EPR,optical and superposition model study of Mn2+ doped L+ glutamic acid

Electron paramagnetic resonance (EPR) study of Mn²⁺ doped L+ glutamic acid single crystal is done at room temperature. Four interstitial sites are observed and the spin Hamiltonian parameters are calculated with the help of large number of resonant lines for various angular positions of external magnetic field. The optical absorption study is also done at room temperature. The energy values for different orbital levels are calculated, and observed bands are assigned as transitions from ⁶A_1g(s) ground state to various excited states. With the help of these assigned bands, Racah inter–electronic repulsion parameters B = 869 cm^?1, C = 2080 cm^?1 and cubic crystal field splitting parameter Dq = 730 cm^?1 are calculated. Zero field splitting (ZFS) parameters D and E are calculated by the perturbation formulae and crystal field parameters obtained using superposition model. The calculated values of ZFS parameters are in good agreement with the experimental values obtained by EPR.     

An accurate,global, ab initio potential energy surface for the H+ 3 molecule

A new global, ground-state, Born-Oppenheimer surface is presented for the H⁺ ₃ system. The energy switching approach has been used to combine different functional forms for three different regimes: a spectroscopic expansion at low energy, a Sorbie-Murrell function at high energy and known long-range terms combined with accurate diatomic potentials at large separations. At low energies we have used the ultra high accuracy ab initio data of Cencek et al. (1998, J. chem. Phys., 108, 2831). At intermediate energy we have calculated 134 new ab initio energies using a high accuracy, explicitly correlated procedure. The ab initio data of Schinke et al. (1980, J. chem. Phys., 72, 3909) has been used to constrain the high energy region. Two fits are presented which differ somewhat in their behaviour at energies over 45 000 cm^?1 above the H⁺ ₃ minimum. Below this energy, the fits reproduce each set of ab initio data close to their intrinsic accuracy. The ground state surface should provide a suitable starting point for renewed studies of the near-threshold photodissociation spectrum originally reported by Carrington et al. (1982, Molec. Phys., 45, 753).     

Electronic structure and activation energy of hydrogen in NEG alloy using nonlinear response theory

Electronic structure of hydrogen in NEG alloy [Zr_0.70V_0.246Fe_0.054] is calculated by using nonlinear response theory [Kohn and Sham,Phys. Rev. A140,1133 (1965)]. The configurational energy is calculated by assuming the ideal hcp structure for NEG alloy. The calculated configurational energy predicts that hydrogen prefers octahedral (0)-site in NEG alloy.s-Type shallow bound state of energy -1.580 x 10^-5 Ryd. suggests that hydrogen does not form NEG hydride and it stays as a free ion in NEG alloy. This conclusion confirms the prediction of Tripathiet al.     

A comparative study of the energy migration and energy transfer in highly doped Nd:YAG single crystal

The donor-donor (D-D) energy migration interaction parameter C_DD in high-concentration Nd³⁺-doped YAG laser crystal is estimated, for the first time, by using the Yokota-Tanimoto (Y-T) model and the spectral overlap model (SOM) of Kushida. Firstly, the experimental luminescence decay curves of ⁴F_3/2 state of Nd³⁺ ions in YAG laser crystal at room temperature for 2.0 and 3.0 at% Nd³⁺ concentrations reported by Mao are fitted successfully by using the Y-T model and the parameter C_DD is obtained to be 1.50×10⁻³⁹ cm⁶/s. Secondly, the parameter C_DD is also directly calculated by using the SOM of Kushida: C_DD is calculated to be 2.73×10⁻³⁹ cm⁶/s. By comparing the energy migration interaction parameter C_DD and the donor-acceptor (D-A) energy transfer interaction parameter C_DA (1.794×10⁻⁴⁰ cm⁶/s), it is concluded that energy migration rate between Nd³⁺ ions in YAG laser crystal was about 11 times larger than energy transfer rate, and that energy migration would play a very important role in high-concentration Nd³⁺ -doped YAG laser crystal.     

Calculation on the CO2 influences on the structure,stability of (MgO)m (m = 1–6) clusters

ABSTRACT

The concentration of carbon dioxide (CO₂) has a significant influence on the morphology of thermal decomposition products of magnesite. So, structures, stabilities and adsorption mechanisms of (MgO)_m (m?=?1–6) clusters by one or two CO₂ molecules were calculated by the GGA-PW91 method. The results show that the stability of the considered clusters is (MgO)_m(CO₂)₂ clusters > (MgO)_m(CO₂) clusters > (MgO)_m clusters by the average binding energy. Certain low-lying isomers of (MgO)_m(CO₂) and (MgO)_m(CO₂)₂ clusters which have an isolated O atom are deviating from the cluster center which possess higher kinetic activity. (MgO)_m clusters prefer to adsorb a CO₂ molecule, while (MgO)₃(CO₂) clusters prefer to adsorb a CO₂ molecule rather than the neighbors. Magnesite is difficult to transit to (MgCO₃)₂ clusters at room temperature. However, magnesite will spontaneously transit to (MgO)₂ clusters and further transit to MgO crystal which need to adsorb more energy at 700?K.     

从头计算法计算Yb3+掺杂钽酸盐的晶体场参数和能级结构

介绍了一种基于从头计算的DV-Xα方法和有效哈密顿量模型,它可以计算晶体中掺杂离子的晶体场参数和旋轨耦合参数,尤其适合计算低对称性的晶体.对于低对称性的晶体,参数的数目比能级的数目多,因此通过实验能级拟合确定所有的参数不太准确,而从头计算法可以准确地确定所有的晶体场参数和旋轨耦合参数.首先用这种模型计算了Yb3+掺杂GdTaO4晶体中的晶体场参数和旋轨耦合参数,然后给出了Yb3+在GdTaO4中的能级结构,并分析了Yb3+:GdTaO4的发射谱形成一个连续的发射带.这有利于激光的调谐和锁模激光输出,预言了Yb3+:GdTaO4有望成为新型全固态超短脉冲激光工作物质.同样用这种模型分别计算了Yb3+掺杂YTaO4和ScTaO4中的晶体场参数和旋轨耦合参数,并给出了Yb3+在YTaO4和ScTaO4中的能级结构,得到了与Yb3+:GdTaO4晶体类似的结论.     

Investigations of the spin-Hamiltonian parameters for Er3+ ions in AlN crystal

The spin-Hamiltonian parameters (g factor g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) for Er³⁺ ion at the trigonal Al³⁺ site of AlN crystal are calculated by diagonalising the 52 × 52 energy matrix. The matrix are related to the ground mutiplet ⁴I_15/2 and the first to third excited multiplets ⁴I_13/2, ⁴I_11/2 and ⁴I_9/2 for 4f¹¹ ions in trigonal crystal field under an external magnetic field. The crystal-field parameters used in the matrix are obtained from the superposition model and the local lattice relaxation due to the substitution of Er³⁺ for Al³⁺ is considered. The calculated spin-Hamiltonian parameters are in reasonable agreement with the experimental values and the signs of hyperfine structure constants are suggested. The results are discussed.     

Theoretical study of ground and excited state potential energy surfaces for the Ca+-H2 complex

The potential energy surfaces of the Ca⁺-H₂ complex are calculated using the internally contracted multireference CI method (ICMR CI) and complete active space SCF (CAS SCF) reference wave functions. The calculations involve both the ground and the excited states correlating to (3d)²D and (4p)²P Ca⁺ terms and are carried out for C_∞v and C_2v configurations. Anisotropy of the potential surfaces has also been analysed by computing the interaction energy for some representative points as a function of the angle between the H₂ molecular axis and the Ca⁺—centre of mass of H₂ bond axis. The calculations have revealed the existence of a conical intersection of the lowest excited (3d)²B₂ potential surface with the ground state one. The obtained global energy minimum of the (3d)²B₂ potential surface lying 0.683 eV below the asymptote indicates a possible stabilization of the Ca⁺-H₂ complex towards formation of an exciplex in the (3d)Ca⁺-H₂(v = 0) collision process. The dependence of the vibrational energy levels of H₂ on the distance from Ca⁺ in the C_2v configuration has also been studied.     

Size-dependent energy scales in the ideal fermi gas

Abstract

The simplest model for the electronic properties of small metal particles is an ideal Fermi gas confined to a finite volume. When the confining region of size L has a regular shape such as a sphere or a cube, there are two distinct scales of energy which characterize the spectrum of eigenvalues near the Fermi energy E_F ≡ ?² k ² _f/2m. The inner scale δ ～ E_F /(k_FL)² is the mean spacing between successive energy levels, while the outer energy scale Δ ～ E_F /(k_FL) describes clustering of several levels, or shell structure. Consequences for the behaviour of thermodynamic properties are investigated. There are three regimes of temperature T: normal metallic (T > Δ), shell-metallic (δ < T < Δ) and semiconductor-like (T < δ). Finally, if the shape of a hard-walled container is allowed to vary so as to minimize the energy, it is argued that the optimal shape fluctuates between spherical and distorted as L is changed.     

Comparative analysis of energy-level splitting of Pr3＋ doped in LiYF4 and LiBiF4 crystals： a complete energy matrix calculation

Based on the combination of Racah's group-theoretical consideration with Slater's wavefunction, a 91 × 91 complete energy matrix is established in tetragonal ligand field D_2d for Pr³⁺ ion. Thus, the Stark energy-levels of Pr³⁺ ions doped separately in LiYF₄ and LiBiF₄ crystals are calculated, and our calculations imply that the complete energy matrix method can be used as an effective tool to calculate the energy-levels of the systems doped by rare earth ions. Besides, the influence of Pr³⁺ on energy-level splitting is investigated, and the similarities and the differences between the two doped crystals are demonstrated in detail by comparing their several pairs of curves and crystal field strength quantities. We see that the energy splitting patterns are similar and the crystal field interaction of LiYF₄:Pr³⁺ is stronger than that of LiBiF₄:Pr³⁺.     

Theoretical investigation of the optical spectra and g factors for Cu2+ in dioptase

The optical spectra of Cu²⁺ in dioptase are calculated using crystal-field theory. Good agreement between measured and calculated energy values is obtained under D _4h point-symmetry approximation. The electron paramagnetic resonance g factors, g _// and g _⊥, are also investigated from high-order perturbation formulae. The local structure of Cu²⁺ in dioptase is obtained using these formulae. Theoretical results are in perfect agreement with experimental findings.     

Relaxation of excited states of Tm3+ and Tm3+-Eu3+ energy transfer in YVO4 crystal

Tm³⁺-Eu³⁺ energy transfer processes and relaxation dynamics of the ³ H ₄ and ³ F ₄ excited states of Tm³⁺ ions in 1 at. % Tm³⁺, 5 at. % Eu³⁺:YVO₄ single crystal were studied. Contribution of Tm³⁺-Eu³⁺ energy transfer reduces effectively the lifetime of terminal level in a potential ³ H ₄–³ F ₄ laser transition at around 1.48 μm. Adverse quenching of the ³ H ₄ emission by Eu³⁺ ions is found to be less efficient than that reported for Tm³⁺ + Tb³⁺ system in YVO₄. The classical Inokuti–Hirayama model accounts well for an experimental decay curve of the ³ H ₄ emission recorded for co-doped crystal. Stimulated emission cross section for ³ H ₄–³ F ₄ transition of Tm³⁺ at around 1.48 μm was analyzed taking into account the anisotropy of YVO₄ crystal. PACS 42.55.Xi; 42.62.Fi     

Calculations of the optical and EPR spectral data for Cr3+ ion in Y3Ga5O12 crystal from the complete diagonalisation method

The complete diagonalisation (of energy matrix) method is applied in this paper to calculate together the optical and electron paramagnetic resonance (EPR) spectral data for Cr³⁺ ion at the trigonal Ga³⁺ site of Y₃Ga₅O₁₂ crystal. The method is founded on the two-spin-orbit-parameter model where in addition to the contributions from the spin-orbit parameter of central dⁿ ion (i.e., one-spin-orbit-parameter model) in the traditional crystal field theory, those from the spin-orbit parameter of ligand ion via covalence effect is also considered. The calculated results propose that by using only four adjustable parameters, the 12 observed spectral data (nine optical band positions and three EPR parameters g_//, g_⊥ and D) in Y₃Ga₅O₁₂: Cr³⁺ are reasonably explained. The impurity-induced local lattice distortion of Cr³⁺ in Y₃Ga₅O₁₂ crystal is also estimated through the calculations. The results are discussed.     

Er3+：GdVO4中Er3+离子的光谱参数计算和晶场中能级分裂的讨论

对Er³⁺：GdVO₄样品的光谱参数以及Er³⁺在晶场中能级的分裂情况进行了研究. 首先对样品进行了吸收光谱的测量，接着用Judd-Ofelt理论拟合出了Er³⁺在GdVO₄晶体中的强度参量Ω_t，并由此计算了跃迁的振子强度、自发辐射跃迁速率、荧光分支比和积分发射截面. 通过计算结果可以发现有较多能级之间的跃迁都有大于10^-6的振子强度和大于10^-18cm的积分发射截面，并且具有较高的荧光分支比，特别是²H_11/2→⁴I_15/2，⁴S_3/2→⁴I_15/2，⁴F_9/2→⁴I_15/2和⁴I_13/2→⁴I_15/2等几个强发光能级除了具有较大的振子强度和积分发射截面外还有很好的应用前景，因此也更加值得关注. 最后还利用群论讨论了Er³⁺离子在GdVO₄晶场中各能级的分裂情况并对各Stark子能级的J_z混杂情况进行了分析.     

Electronic structure and spectrum of Cr3+ in LiCaAlF6

The electronic structure and spectrum of Cr³⁺ in LiCaAlF₆ are investigated by using the discrete variatitional-local density functional (DV-LDF) method with embedded cluster model. The clusters (CrF₆)^3– withC ₃,D _3d andO _h point group symmetries embedded in the crystal are treated. The one-electron energy levels, densities of states, orbital populations, spin polarization splittings and energies of some terms are calculated. The results show that the relaxation of F^– ions around the Cr³⁺ impurity is inevitable, and that theD _3d andO _h (CrF₆)^3– clusters, with an extended bond-lengthR(Cr–F) chosen to be equal to 1.88 Å can represent this relaxation in a much better way. All the ligand-field transition energies, which are obtained from the transition-state energy and the Griffith parameters, as yielded by a restricted one-electron DV-LDF calculation, compare well with the experimental ones.     

Nucleation and stability of defect clusters: New energetic model in the case of substituted wustites

Abstract

For wüstite Fe_1?z O (z < 0.08) an energetic model accounts for the stability of cubic defect clusters (m/n) which are partly ordered in the crystal. The Gibbs energy G_T (N) associated with clusters, including their distorted envelope, is expressed as a sum of a volume term in N ³ and of surface terms in N ⁴; N is the number of bonds characteristic of the cluster size. In the case of a (10/4) type cluster, this energy is negative and minimum for N_m ranging between 4 and 5, when the volume and surface energies range between specific values. Using simple assumptions, a volume energy ?0.80 eV per vacancy is found in accordance with the value of stabilization energy calculated by theorists for the (10/4) cluster. The substitution of Fe²⁺ by Ca²⁺ should lead to a decrease of cluster size; this has been recently suggested by neutron diffraction studies.     

A quasi-classical trajectory study of the Cl + H<Subscript>2</Subscript> (D<Subscript>2</Subscript>) reaction on a new BW3 potential energy surface

Molecular reaction dynamics of Cl + H₂ (D₂) has been studied on the latest analytical potential energy surface called BW3 using the Monte Carlo quasi-classical trajectory method. Excitation functions, differential cross sections and angular distributions of HCl and DCl products have been calculated. The excitation functions of the Cl (²P_3/2) + n-H₂ and Cl(²P_3/2) + n-D₂ reactions are also studied. The results are compared with those of quasi-classical trajectory [M. Alagia et al.: Phys. Chem. Chem. Phys. 2 (2000); F. J. Aoiz et al.: J. Phys. Chem. 100 (1996)], quantum mechanical (QM) calculations [F. J. Aoiz et al.:J. Chem. Phys. 115 (2001)] and experimental data [S. H. Lee et al.: J. Chem. Phys. 110 (1999); F. Dong et al.: J. Chem. Phys. 115 (2001)]. Discussions are given to some new results.     

Crystal field analysis of energy level structure of the Cr2O3 antiferromagnet

A detailed analysis of the energy level structure of the six-fold coordinated Cr³⁺ ion in the chromium oxide Cr₂O₃ is performed using the exchange charge model of the crystal field theory. Parameters of the crystal field acting on the Cr³⁺ optical electrons are calculated from the crystal structure data for the [CrO₆]⁹⁻ impurity center. The energy levels obtained are compared with the experimental absorption spectra for the considered crystal; a good agreement with experimental data is demonstrated. One possible explanation for the ultraviolet p₁ absorption band is proposed based on the results of crystal field calculations.     

The structure and properties of the stepped surfaces of MgO and NiO

The static lattice computer simulation method has been used to study the structure and properties of high index faces of MgO and NiO. The (10_n) series of faces can be considered as stepped (001) surfaces and have been studied for both materials. In addition, the (403) and (302) faces which can be considered as stepped (101) surfaces have been studied for NiO. The calculated energies for steps on the (001) face are 3.72×10^?10 and 3.62×10^?10 J m^?1 for MgO and NiO respectively. The NiO energy also requires correction for the crystal field splitting. The energy of steps on the (101) surface is at least an order of magnitude lower. The interaction between the steps is repulsive but of short range. The large variation of surface energy with angle indicates that torque terms cannot be neglected in the analysis of thermal grooving experiments. The step structure is modified by substantial ionic displacements leading to an obtuse step angle. The structure is qualitatively similar for both NiO and MgO. The large distortions are likely to modify the step properties from those deduced by consideration of only the ideal geometry.