The reverse shift of the EPR line of paramagnetic centers coupled to species with a fast paramagnetic relaxation

The EPR spectrum of the spin 1/2 paramagnetic centers with a relatively slow relaxation is considered in the case when they are coupled via the Heisenberg exchange interaction to partners which have short times of the longitudinal and transverse paramagnetic relaxation. Under these conditions only the EPR line of paramagnetic centers with a relatively slow relaxation is detectable in experiment. The shape of this line is analyzed by solving numerically kinetic equations for the spin density matrix for simple model systems. Depending on a ratio between the exchange integral and the paramagnetic relaxation rates of partner spins, the EPR line shifts in opposite directions. For moderate relaxation rates, as the relaxation rates decrease, the EPR line shifts toward the gravity center of the total EPR spectrum. In the case of extremely fast relaxation, as the relaxation rates decrease, the reverse shift of the EPR line is expected, the line shifts away from the gravity center of the total EPR spectrum. This type of the non-monotonous line shift was experimentally observed for the monocrystal of [CuNd₂(C₄O₄)₄(H₂O)₁₆] · 2H₂O when relaxation rates were changed by temperature variation.     

A contribution to the theory of the exchange narrowing of spectral lines

The intensity distribution of a composite spectral line with exchange narrowing is calculated by integrating a matrix expression derived by Anderson     

Temperature-induced broadening of the emission lines from a quantum-dot nanostructure

We report on the effect of temperature fluctuations on the midinfrared electroluminescence from a cascade of coupled AlInAs quantum dots and GaAs quantum wells. The observed line width is significantly broadened with increasing temperature. We then present our theoretical results on homogeneous line broadening due to temperature fluctuations for our experimental system. Our numerical simulations clearly indicate that, temperature fluctuations can account for the observed finite width of the emission lines at high-temperatures.     

High-frequency (94.9 GHz) EPR spectroscopy of paramagnetic centers in a neutron-irradiated sapphire single-crystal fiber

Initial results are reported of an EPR study, conducted at 94.9 GHz, of a thermally annealed, neutron-irradiated white sapphire (α-Al₂O₃) single-crystal fiber. The optical centers produced in sapphire by neutron irradiation and thermal annealing are of interest for optical technologies involving the phenomenon of spectral hole-burning. While these centers have been modeled as consisting of electrons localized at anion vacancies, experimental tests of this model have been very limited. EPR spectroscopy — a choice technique for elucidating structural details of such color centers — reveals signals from numerous paramagnetic centers in this material. The predominant signals, with amplitudes a hundredfold greater than any other signals, derive from three closely related, highspin-multiplicity centers. These centers do not, however, derive from radiation-induced lattice defects: they are readily identified as Cr³⁺ (S = 3/2) and a pair of crystallographically equivalent Fe³⁺ (S=5/2) impurity ions. The advantages of high-frequency-EPR instrumentation in facilitating this identification are presented and discussed in detail. These advantages include enhanced sensitivity for this volume-limited, fiber sample. Moreover, the analysis of the spectra — entailing spectral assignments, evaluation of the spin-Hamiltonian parameters, and spin-counting — is greatly facilitated when the Zeeman interaction is dominant.     

Temperature dependent exchange narrowing of line width in EPR on interacting donors in germanium and silicon

An expression has been derived for the temperature dependent exchange integral for a pair of interacting shallow donors substituted in a dielectric material. This has been achieved by taking into account the phonon induced mixing of excited orbital states with the ground orbital states of the impurity centres. The results have been used to explain the temperature narrowing of the EPR line width observed by some workers in phosphorus doped germanium at low temperatures. Good agreement between theory and experimental results seems to favour the proposed mechanism in preference to one of motional narrowing due to hopping.     

Oxygen effect on spin relaxation of the surface paramagnetic centers in carbon chars

The original technique with the use of longitudinal electron paramagnetic resonance detection was applied to measure longitudinal and transversal electron relaxation times (T ₁ andT ₂) in specially prepared carbon chars containing paramagnetic centers (PCs) on the surface. The results not only confirmed the conclusions of our previous studies but enabled us to determine the exchange integral and clarify the mechanism of interaction between the PCs and adsorbed molecular oxygen O₂. A spin relaxation mechanism was also suggested which takes into account both the exchange and dipolar interactions in the systems combining PC and O₂. The value of the exchange integral between PCs as well as its functional dependence on the oxygen content were estimated.     

Generation and EPR characterization of color centers at the surface of ionic oxides

The activation of the surface of high surface area alkali-earth oxides to form surface color centers is described in detail. Two methods (the radiative coloring and the additive coloring by means of vapors of low ionization energy metals) have been adopted to form surface electron excess centers of various complexity. The EPR technique is essential to describe the nature and the structural features of the paramagnetic centers. The electron rich surface quickly reacts with a variety of adsorbed molecules forming adsorbed anions and radical anions. Two examples of such reactivity are illustrated in the final part of the paper.     

Effect of oxygen diffusion on the relaxation parameters of paramagnetic centers on the surface of alumina

The present article is concerned with theoretical consideration of the effect of oxygen diffusion on the relaxation parameters of paramagnetic centers (PC) located on a surface. Proceeding from a model of dipole-dipole interaction between paramagnetic molecules of O₂ and PC that is modulated by translational diffusion motion of O₂ molecules in the volume and on surface of an adsorbent and in its pores, we give an explanation for the observed dependence of the intensity of the EPR signal on the pressure of O₂. In the present case, diffusion in the volume is the decisive factor. We also managed to explain the presence of an SHF field power threshold for the indicated phenomenon to be observed. In the calculations we used correlation functions obtained earlier for diffusion in a volume. Institute of Physicoorganic Chemistry, National Academy of Sciences of Belarus, 13, Surganov St., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 230–235, March–April, 1998.     

Mechanisms of broadening of emission lines of a laser-induced plasma generated on the surface of rigid targets

This article reports the results of an experimental investigation of the basic mechanisms that cause the broadening of emission lines of a laser-induced plasma generated on the surface of rigid targets in a gaseous atmosphere. The contribution of Stark and resonance mechanisms of emission-line broadening for the resonance doublet Al I 3s²3p²P_3/2,1/2-3s²4s²S_1/2 and triplet Mg I 3s3p³P _0,1,2 ⁰ -3s4s³S₁ is discussed. The concentrations of neutral Al atoms in the laser-induced plasma are evaluated by the magnitude of resonance line broadening. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 234–237, March–April, 2000.     

Fluctuation-induced broadening of Mn2+ EPR lines in the incommensurate phase of Rb2ZnCl4 crystals

The EPR spectra of Mn²⁺ ions in Rb₂ZnCl₄ crystals is investigated in the vicinity of the transition from the paraelectric phase to an incommensurate modulated phase. When these crystals are cooled below the transition temperature T _i =304 K, a splitting of the resonance lines is observed in the singular spectrum. A one-harmonic model is used to discuss the contributions that fluctuations in the amplitude and phase of the incommensurate displacement wave make to the local width of the singular spectra. It is shown that anomalies in the local width of the low-temperature singular peaks observed in the vicinity of T _i are caused by amplitude fluctuations. Fiz. Tverd. Tela (St. Petersburg) 41, 1668–1674 (September 1999)     

Electron paramagnetic resonance (EPR) and photo-EPR studies of aggregate centers with two iron atoms in silicon

We have investigated iron by EPR (electron paramagnetic resonance) and photo-EPR in initially boron doped silicon samples in which the iron concentration exceeded the boron content. A new EPR spectrum, showing orthorhombic-I symmetry was observed and could be fitted by an effective spin Hamiltonian with the parameters S=3/2, g=2.07, and E/D=0.68. We identify this spectrum as a new modification of a Fe₂B center which has the same symmetry but different configuration of the constituent atoms.Furthermore, we were able to determine the donor level of the old Fe₂B center to E=E_V+0.57±0.02 eV above the valence band.We have also investigated the Fe₂ donor. According to our straightforward interpretation the energy levels of the transitions from Fe₂⁺ to Fe₂⁰ and from Fe₂⁰ to Fe₂⁺ were determined as E=E_V+0.61±0.02 eV and E=E_C-0.67±0.02 eV, respectively, suggesting a lattice relaxation on electron capture, which is unusual for transition metal centers in silicon. PACS 71.55.Cn; 76.30.Fc     

ESR determination of lipid translational diffusion coefficients at low spin-label concentrations in biological membranes,using exchange broadening,exchange narrowing,and dipole-dipole interactions

Spin-spin interactions between phosphatidylcholine nitroxide spin labels in fluid-phase dimyristoyl phosphatidylcholine bilayers have been studied in the concentration range 0.1–2.0 mol%. The total line broadening has been separated into its Gaussian and Lorentzian components. The Gaussian linewidth exhibits exchange narrowing of the unresolved proton hyperfine structure, with an approximate quadratic concentration dependence for the square of the linewidth, over this spin-label concentration range. The Lorentzian linewidth has a linear dependence on the spin-label concentration over the same range. The temperature dependence of the gradient of the Lorentzian broadening with respect to spin-label concentration is biphasic over the range 30–80°C, indicating a dominant contribution from magnetic dipole-dipole broadening at the lower temperatures. The contributions from the dipole-dipole and exchange interactions to the Lorentzian broadening have been separated by fitting the temperature dependence using an unconstrained optimization procedure and assuming a single effective activation energy for both diffusion-controlled processes. The lipid lateral diffusion coefficient has been determined from the concentration dependence of both the exchange broadening of the nitrogen hyperfine structure and the exchange narrowing of the proton hyperfine structure using a two-dimensional lattice diffusion model, and also from the concentration dependence of the dipole-dipole interaction using a three-dimensional Fick's law diffusion model. All three methods yield consistent values for the diffusion coefficient, although because of the approximate nature of some of the theoretical models the exchange broadening is expected to give the most reliable results. An experimental protocol for determining the actual spin concentration in the membrane sample is also presented.     

Radiation-power broadening of the Na-D lines in a vapour cell

We have measured the time-resolved fluorescence spectrum of both Na-D lines in a vapour cell. The atoms were excited with a spectrally broadband pulsed dye laser, tuned to resonance. The width of the Na-D lines turned out to be a linear function of the laser intensity, as is predicted by a simple theory based on lifetime considerations. The experimental results are in quantitative agreement with this theory.     

Identification of paramagnetic centers and the dating of cave dripstones by electron paramagnetic resonance

Dating of samples of cave dripstones collected from the Jaskinia Czarna (the Polish Tatra Mountains) and fossil bones was carried out by means of electron paramagnetic resonance (EPR). By this method the age of the sample is found as the ratio of the total dose (TD) to the annual dose rate (D _a). TD is established on the basis of EPR spectra, whileD _a is the sum of the internal dose rate, calculated on the basis of the concentration and activity of radioactive nuclides in the sample, and the external dose rate measured at the site of the sample collection by a calibrated γ-radiation probe. Two models of uranium accumulation were used: the model of linear accumulation and the model on the basis of the assumption of a disturbed radiation equilibrium in the uranium series. The results were compared with the age determined by the uranium-thorium method. Moreover, by the computer resolution enhancement method the dynamics of paramagnetic centers in calcite annealed in nitrogen atmosphere was studied in the range of 283–873 K, and the results were compared with earlier results for calcite annealed in the air. On the basis of the study of paramagnetic center dynamics in dripstone calcite it was established that the radicals responsible for the EPR signals were CO₂⁻ in different symmetries of the crystalline field, CO₃³⁻ in the axial symmetry, CO₃⁻ in the axial and isotropic symmetry and HCO₃²⁻. The results implied that samples to be subjected to EPR dating should be kept at room temperature.     

Electron paramagnetic resonance (EPR) and spin-lattice relaxation of trigonal Co2+ centers in ZnS single crystals

Single crystals with a ZnS structure are of great practical value, in which connection they are studied in detail by different methods including the spectroscopic. In particular, such crystals activated by ion impurities are investigated in detail by the EPR method. However, the spin-lattice relaxation (SLR) of such systems has hardly been studied. Papers studying the SLR of cubic Mn²⁺ centers [1] and trigonal Nd³⁺ and Yb³⁺ [2] centers in ZnS did only appear recently.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 141–143, August, 1978.The authors are grateful to A. I. Ryskin for providing the single crystals for the investigation and to R. Yu. Abdulsabirov for assistance in performing the experiments.     

Stark broadening of lines from multiply-charged carbon ions in a high-density arc plasma

Profiles of C(III) and C(IV) lines emitted in the VUV and visible regions from a pulsed arc (N_e=1.5×10¹⁸ cm^-3, T_e=5.3×10⁴K) have been measured with time resolution. The experimental results are in satisfactory agreement with the predictions of electron impact theory for (3-3) and (3-4) transitions for which the energy separation between the upper level and the nearest perturbing level is smaller than the electron thermal energy. For lines from (2-2) transitions for which the plasma is optically thick, damping constants determined experimentally agree with theoretical values within a factor of two.     

Second-order effects on the EPR spectra of exchange-coupled pairs of paramagnetic ions

The paper gives a spin-hamiltonian formalism for a pair of exchange-coupled paramagnetic ions, operating within the single S-multiplet of total spin and in which coupling among the different multiplets is taken into account with a second-order perturbative approximation. Arbitrary values of the coupled spins S₁ and S₂ are considered and a tensorial form for the various contributions to the pair energy is assumed, without restrictions on principal values and directions. That allows us to point out some new EPR aspects related principally to the role of local and exchange anisotropies.     

Theory of the pressure broadening and shift of spectral lines

The broadening and shift of spectral lines caused by the interaction of the emitting (or absorbing) atoms with surrounding particles in the plasma are commonly observed in laboratory and astronomical spectra. A detailed analysis of the observed line shapes can give information about the density and temperature of the plasma, and in cases where the interaction between emitter and perturber is not known, some information on its form may be deduced.     

Investigation of the broadening of spectral lines in a pulse plasma accelerator

Journal of Applied Spectroscopy -