Some peculiarities of the EPR spectra of radiation-induced defects in LiF and NaF crystals

To refine the structure of radiation-induced defects, a comparison is made of the EPR spectra of LiF and NaF crystals with various constituent impurities γ-irradiated with different doses, as well as the experimentally measured and calculated intensity distributions of HFS lines in the EPR spectra of these crystals. It is shown that the F-centers, whose model represents an ionized quasimolecule consisting of two metal atoms on each side of a halide vacancy, are responsible for the broad band observed independently of the impurity composition of the crystals and irradiation dose, while the HFS is caused by defects based on interstitial halide atoms. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 455–459, May–June, 1998.     

EPR study of Mn2+ ions adsorption on silica gel

The paper deals with the study of the spectra of the electron paramagnetic resonance (EPR) of the Mn²⁺ ions adsorbed on the silica gel. Further on there have been studied the changes in the EPR spectrum of the Mn²⁺ ions adsorbed on the silica gel that are occuring as a result of dehydratation of the introduced samples, as well as the spectra changes of these samples that have arisen owing to the natural adsorption of the water vapour from the air under normal physical conditions.At low concentrations of the Mn²⁺ ions adsorbed on the silica gel, the EPR resonance spectrum contains six hyperfine structure (HFS) components the widths of which enlarge towards higher magnetic fields and do not change with the increase of the Mn²⁺ ions concentration.The spectrum further contains a system of lines that have been identified as the so-called forbidden transitions, when simultaneously with the orientation of the electron spin the orientation of the nuclear spin changes as well. This spectrum corresponds to the Mn²⁺ ions adsorbed in the individual positions considerably far from each other, without mutual dipole-dipole interaction.During further increase of the adsorbed Mn²⁺ concentration, there appears in the EPR spectrum, besides the above-described HFS, a broad symetric line the width of which does not change with increasing concentration, except for high concentrations when it becomes narrower probably due to the exchange interaction. This signal corresponds to the Mn²⁺ ions adsorbed in clusters, probably in the silca gel pores, when the HFS gets smudged as a result of dipole-dipole interaction.It appears that the width of the EPR spectrum broad line is dependent upon the temperature at which the dehydratation process was passing. At the adsorption of water vapour from the air under normal conditions on the dehydrated sample, there arise expressive changes in the EPR spectrum shape. The broad line decomposes in 6 HFS components, the widths of which change with duration time of the water vapour adsorption. Since the HFS of the individually adsorbed Mn²⁺ ions does not change after the dehydratation and the ensuing water vapour adsorption, the EPR spectrum of these samples becomes a superposition of two hyperfine structures coresponding to the two inequivalently bound groups of Mn²⁺ ions.     

New Data on the Nature of the EPR OK1 and N3 Centers in Diamond

The well-known diamond electron paramagnetic resonance (EPR) OK1 and N3 defects are discussed in the context of incorporation of heavy metal, supposedly Ti, in the double semivacancy and substitutional structures, respectively. X-ray phase analysis of inclusions of the growth medium, eclogite, in the diamonds under study shows an atypically high concentration of titanium compounds. To support proposed models, the hyperfine structure (HFS) from ¹³C in the OK1 center has been analyzed more precisely. The data obtained are consistent with attributing additional satellites around the main EPR lines to HFS from ⁴⁷Ti and ⁴⁹Ti. A double semivacancy model describes the directions of g and A tensor components of the OK1 center. The features of infrared and photoluminescence spectra related to the OK1/S1 and N3/440.3 nm centers are presented.     

A New EPR Data on the MA1 and NIRIM8 (NP1) Phosphorus-Related Centers in Synthetic 1b Diamonds

Microdiamonds grown under high-pressure and high-temperature conditions from a P–C medium at different temperatures have been studied by electron paramagnetic resonance (EPR). Two paramagnetic centers P1 and MA1 were observed in microdiamonds grown at 1,873 K. Analysis of weak lines around these centers suggests that they may be due to ¹³C hyperfine structure (HFS) of the MA1 center. The calculated s/p hybridization parameter for this carbon atom (C₁) was similar to that for the undistorted lattice. At a growth temperature of 1,973 K, in addition to MA1 and P1, a new center, labeled NP1, with HFS from nitrogen and phosphorus atoms and with EPR parameters similar to NIRIM 8 was identified. The NP1 (NIRIM8) centers have an electron spin S = 1/2. We propose that nitrogen–phosphorus defects are created through subsequent migration of nitrogen atoms towards phosphorus atoms upon increasing of the growth.     

Effects of F2 Color Center Spatial Distribution on the Emission Properties of Optical Microcavities Based on LiF Films

Highly stable F₂ color centers are very efficiently produced in lithium fluoride (LiF) by electron beam irradiation at room temperature. We have fabricated optical microcavities in which the active medium is a low-energy electron beam irradiated LiF film, whose optical thickness is comparable with the peak wavelength (～668nm) of the F₂ broad photoluminescence band. By selecting the proper electron beam energy, one can control the F₂ color center depth distribution. This distribution influences the photoemission angular distribution of the microcavity, whose resonance properties are determined by the coupling of the depth profile of the defects with the pump electromagnetic field and microcavity modes.     

Types of paramagnetic centers in Cu2+ complexes with model neuromelanins

Electron paramagnetic resonance (EPR) spectroscopy was used to examine free radical properties of model neuromelanins obtained by oxidative polymerization of noradrenaline, adrenaline and dopamine. We compared the lineshape of the experimental spectra, type and concentration of free radicals in the analyzed samples. The effect of different concentrations of Cu²⁺ on free radicals in melanins was studied. The total concentration of free radicals (about 10¹⁸ to 10¹⁹ spin/g) in the studied melanins increases as follows: adrenaline-melanin < dopamine-melanin < noradrenaline-melanin. EPR spectra of dopamine-melanin and adrenaline-melanin were a single EPR line (ΔB _pp, 0.50 and 0.55 mT, respectively). o-Semiquinone free radicals with the characteristicg-value of 2.0040 exist in these melanins. EPR spectra of noradrenaline-melanin were a superposition of two lines (ΔB _pp, 0.45 and 0.81 mT). o-Semiquinone free radicals were responsible for the narrower component. Nitrogen free radicals with ag-factor of 2.0030 were probably responsible for the broader component. Paramagnetic copper ions quenched the EPR signals of melanin free radicals in the studied samples. For melanin-Cu²⁺ complexes, broad EPR lines (ΔB _pp, 10–32 mT) of copper ions with ag-value of about 2.1 appeared. The influence of the microwave power on the EPR spectra of these complexes demonstrated the fast spin-lattice relaxation in the copper system in melanins.     

New photoluminescence defect spectra in silicon irradiated at 100 K: Observation of interstitial carbon?

We report photoluminescence spectra of defects in irradiated silicon which are stable below room temperature. No-phonon lines (STI)⁰ at ≈ 856 meV, ST2 at ≈ 1115 meV, and ST3 at ≈ 1126 meV are observed along with a broad emission band extending from 0.7 to 1 eV. The ST1 defect studied in detail is a deep hole trap at ≈ E_v + 0.25 eV, which in addition can bind an electron loosely. Piezospectroscopy shows that the defect is essentially <100 > axial symmetric with slight distortion to C_1h. The absence of Zeeman splittings confirms the deep hole binding in an axially symmetric potential. The independence of dopants, the annealing behaviour, and comparison to EPR and IR active defects suggest a correlation of the ST1 defect with interstitial carbon.     

Infrared tunable diode laser spectra of lithium fluoride at high temperatures

At temperatures up to 1300 K the high resolution spectrum has been measured for the 1-0 through 7-6 vibrational transitions of ⁷LiF and the 1-0 through 8-7 vibrational transitions of ⁶LiF. These infrared ro-vibrational measurements have been combined with microwave measurements taken from the literature to obtain a set of ten Dunham potential constants that reproduce all the measurements for both isotopic species to within their experimental uncertainty. The band center for the v = 1-0 transition is 894.34104±0.00020 cm^?1 for ⁷LiF and 946.12074±0.00024 cm^?1 for ⁶LiF.     

Thermally stimulated transformation of the EPR spectra in γ-irradiated bone tissue

γ-irradiated bone tissue annealed at different temperatures is studied by EPR. In powder samples the annealing-induced changes of the EPR spectra are mainly determined by sharp growth of a signal caused by the products of thermal decay of an organic component in the tissue. This signal considerably complicates the analysis of the EPR spectrum shape for the annealed samples. On the other hand, in the spectra of the plates of bone tissue a change in the lineshape of the EPR signal from CO₂⁻ radicals after annealing is more pronounced than in powder, and it is attributed to the change in the contributions from axial and orthorhombic CO₂⁻ radicals. The analysis of different radicals concentrations vs. annealing temperature revealed above 210 °C the temperature-induced transformation of orthorhombic CO₂⁻ radicals into axial ones in bone tissue which (together with organic component) is responsible for the changes of EPR lineshape at annealing.     

Small-molecular white organic light-emitting devices employing 2, 5, 2′, 5′-tetra (<Emphasis Type="Italic">p</Emphasis>-trifluoromethylstyryl)-biphenyl as single-emitting component

We demonstrate a promising single layer white light-emitting device using a dimeric trimeric phenylenvinylene derivative as emitting layer. The broad electroluminescence emission band is composed of blue component from singlet excited state of individual 2, 5, 2′, 5′-tetra (p-trifluoromethylstyryl)-biphenyl molecule and long-wavelength electromer emission in electroluminescence. Therefore, white-light emission can also be obtained with a typical three-layer structure of ITO/NPB (50 nm)/TFM-TSB (50 nm)/Alq₃ (30 nm)/LiF/Al device. The maximum brightness of this device is 809 cd/m² at 217 mA/cm² and 13 V, and the maximum luminous efficiency is 1.49 cd/A at 11 mA/cm² and 8 V.      

Optical Spectra of Gamma-Irradiated LiF Crystals with Anisotropic Lithium Nanoparticles

The optical response of Li anisotropic defects and nanoparticles from three faces of cubic LiF crystals with 10-mm edges to γ radiation from a ⁶⁰Со source with doses of 10⁵–10⁹ R at 320 K in air is studied. After a maximum dose of 10⁹ R, scanning electron microscopy revealed submicron metal lithium flakes ordered into long parallel nanofilaments. The resonance absorption band, which shifts from 272 to 295 nm with increasing dose, is assigned to lithium nanocolloids in the (110) plane. The band at 202–225 nm observed only from the (100) face narrows at doses below 10⁵ R and is assigned to diatomic fluorine molecules in interstitial sites. The appearance of the 209–212–215-nm triplets at 2.7 × 10⁵ R from the (100) plane and the 211–213–215-nm triplets at 0.9 × 10⁵ R from the (010) plane is related to radiation-induced axially symmetric bipolarons with a high oscillator strength. The splitting of the 445-nm band into a 438–445–450-nm triplet is observed only for the (010) plane after a dose of 10⁶ R and is related to surface plasmon polaritons (longitudinal and transverse) of elongated Li nanoparticles. The (F₂–F₃) complexes are oriented in the most imperfect (111) plane and are seen along three cubic axes.     

Pulsed High-Frequency EPR Investigation of Gadolinium-Doped PbZrO3 and PbTiO3

Using a resonator-free setup, pulsed high-frequency (240 GHz) electron paramagnetic resonance (EPR) experiments on gadolinium-doped PbTiO₃ and PbZrO₃ samples have been performed. It could be demonstrated that echo-detected EPR spectra can be recorded routinely from these materials. These compounds are highly absorptive at microwave frequencies, thus preventing the use of microwave resonators at very high frequencies. As echo-detected EPR allows us to record the EPR absorption directly, the effect of relative suppression of broad unstructured spectral components in conventional field-derivative EPR is avoided. The analysis of the high-frequency EPR spectra indicated that Gd³⁺ ions are additionally also positioned at highly distorted sites. This might indicate that charge compensation leads to the formation of closely correlated Gd³⁺-V _Pb′′-Gd³⁺ defects under high doping conditions in addition to Gd³⁺ inserted substitutionally at Pb²⁺ sites with undistorted oxygen and lead neighboring shells. For the orthorhombic low-temperature phase of PbZrO₃ two crystallographically inequivalent Pb²⁺ sites of equal abundance are present. The contribution of Gd³⁺ inserted substitutionally at these sites could be confirmed.     

Photochromic behaviour of doped Bi4Ge3O12 single crystal scintillators

The changes induced by ultraviolet (UV) illumination on the optical absorption and electron paramagnetic resonance (EPR) spectra of Bi₄Ge₃O₁₂ single crystals, doped either with Fe (and Gd) or Mn, have been followed at room temperature (RT). In both crystals several overlapping optical absorption bands develop under UV illumination, covering from 0.7 eV up to the band edge of the matrix. The optical damage can be bleached by heating the samples above RT or by illumination with visible light. Although these optical changes temporarily correlate with the variation of the Gd³⁺ and Mn²⁺ concentrations, it has been concluded that other defects are present and partially responsible for the optical damage.     

Thermoluminescence, structural and magnetic properties of a Li2O-B2O3-Al2O3 glass system doped with LiF and TiO2

A Li₂O-B₂O₃-Al₂O₃ glass system, un-doped and doped with LiF, and/or TiO₂ was synthesized by the fusion method and its physical properties were investigated by thermoluminescence (TL), X-ray diffraction (XRD), electron paramagnetic resonance (EPR), atomic force microscopy (AFM) and differential thermal analysis (DTA). The samples were subjected to γ-rays from a colbalt-60 (⁶⁰Co) source. These techniques provided evidence of LiF and LiF doped with Ti crystal formation in the glass system. A TL glow peak at about 433 K was sensitive to ⁶⁰Co γ-rays and showed good linearity with doses and consequently could be used to quantify radiation doses.     

EPR of neutral vacancy-helium centers in silicon

In this letter we report the first observation by EPR of helium-associated defects in solid. Two new (S = 1) EPR spectra, labeled Si-AA5 and Si-AA6, arise from the association of helium and intrinsic defects in crystalline silicon. Both are produced by helium ion implantation at room temperature and are stable to 180°C. By implanting ³He isotope, the Si-AA6 ³He hyperfine spectrum has been observed. Both centers are tentatively identified as vacancy-helium centers in neutral charge states.     

Protons and other defects in Fe-doped KTaO3

The properties of defects in Fe-doped KTaO₃, both in the form of single crystals and polycrystalline ceramics, have been investigated for a wide range of Fe concentrations. The techniques employed included infrared (IR) absorption, electron paramagnetic resonance (EPR), and ac electrical conductivity together with complex-impedance analysis. Samples were pretreated (at 900° C) in water vapor to introduce protons which take the form of OH^· _O defects, and were also treated at various oxygen partial pressures. A calibration of the OH^– IR absorption band was carried out with the aid of a deuterium nuclear-probe method. EPR showed cubic and axial Fe³⁺ spectra, but only the axial spectrum appeared for crystals with high Fe concentrations. Pre-dominantly proton conductivity was observed for samples treated in reducing atmospheres, and a proton-hopping activation energy of 0.73 eV was deduced. For samples treated in high oxygen pressures, however, hole conduction dominated. Evidence for proton interaction effects was also found, but the nature of the traps is not clear.     

Structure and energy level of native defects in as-grown and electron-irradiated zinc germanium diphosphide studied by EPR and photo-EPR

The properties of defects in as-grown p-type zinc germanium disphosphide (ZnGeP₂) and the influence of electron irradiation and annealing on the defect behavior were studied by means of electron paramagnetic resonance (EPR) and photo-EPR. Besides the well-known three native defects (V_Zn, V_P, Ge_Zn), an S=1/2 EPR spectrum with an isotropic g=2.0123 and resolved hyperfine splitting from four equivalent I=1/2 neighbors is observed in electron-irradiated ZnGeP₂. This spectrum is tentatively assigned to the isolated Ge vacancy. Photo-EPR and annealing treatments show that the high-energy electron irradiation-induced changes in the EPR intensities of the zinc and phosphorus vacancies are caused by the Fermi level shift towards the conduction band. Annealing of the electron-irradiated samples induces a shift of the Fermi level back to its original position, accompanied by an increase of the EPR signal associated with the V_Zn⁻ and a proportional increase of the EPR signal assigned to the V_P⁰ under illumination (λ<1 eV) as well as generation of a new defect. The results indicate that the EPR spectra originally assigned to the isolated V_Zn⁻ and V_P⁰ are in fact associated defects and the new defect is probably the isolated phosphorus vacancy V_Pi.     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

Fundamental analysis of defects induced by energetic ions implantation in AgCl

The effects of the energetic ions implantation in AgCl can be related to two types of perturbation due to: the electronic energy losses observed, essentially with H⁺ (2 MeV) implantation and the collision energy losses illustrated in K⁺ (0.5 MeV) implantation.

The ionization influence is essentially the sensibilization of AgCl (print-out) at RT and at LNT. We discuss the the photolysis mechanism regarding the one postulated under action of light.

The defects related to collision losses are characterized by a new optical absorption band with a characteristic length independent of the implanted ion (Na⁺ and K⁺). The nature of those defects are analysed by thermal and optical bleaching and by EPR. Those collision effects do not contribute to the print-out phenomena.     

Spectral studies on Cr ions doped in sodium-lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and emission spectra of Cr³⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xCr₂O₃ (x=0.5, 2.0, 3.0, 4.0 and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g values at g≈4.55 and g≈1.97. The EPR spectra of x=3.0 mol% of Cr₂O₃ in sodium-lead borophosphate glass sample were studied at various temperatures (295-123 K). The intensity of the resonance signals increases with decrease in temperature. The optical absorption spectrum exhibits four bands characteristic of Cr³⁺ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibit one broad band characteristic of Cr³⁺ ions in octahedral symmetry. This band has been assigned to the transition ⁴T_2g (F)→⁴A_2g (F). Correlating EPR and optical data, the molecular bonding coefficient (α) has been evaluated.