Temperature dependence of the anomalous emission line intensities in57Co:LiTaO3-initial population and relaxation

Mössbauer spectra of⁵⁷Co: LiTaO₃ single crystals were recorded in an external longitudinal magnetic field of 6 T at different temperatures between 4.2 K and 150 K. The spectra were taken at two different orientations of the crystallographicc-axis relative to the magnetic field. The line intensities of the Fe³⁺-subspectra show a temperature dependent anomalous population of the three Kramers doublets of the Fe³⁺ spinS=5/2 ground state. A relaxation broadening is observed at higher temperatures, which cannot successfully be reproduced within a relaxation model taking into account only the six lowest lying electronic states.     

Non-equilibrium population and slow relaxation of Fe2+ in57Co:LiNbO3 and57Co:LiTaO3 single crystals

The Fe²⁺ fraction observed in the Mössbauer emission spectra of⁵⁷Co:LiNbO₃ and⁵⁷Co:LiTaO₃ exhibits both slow electronic relaxation and nonthermal populations of them _s sublevels of the⁵A_lg orbital singlet ground state at low temperatures (T<15 K) in high magnetic fields. The relaxation rates depend on temperature and on the angle ? between the magnetic field and the crystallographicc-axis.     

Mössbauer emission line intensities in the limit of slow relaxation

Mössbauer emission spectra of multi-level systems depend upon the initial population of those states after nuclear decay and upon the relaxation matrix. General formulae are derived for the line intensities in the limit of slow relaxation, both for time-integral and for time-differential measurements.     

Spectral line intensities of Yb3+ ion in LiNbO3 crystals

Optical spectra of absorption and luminescence of LiNbO₃:Yb³⁺ crystals are investigated experimentally and theoretically. Within the framework of the point charge approximation of crystal field, the Stark problem is solved: crystal field parameters and wave functions of Stark states are determined. Line strengths due to indirect electric-dipole and magnetic-dipole inter-Stark transitions are calculated and values of the Judd-Ofelt parameters are determined. Values of the branching ratios of luminescence. Einstein coefficients and, lifetime of the excited sublevel are calculated. A satisfactory agreement of calculated results with experimental data is obtained.     

Field and temperature dependence of electronic states populations and spin relaxation in Mössbauer absorption spectra of LiNbO3:57Fe(III)

Single crystal LiNbO₃:⁵⁷Fe(III) Mössbauer absorption spectra at 3 Tesla and 120...300K can be fitted using a dynamical spin Hamiltonian according to spin-lattice relaxation. At high fields (0.9...7 Tesla) and 4.2K the spectra can be fitted using an electronuclear spin Hamiltonian. The area of the subspectra due to each electronic state is not in accordance with a Boltzmann distribution at the bath temperature. From the population we calculate spin temperatures which depend linearly on the external magnetic field.     

Magnetic field dependence of cyclotron-resonance line broadening

We report that for non-degenerate electrons an electron-impurity interaction with a screening radius larger than the cyclotron orbit radius yields a cyclotron-resonance line broadening, which decreases with increasing magnetic field. This is consistent with most measured results.     

Magnetic field effect on indole exciplexes: an anomalous dielectric dependence

Individual exciplex formation between various aromatic hydrocarbons, anthracene, pyrene, all-s-trans-1,4-diphenylbuta-1,3-diene and a heteroaromatic amine, 1,2-dimethylindole, was investigated by steady-state fluorescence and magnetic field effect (MFE). A comparative study was carried out with two other exciplex systems 9-cyanophenanthrene-1,2-dimethylindole and 9-cyanophenanthrene-N-methylindole. The extent of charge transfer and dielectric dependence of MFE reveals the potential role of specific interactions related to exciplex geometry.     

The temperature and light intensity dependence of photorefraction in LiNbO3

In the paper the dependence of the photorefraction (PhR) in LiNbO₃ and LiNbO₃−Fe (0.1 wt%, 0.3wt%) crystals on light intensity (within 10¹⁶–10²³ quanta·cm⁻²·s⁻¹ at wavelengths 496.5 nm and 600 nm) and temperature (in the region 100–500 K) is studied. For all the crystals the limiting values of PhR are similar and atT=293 K Δn _sat ^lim ≈3·10⁻³. In LiNbO₃ the temperature dependence of PhR in the range 100–500 K requires to take into account at least two trapping centres.     

Magnetic field dependence of the superconducting fluctuation contribution to NMR-NQR relaxation

The dependence of the Maki-Thompson and of the density of states (DOS) depletion contributions from superconducting fluctuations to nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) relaxation is derived in the framework of the diagrammatic theory, applied to layered three-dimensional (3-D) high-T_c superconductors. The regularization procedure devised for the conductivity (A. I. Buzdin, A. A. Varlamov: Phys. Rev. B58, 14195, 1998) is used in order to avoid the divergence of the DOS term. The theoretical results are discussed in the light of NMR-NQR measurements in YBCO and compared with the recent theory (M. Eschrig et al.: Phys. Rev. B59, 12095, 1999), on the basis of the assumption of a purely 2-D spectrum of fluctuations.     

Magnetic field dependence of spin-spin relaxation in cerium- and neodymium magnesium nitrate

The magnetic field dependence of spin-spin-relaxation in R₂Mg₃(NO₃)₁₂·24H₂O monocrystals with R=Ce and¹⁴²Nd has been measured and ab-initio calculated. Experimental and theoretical results are in good agreement.     

Magnetic field dependence of the spin relaxation rate in optically oriented mercury vapour

The magnetic field dependence of the relaxation rate of ¹⁹⁹Hg has been investigated at low fields in the presence of Ne or N₂. It is shown that there is a correlation time, in the interaction of the Hg nuclear spin with the walls, that is equal to the time between wall collisions. The diffusion coefficients of Hg into N₂ and Ne are evaluated, and the possibility of investigating other correlation times is discussed.     

Calculation of emission spectra in terms of a stochastic relaxation model: Fe3+ in LiNbO3

Mössbauer emission spectra of LiNbO₃:⁵⁷Co single crystals at 100 K in a magnetic field of 4 T show Fe³⁺ line intensities corresponding to a nearly Boltzmann population of the⁶A_1g Zeeman sublevels. Supposing that this is due to a spin-lattice relaxation in the ground state, no relaxation matrix can reproduce the shape of the spectrum. We conclude that the initial populations are temperature dependent due to spin-lattice relaxation within the \(\Gamma _6 ^T \) excited doublet.     

The temperature dependence of the57Fe hyperfine magnetic field distribution in Fe-Ni

The temperature dependence of the⁵⁷Fe hyperfine magnetic field (hmf) in Fe-Ni is stronger than the temperature dependence of the⁵⁷Fe hmf in pure Fe. By analyzing the shape of the⁵⁷Fe hmf distribution, and with the help of experiments with Si in Fe-Ni, we deduce that this anomalous temperature dependence originates from a large thermal sensitivity of the magnetic moments at those Fe atoms with more Ni nearest neighbors. A strong temperature dependence of the recoilfree fraction was also observed in Fe-Ni alloys. We suggest that a large mean square thermal displacement of Fe atoms in Fe-Ni is the cause of the anomalous temperature dependence.     

Photoconductivity and photovoltaic behaviour of LiNbO3 and LiNbO3 waveguides at high optical intensities

The photoconductivity and photovoltaic currents of pure LiNbO₃ and proton exchanged waveguides in LiNbO₃ have been measured as a function of the optical intensity up to about several kW/cm² by the use of surface electrodes. For pure LiNbO₃ the observed dependences are a simple extrapolation of the well known low intensity behaviour. The photoconductivity of proton exchanged waveguides is considerably increased compared with pure LiNbO₃ and the curves are strongly nonlinear in the high intensity region. These results can explain, at least qualitatively, the previously observed characteristic time and intensity dependence of light-induced refractive index changes in this type of waveguides. Both the time and temperature behaviour of the dark conductivity of all proton exchanged waveguides give strong evidence of ionic charge transport in the proton exchanged region.     

Magnetic field dependence of spin-lattice relaxation enhancement using piperidinyl nitroxyl spin-labels

We examined the magnetic resonance properties of 12 paramagnetic piperidinyl nitroxyls in water and plasma solutions. Paramagnetic contributions to proton relaxation times were measured using 10.7 and 100 MHz spectrometers. Proton relaxation enhancement from nitroxyls increased with ascending molecular weight, in plasma solutions versus equimolar aqueous solutions, and with measurements at 10.7 MHz compared to 100 MHz. Relaxation rates were observed to approximately double at 10.7 MHz compared to 100 MHz and from water to plasma solutions. The data indicate that proton spin-lattice relaxation enhancement is magnetic field-dependent, and increases using nitroxyls of large molecular weight and with chemical substitutents that increase the microviscosity of solvent water molecules. The development of nitroxyls for diagnostic MRI will be aided by understanding these in vitro physical characteristics and trends.     

Magnetic field dependence of red emission intensity of ruby at various temperatures

Application of a magnetic field perpendicular to the c-axis of a thin slice of pink ruby at low temperatures reduces the intensity of R-line and vibronic emission. At field intensity H = 6 kG the loss of vibronic emission exceeds 10%. The decay time decreases by a similar fraction. The method used for studying intensity involves on-off modulation of the field, and digital integration. A function which describes the observed fractional reduction of intensity is $\text{F(H) =}\text{AH}{}^2\text{(1 + BH}{}^2\text{)}$ where A and B are nearly constant below 77 K, but decrease rapidly between 100 K and 200 K. The function can be derived by assuming that radiationless transitions to the ground state occur with probability proportional to H² from low-lying vibronic states of ²E. Thermal excitation from these states to states in the neighborhood of ²T₁ is proposed as the mechanism for quenching the magnetic field effects.     

对称陀螺分子NH3的高温谱线强度研究

在直接计算分子配分函数的基础上,将无转动跃迁偶极矩平方近似为一常数,计算了对称陀螺分子NH3,0300a-0000 s跃迁在高温下的线强度.在296 K,计算的分子总配分函数与HITRAN数据库的结果符合很好,只有O.19%的百分误差.计算的跃迁线强度在2000 K和3000 K的高温与HITRAN数据库的结果也符合相当好,最大百分误差分别为-0.65%和-1.77%.这就表明分子配分函数和线强度的高温计算是可靠的.在此基础上,计算被扩展到更高温度,报道了对称陀螺分子NH30300 a-0000 s跃迁在极端高温4000和5000 K的模拟光谱.计算结果对大气分子高温光谱的实验测量和理论研究均有一定的参考价值.     

Effect of external electric field on R line absorption spectra in LiNbO3:Cr3+ crystals

The effect of an external electric field on the R absorption lines of LiNbO₃:Cr³⁺ crystals has been studied by a high-sensitivity differential technique at 77 K. Linear Stark effect has been observed in a field directed along the trigonal crystal axis. No effect of the field was observed when directed perpendicular to this axis. These characteristics of the Stark effect provide a convincing argument for the electric dipole moments of Cr³⁺ centers being oriented along the trigonal C ₃ axis of the crystal and indicate that the chromium centers in LiNbO₃ have C ₃ symmetry. Fiz. Tverd. Tela (St. Petersburg) 39, 2053–2056 (November 1997)