RYDMR spectra for reactions of triplet−triplet annihilation and photogeneration of triplet pairs in molecular crystals

The contribution to the RYDMR spectrum from an elementary event of collision of two triplet excitons during their mutual annihilation is calculated, and the reverse process of photogeneration of a pair of triplet excitons in a molecular crystal are considered. The interaction between the excitons in the cell is assumed weak in comparison with the Zeeman splitting and the fine-structure parameters for the triplet state of each of the molecules. For both the annihilation of excitons and the photogeneration of a pair, the RYDMR signal from a single-crystal sample disappears at certain orientations of the crystal relative to the external magnetic field. This effect was previously experimentally observed for the anthracene?tetracyanobenzene crystal and was explained in the case of stationary optical excitation of the molecule. The results obtained in this paper make it possible to consider an arbitrary, not only stationary, mode of irradiation sample. An interval of angles between the static magnetic field and the crystallographic axes of the crystal within which the lines in the spectrum can disappear is determined. The possibility of using the RYDMR method for studying nanosized objects, for which additional peaks in the RYDMR spectrum may arise, is briefly discussed.     

Dielectric loss relaxation maxima of undoped and calcium doped NaCl crystals between 20 and 400°C

When studying the dielectric loss dependence of undoped (pure) and CaCl₂ doped NaCl crystals on temperature and frequency, some further maxima were observed apart from the wellknown relaxation maxima of calcium complexes. The temperature of these maxima shifts with rising calcium content to lower temperatures (from 259 to 142°C), whereas the value of the maxima does not change with concentration and remains unchanged even with the purest crystals. It seems probable that the observed maxima are due to the relaxation loss of the crystal lattice itself.     

dimer paramagnetic centers in lead germanate crystals doped with iron and halogen (Cl−, Br−, F−) ions

The dimer complexes Fe³⁺-Cl^?, Fe³⁺-Br^?, and Fe³⁺-O^2? in ferroelectric lead germanate crystals doped with iron and annealed in chlorine-, bromine-, and fluorine-containing atmospheres have been studied using the electron paramagnetic resonance method. These complexes are formed by Fe³⁺ ions in the trigonal position of lead and their associated anions located in the interstitial channel of the structure. The positions of the charge-compensating anions in the channel have been discussed based on the analysis of the parameters of the spin Hamiltonian and their temperature dependence.     

Raman spectra of undoped and uranium doped CaF_2 single crystals

Raman scattering experiments for nominally pure and uranium doped CaF2 single crystals were presented. In all crystals, the Raman active T2g vibration mode of CaF2 was observed, whose frequency shift and full-width at half-maximum (FWHM) broadening correspond well with defects and impurities in CaF2 lattice. Additional Raman peaks develop in nominally pure CaF2 with high etch pits density and U6+:CaF2 crystals. Part of additional Raman peaks in the experimental results, which are assumed due to vibration modes from F- interstitials and vacancies, are in well agreement with the theoretical predications by employing the Green-function formulation.     

Calculation of the elastic scattering properties for cold and ultracold 39K atoms in a triplet state

The elastic scattering properties for collisions between cold and ultracold 39K atoms in a triplet state are investigated. Based on the recent theoretical and experimental results, the improved hybrid potential is presented for a triplet α3∑u^＋ ground state of K2. Our calculated value of the s-wave scattering length a by using the Numerov method for the triplet state is 79.578α0 and found to be in good agreement with the previous ones. The numbers of bound states are supported by the molecular potential. Pronounced shape resonances appear for the l = 3 partial waves for the α3∑u^＋ state. Furthermore, the s-wave scattering cross section, the total cross section and energy positions of shape resonances for the α3∑u^＋ state are calculated.     

Dislocation centers in α-martensite formation and dual couplings of thinly-laminated martensite crystals

The spectrum of observable dual couplings of crystals of thinly-laminated martensite with habiti {3 15 10} is discussed within the framework of the concept of the dislocation formation of -martensite crystals. A spectrum of dislocation centers of formation arising together with the developing martensite crystal is proposed. Here it is essential to divide the arising dislocation centers of formation into two groups associated with different displacement mechanisms. This difference permits understanding of the difference in the frequency of observation of couplings and the introduction of the concept of a comb structure — a set of secondary parallel crystals of martensite, associated with the same initial crystal.Urals Forestry Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 64–67, June, 1994.     

Ionoluminescence and formation of color centers in α-Al2O3 single crystals under proton irradiation

This paper describes results of experimental studies on radiation defects in nominally pure single crystals of corundum in two initial states: α-Al₂O₃ with an unperturbed lattice and α-Al₂O₃:C with a high concentration of anion vacancies. Defects were identified from optical absorption spectra, ionoluminescence, pulsed cathodoluminescence and photoluminescence spectra. It is shown that mostly color centers of the F- and F⁺-types are formed in the α-Al₂O₃ lattice under irradiation with 5,7 MeV protons.     

ENDOR on the triplet state of the primary electron donor in the photosynthetic bacteriumRhodobacter sphaeroides — One step forward in a still unfinished story

The electronic structure of the primary electron donor (D) of the photosynthetic bacteriumRhodobacter sphaeroides is investigated by ENDOR in the photoinduced triplet state of D. Hyperfine (hf) splittings of the triplet state measured on frozen solutions are given and compared to the results obtained earlier (Lendzian F., van Willigen H., Sastry S., Möbius K., Scheer H., Feick R.: Chem. Phys. Lett.118, 145 (1985). The hf splittings found are consistent with a model of an asymmetric spin density distribution over the two bacteriochlorophyll molecules which constitute D, suggesting a mirror image symmetry of HOMO and LUMO coefficients. The could be relevant for electron transfer, in particular unidirectionality in the reaction center (RC). The first triplet state ENDOR experiments on single crystals of RC’s are also reported.     

Kinetics of accumulation and decay of paramagnetic centers in γ-irradiated doped phosphate glasses

The dose dependences and decay kinetics of PO ₄ ^2? and PO ₃ ^2? paramagnetic centers and radiation-reduced europium Eu^(3+)? in γ-irradiated phosphate glasses of composition 4Na₂O · La₂O₃ · 7P₂O₅ doped with europium are studied. The data obtained are discussed within the trapping volume model in the approximation of short irradiation duration. It is shown that the trapping volume parameter v decreases with increasing irradiation dose. A physical interpretation of the parameter v is proposed.     

Study on the in—plane electrical resistivity and thermoelectric power in single crystals of La2—xBaxCuO4

The in-plane electrical resistivity and thermoelectric power have been measured on single crystals of La_2-xBa_xCuO₄ at around x=0.125. The room temperature resistivity and thermopower have their maximum values at x=0.125, indicating that the carrier concentration is the minimum and the carriers are most strongly localized at x=0.125. The observed semiconductor-like behaviour can be well described by the weak-localized quasi-two-dimensional state. The steep rise in electric resistivity of the sample at x=0.125 below 70K is attributed to the formation of static stripe-order of holes and spins, which are pinned by the low-temperature tetragonal (LTT) structure, as discovered in La_1.48Nd_0.4Sr_0.12CuO₄. The temperature dependence of electric resistivity below 70K is still well described by the formula ρ∝ lnT. A definite change in the slope of thermopower is observed at the low-temperature orthorhombic-LTT structural phase transition temperature. The origin of the 1/8 anomaly is discussed in the text.     

g-factors in the ground state and in the γ-bands in 160,162,164Dy

The g-factors of some members of the ground state band and of the 2⁺ state in the %-vibrational band have been measured in ^160,162,164Dy using the Coulomb Excitation Transient Field technique, induced by ⁵⁸Ni projectiles at 230, 210 and 217 MeV, respectively. The g-factors in the ground state band are consistent with a constant value, while that of the 2⁺_% states is about 20% larger in average than those in the ground state band. Results are discussed in the frame of the systematics in this nuclear region.     

Low-temperature optical spectra and zero-phonon transition of color centers in γ-rayed lithium fluoride crystals

The zero-phonon lines on R_2, R~ , N_1 and R′_2 centers and some new narrow lines have been observed in bivalent-metal-doped and air-grown LiF crystals irradiated by γ-ray in the temperature range of 9.5—130K. The spectral properties and thermostabilities of the lines are investigated systematically at different temperatures.     

Optical properties of (F2 +) H centers in NaCl:OH− crystals

Abstract

In this work we present new data on the formation and destruction of the laser active (F₂ ⁺)     

Analysis on suitability of pure and α-Histidine doped KDP crystals in high speed applications

Ferroelectric Potassium Dihydrogen Phosphate (KDP) crystals are widely used in modern short wavelength laser techniques, non-linear and integrated optics. The pure and α-Histidine doped KDP crystals were procured from SRM Institute of Science and Technology, Chennai. The presence of additive in the doped KDP crystal is confirmed through X-Ray diffraction patterns and the lattice parameters were evaluated. Also, Fourier Transform Infrared spectrum confirms the presence of α-Histidine in the doped crystals. The dielectric behaviour of the pure and doped crystals have been studied in the microwave region using K-band microwave bench equipped with the Gunn oscillator and guided with rectangular wave guide.     

Determination of charge-compensated C3v (Ⅱ) centers for Er3+ ions in CdF2 and CaF2 crystals

A unified theoretical method is established to determine the charge-compensated C3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 crystals by simulating the electron paramagnetic resonance(EPR)parameters and Stark energy levels.The potential(Er³⁺–F^?–O₄^2?)and(Er³⁺-F₇^?-O₄^2?)structures for theC3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 crystals are checked by diagonalizing 364×364 complete energy matrices in the scheme of superposition model.Our studies indicate that the C3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 may be ascribed to the local(Er³⁺-F^?-O₄^2?)structure,where the upper ligand ion F?undergoes an off-center displacement by?Z≈0.3?A for CdF2 and?Z≈0.29?A for the CaF2 along the C3 axis.Meanwhile,a local compressed distortion of the(ErFO4)6?cluster is expected to be?R≈0.07?A for CdF2:Er3+and?R≈0.079?A for CaF2:Er3+.The considerable g-factor anisotropy for Er3+ions in each of both crystals is explained reasonably by the obtained local parameters.Furthermore,our studies show that a stronger covalent effect exists in the C3v(Ⅱ)center for Er3+in CaF2 or CaF2,which may be due to the stronger electrostatic interaction and closer distance between the central Er3+ion and ligand O2?with the(Er³⁺-F^?-O₄^2?)structure.     

Similarity of the Fermi surface in the hidden order state and in the antiferromagnetic state of URu₂Si₂

Shubnikov-de Haas measurements of high quality URu2Si2 single crystals reveal two previously unobserved Fermi surface branches in the so-called hidden order phase. Therefore, about 55% of the enhanced mass is now detected. Under pressure in the antiferromagnetic state, the Shubnikov-de Haas frequencies for magnetic fields applied along the crystalline c axis show little change compared with the zero pressure data. This implies a similar Fermi surface in both the hidden order and antiferromagnetic states, which strongly suggests that the lattice doubling in the antiferromagnetic phase due to the ordering vector Q(AF)=(001) already occurs in the hidden order. These measurements provide a good test for existing or future theories of the hidden order parameter.     

Experimental observation of “configuration” modes of bistable centers in CdF2:In crystals

New “configuration” modes, which were predicted by us for CdF₂:In crystals, have been revealed at the frequencies ν₁ ≈ 32.4 cm^?1 and ν₂ ≈ 96.3 cm^?1 for deep and shallow impurity states, respectively. The frequencies of these oscillations exactly correspond to the potential-energy curves calculated by us for shallow and deep states of In with regard to the reduced mass M = 2m ₁ m ₂/(m ₁ + 2m ₂) of the In ion (m ₁) and two F ions (2m ₂) per primitive fluorite cell. This correspondence confirms the correct choice of the height of the potential barrier between the impurity states of In in CdF₂ (0.02 eV), which was used in the calculations. The dielectric contributions of the noted modes were determined, which made it possible to calculate the concentrations of In impurity ions in the deep (N ₁) and shallow (N ₂) states. The obtained ratio N ₂/N ₁ ≈ 2 directly indicates that photoionization of deep In centers leads to the formation of a doubled number of shallow centers and that two electrons are localized in the deep state of the In ion; such behavior is characteristic of DX centers. A photoinduced increase in the real (ε′) and imaginary (ε″) parts of the dielectric constant has been found (at a frequency of 25 cm^?1, Δε′ ≈ 0.2 and Δε″ ≈ 0.06). These changes correspond to the changes in the dielectric contributions of the configuration modes under illumination. A photoinduced decrease in the lattice reflection of CdF₂:In, related to the impurity lattice modes, has also been revealed.     

Optically detected EPR effect in the ã3 Au triplet state of the oxalylfluoride molecule excited to the 413, 423 and 431 rotational levels of the 00(Ã1 Au ) vibronic state

In the present study, resolved OD EPR spectra were measured for the 4₃₁, 4₂₃, and 4₁₃ levels of the ù A_u 0⁰ vibronic state. Values of the fine and hyperfine constants were estimated from an analysis of these spectra for the triplet rotational levels, coupled by the intramolecular interactions with the singlet levels studied. It was shown that the microwave power dependence differs significantly for different lines of the OD EPR spectrum. This difference can be explained by a model where the microwave field saturation effect is observed for different OD EPR lines at different power values of the microwave field. The decay profile can be fitted by a biexponential function in the presence of both magnetic and microwave fields. The observed data were analysed using the electron and nuclear spin decoupling mechanism in the limit of low level density.     

Order—Disorder phase transformation in ZrS2 single crystals

Order-disorder phase transformation has been observed in ZrS₂ single crystals on annealing them at a temperature of 400 ± 10°C. Loss of sulphur, resulting in its deficiency, on annealing of crystals is thought to be the cause of the observed phase transformation. Evidence in its support, based on X-ray and electron diffraction results, is advanced.