 undergo temperature-induced low-spin ↔ high-spin transitions. The nature of the transition depends on the extent of hydration and for salts of both cations the singlet state is generally stabilized as the extent of hydration increases. Hydrogen bonding effects are believed to be responsible for this. For anhydrous [Fe( bpp )2] [BF4]2 the transition is discontinuous and associated with hysteresis with Tc ↓ 173 K for decreasing temperature and Tc ↑ 183 K for increasing temperatures. The transition to the singlet state species is complete at low temperatures provided that the cooling rate is relatively slow. Rapid cooling to 77 K results in the trapping of a fraction of metastable quintet state species. For all other species containing either [Fe(3pp)3]2+ or [Fe( bpp )2]2+ the spin transition is continuous. Spectral data for [NiN6]X2 complexes establish an order of field strengths for the ligands pi < 3pp < 1pp < bpp , which, for the bidentate species only, is consistent with the observed electronic properties of the corresponding [FeN6]X2 complexes.

Effect of pair hopping and magnitude of intra-atomic interaction on exchange-mediated superconductivity

Abstract: It is shown that the kinetic exchange Hamiltonian for a partially filled narrow band contains a term responsible for hopping of neighboring pairs of electrons in the singlet spin state. In this way we introduce explicitly the dynamics of the resonant valence bond into the formalism describing the exchange-mediated high-${T}_{c}$ superconductivity. The role of the magnitude of the intraatomic Coulomb interaction on the onset of superconducting phase is also discussed.

Alternative spin states in synthetic analogs of biological clusters: spin-quartet ground states and structures of tetrakis(benzenethiolato)tetrasulfidotetraferrate(3-) and tetrakis(benzenethiolato)tetraselenidotetraferrate(3-) as their tetramethylammonium salts

Abstract: (Me 4 N) 3 [Fe 4 S 4 (SC 6 H 5 ) 4 ]•2 MeCN cristallise dans le systeme monoclinique, groupe d'espace C2/c et sa structure est affinee jusqu'a R=0,0444. (Me 4 N) 3 [Fe 4 Se 4 (SC 6 H 5 ) 4 ]•2Me CN cristallise dans le systeme orthorhombique, groupe d'espace Fdd 2 avec R=0,0332

Thermodynamics of Cobalt (II, III) Oxide (Co3O4): Evidence of Phase Transition

Abstract: The Gibbs' energy change for the reaction, 3CoO (r.s.)+1/2O2(g)→Co3O4(sp), has been measured between 730 and 1250 K using a solid state galvanic cell: Pt, CuO+Cu2O|(CaO)ZrO2|CoO+Co3O4,Pt. The emf of this cell varies nonlinearly with temperature between 1075 and 1150 K, indicating a second or higher order phase transition in Co3O4around 1120 (±20) K, associated with an entropy change of ∼43 Jmol-1K-1. The phase transition is accompanied by an anomalous increase in lattice parameter and electrical conductivity. The cubic spinel structure is retained during the transition, which is caused by the change in CO+3 ions from low spin to high spin state. The octahedral site preference energy of CO+3 ion in the high spin state has been evaluated as -24.8 kJ mol-1. This is more positive than the value for CO+2 ion (-32.9 kJ mol-1). The cation distribution therefore changes from normal to inverse side during the phase transition. The transformation is unique, coupling spin unpairing in CO+3 ion with cation rearrangement on the spinel lattice, DTA in pure oxygen revealed a small peak corresponding to the transition, which could be differentiated from the large peak due to decomposition. TGA showed that the stoichiometry of oxide is not significantly altered during the transition. The Gibbs' energy of formation of Co3O4 from CoO and O2 below and above phase transition can be represented by the equations:ΔG0=-205,685+170.79T(±200) J mol-1(730-1080 K) and ΔG0=-157,235+127.53T(±200) J mol-1(1150-1250 K).

Spin-polarised electronic structure of disordered BCC FeCo alloys from LCAO-CPA

Abstract: LCAO-CPA theory is applied to substitutionally disordered ferromagnetic BCC FeCo alloys. Calculations are based on charge and spin self-consistent LCAO parameters determined for the pure component metals within the local spin density functional theory. Effects of charge and spin self-consistency on the alloy magnetisation are treated in an ad hoc manner. Mechanisms determining the concentration dependence of the local magnetic moments are explained. The correlation of magnetic moment formation and maintenance of local charge neutrality is discussed. At Fe sites the exchange splitting is enhanced by a hybridisation-driven local reoccupation within the spin sub-bands. This causes the observed increase of the Fe moment by about 30%. Numerical results are presented for the saturation magnetisation, local magnetic moments, d-band population and specific heat coefficient as functions of concentration. Qualitative agreement was obtained with available experimental data. Numerical results for the concentration dependence of the total and component density of states are shown. Quite different degrees of disorder are found for both spin states. The majority spin states are typical rigid-band-like, whereas a strong influence of alloy disorder on the minority spin states is observed. This causes a strong broadening of the minority spin Bloch spectral functions and Fermi surface.

Mechanism of nuclear spin-lattice relaxation in insulators at very low temperatures.

Abstract: Nuclear spin-lattice relaxation by paramagnetic impurities in insulating crystals is expected in conventional theory to be frozen out in the millikelvin temperature region owing to the freezing of the electron-spin orientation into the lowest-energy spin state in the presence of a static field. Experimental relaxation times are much shorter than the conventional theory predicts. The authors show that by extending the normal relaxation theory to include the slight change in electron-spin quantization arising from a nuclear spin flip, the electron effectively gains a new degree of freedom, which we call wobble, which takes much less energy to excite than a complete electron-spin flip. The electron-spin orientation is, therefore, effectively unfrozen even at millikelvin temperatures. This new mechanism is expected to dominate in high field at temperatures of tens of millikelvin and below.

Characteristic properties of high- and low-spin-state five-coordinate .sigma.-bonded aryl-, alkyl-, and perfluoroaryliron(III) porphyrins: proton NMR, ESR, Moessbauer, and magnetic studies

Abstract: The magnetic behavior of /sigma/-alkyliron(III) (or /sigma/-aryliron(III)) porphyrins is studied in solution and in solution and in solid state by various spectroscopic methods: /sup 1/H NMR, ESR, and /sup 57/Fe Moessbauer spectroscopies. Variable-temperature magnetic susceptibility measurements were also performed on these complexes. The iron(III) atom of these compounds is present in different spin states. These are high (S = 5/2) or low spin (S = 1/2) at room temperature. Several factors can affect the spin state, the major parameter being the nature of the axial ligand. The perfluoroaryl axial groups (C/sub 6/F/sub 4/H, C/sub 6/F/sub 5/) lead to complexes in a pure high-spin state whatever the temperature. In contrast, the (P)Fe(R(Ar)) complexes where R = CH/sub 3/, Ar = C/sub 6/H/sub 5/ and p-MeC/sub 6/H/sub 4/, and P = OEP, TPP, T(m-Me)PP, T(p-Me)PP, T(p-Et/sub 2/N)PP, and (/beta/-CN)/sub 4/TPP behave differently and are in a low-spin state at room temperature. However, for the latter compounds in frozen solution or in the solid state, some high-spin sites are observed by ESR spectroscopy. The amount is critically dependent on the nature of the axial and porphyrin ligands. Moreover, the solvent matrix appears very important. In the solid state and on the basis ofmore » ESR data, the spin mixture could be slightly modified by grinding of the crystalline sample, leading to high-spin entity increase. The effect of these parameters is examined with respect to the nature of the metal-porphyrin and metal-ligand bond. Correlations between stereochemistry, spin state, and the nature of ligands are discussed. 64 refs., 9 figs., 8 tabs.« less

Identification of spin, charge states and optical transitions of vanadium impurities in GaAs

Abstract: With optically detected electron spin resonance the spectra of the two charge states of vanadium V2+(3d3) and V3+(3d2) in GaAs were investigated. A novel magneto-optical technique was used for the first time to determine the spin states of both configurations. The spin of the 3d3 configuration of V2+ is found to be a low spin S=1/2 and not a high spin S=3/2 according to Hund's rule. This confirms recent theoretical predictions. The optical transitions of each charge state could be determined separately and assigned to each spin state.

Light-induced formation of metastable high-spin states in [Fe(mtz)6](CiO4)2

Abstract: [Fe(mtz)6](CiO4)2 (mtz=1-methyltetrazole) is a spin crossover compound with two different iron(II) lattice sites. Only one of them (lattice site A) shows thermally induced high spin (HS) ⇌ low spin (LS) spin transition. The LIESST effect (Light-Induced Excited Spin State Trapping) can be observed below 50 K. Complex molecules in B-sites remain in HS state at all temperatures. At ∼ 20 K irradiation with red light causes a partial conversion to another HS species, HS(C), with also practically infinite lifetime.

Exciton Parameters and Electron Miniband Structure of GaAs/AlGaAs Superlattices

Abstract: The linear optical properties of short-period GaAs/AlGaAs superlattices (SL) with three-dimensional conduction minibands are studied. Using spectroscopy of p-polarized reflectance at oblique incidence the enhancement of exciton resonance features and effect of lineshape inversion at Brewster angle for the first time are observed. The heavy- and light-hole exciton oscillator strength is found to decrease with shortening of the SL period. Using the optical orientation technique the splitting of electron minibands linear in wave vector is measured for the first time. An essential increase of the photoelectron spin polarization is observed in the presence of magnetic field in Faraday configuration. The magnetic field effect is described in terms of the suppression of the electron spin relaxation connected with the k-linear splitting of miniband spin states. [Russian Text Ignored].

ESR Spectroscopy of Electrode Processes

Abstract: Electron spin resonance (ESR) is an attractive technique for the identification and study of species containing an odd number of electrons (radicals, radical cations and anions, and certain transition metal species) The experiment is based on the fact that whereas in the absence of a magnetic field, the two possible spin states, \( + \frac{1} {2},\, - \frac{1} {2} \), of an unpaired electron have identical energies, this degeneracy is lost when a field is applied ESR spectroscopy involves the flipping of the spin between the two now different energy levels, an act which is brought about by the absorption of microwave radiation As will be shown below, the presence of magnetic nuclei in the molecule is revealed as hyperfine structure in the ESR transition, which thus may be used to provide a positive identification of the odd-electron species Since each magnetic nucleus contributes (at least in principle) to the hyperfine structure, a rather more intimate insight into molecular identity emerges than from, for example, UV-visible spectroscopy It is this high information content of ESR spectroscopy, together with the sensitivity of the technique (radical concentrations on the order of 10−8 M may be observed with standard equipment(1)), that has made ESR the method of choice for investigating complex electrode reactions which proceed via radical intermediates Other advantages are that the specificity towards paramagnetic species is advantageous in some cases, that the technique is nonperturbing (there is no activation or destruction of the sample), and that absolute concentrations may be measured (albeit with limited accuracy)

High spin levels populated in multinucleon-transfer reactions with 480 MeV 12C.

Abstract: Two- and three-nucleon stripping reactions induced by 480 MeV /sup 12/C have been studied on /sup 12/C, /sup 16/O, /sup 28/Si, /sup 40/Ca, and /sup 54/Fe target nuclei. Discrete levels are fed with cross sections up to 1 mbsr for d-transfer reactions and 1 order and 2 orders of magnitude less for 2p- and /sup 3/He-transfer reactions, respectively. These reactions preferentially populate high spin states with stretched configurations. Several spin assignments were known from transfer reactions induced by lighter projectiles at incident energies well above the Coulomb barrier. In the case of two-nucleon transfer reactions, the energy of these states is well reproduced by crude shell model calculations. Such estimates are of use in proposing spins of newly observed states especially as the shapes of the measured angular distributions are independent of the final spin of the residual nucleus.

The state multipoles of the n = 2 state of atomic hydrogen

Abstract: A determination of all the state multipoles of the n=2 excited state of atomic hydrogen has been made. The kinematical conditions were a crossed electron-atom-beam geometry in which the incident electron energy was 350 eV and the scattering angle 3'. The experimental technique measures the scattered 10.2 eV energy loss electrons in coincidence with the 10.2 eV photons from either the 2P state or the delayed Stark-quenched 2S-state decay. An external electrostatic field mixes the S and P states, permitting the observation of their coherence by measuring the linear and circular photon polarisations, while in-plane angular correlations for a zero electric field determine the P-state multipoles. The state multipoles were deduced from a least-squares fit of the measured data to the quantum mechanical expressions determined for a model of an S-P (mj=1/2) superposition excited state with averages over the singlet and triplet spin states.

Spectra of electron-beam pumped XeF lasers

Abstract: An analysis is presented of the lasing spectrum of the rovibronic XeF(B–X) transition as observed in an electron-beam pumped laser. The intricate spin–split rotational and vibrational structure in the 351- and 353-nm lasing emission is investigated, and insight is gained on the processes responsible for the vibrational, rotational, and spin inhomogeneities manifested in the spectra. Explanations of the data are developed in terms of near-resonant vibration–rotation energy transfer, rotational relaxation rate differences in the B and X states, rapid dissociation of the rotational resonances near the dissociation limit of the ground state, relatively slow collisional spin relaxation, and optical coupling of spin states. The implications for narrowband extraction in the 351- and 353-nm bands are noted. The rotational relaxation rates of the X and B states of XeF are estimated in the Appendix.

Structure of high spin states in83Y

Abstract: We report on the first in-beam study of high spin levels in83Y which were established up to 9.0 MeV excitation energy and probable spin of 41/2+ resp. 27/2− by means of the reaction58Ni(28Si, 3p). Ten lifetimes and six lifetime limits in the 10−12–10−9 s range were determined in two recoil distance Doppler shift experiments. The positive parity yrast states form ag 9/2 Coriolis decoupled band with partial alignment, near-rigid rotor moment of inertia and deformationβ 2=0.29. The negative parity yrast band has very similar collective properties; it shows a pronounced band crossing at rotational frequency ħω≈0.40 MeV which we associate with twoquasiparticleg 9/2 proton alignment. At 2.56 MeV excitation, a second ΔI=1 band starting with 17/2− was found. On the basis of the similarity to85Y and the very weakE2 decay of this state, we suggest that this band has (3qp) configuration with an alignedg 9/2 neutron pair. The lifetimes of the lowest 2+ and 4+ state in84Zr populated in the reaction58Ni (28Si, 2p) were determined to be τ(2)=17.8(11) ps resp. τ(4)=3.5(4) ps.