TL, OSL and ESR studies on beryllium oxide

Electron spin resonance (ESR) studies were carried out to identify the defect centres responsible for the thermoluminescence (TL) and optically stimulated luminescence (OSL) processes in BeO phosphor. Two defect centres were identified in irradiated BeO phosphor by ESR measurements, which were carried out at room temperature and these were assigned to an O⁻ ion and Al²⁺ centre. The O⁻ ion (hole centre) correlates with the main 190 °C TL peak. The Al²⁺ centre (electron centre), which acts as a recombination centre, also correlates to the 190 °C TL peak. A third centre, observed during thermal annealing studies, is assigned to an O⁻ ion and is related to the high temperature TL at 317 °C. This centre also appears to be responsible for the observed OSL process in BeO phosphor.     

ESR studies of sepiolites

The electron spin resonance (ESR) spectra for each beige and brown sepiolites from Polatli-Ankara, Turkey, a clay mineral of Mg₈Si₁₂O₃₀(OH)₄(OH₂)₄(H₂O)₈, at 77 K have their own single characteristic peak atg ≈ 1.99, which begins to anneal from 437 and 536°C, respectively. The sextet lines of Mn²⁺ were also observed in addition to two peaks (g = 4.4 and 5.0) associated with Fe³⁺. A high-quality sepiolite called “meerschaum” from Sepetci-Eskisehir, Turkey, has two characteristic peaks atg ≈ 1.99, which begins to anneal from 419°C, and atg ≈ 2.03. ESR intensities of the signal atg ≈ 1.99 for beige, brown sepiolites and meerschaum are enhanced by γ-ray irradiation to give equivalent dosesD _E = 11.8 ± 3.4, 4.7 ± 2.3 and 4.6 ± 1.2 kGy, respectively. The ESR ages obtained by assessing the annual dose rate from the content of²³⁸U,²³²Th and⁴⁰K determined by γ-ray spectroscopy are 1.8 ± 0.8, 0.7 ± 0.5 and 0.9 ± 0.2 Ma, respectively, falling into the Pliocene Epoch in geological time scale in agreement with the stratigraphy.     

ESR studies of diamond powders

The ESR spectrum of two defect centres were observed in finely ground diamond powders (0.5 to 30 μm size). The one centre has been seen before in irradiated and annealed single crystals of natural diamonds (the O₁ centre) while the second one has only been seen in synthetic diamonds grown from a nickel-iron melt and is only observable below 250 K. From the study of the variation of the intensity of these lines with size of particles it could be concluded that the O₁ centre lies just beneath the surface of these particles and the S centre at least 0.5 μm below the surface.     

ESR studies of mylonite at San Andreas Fault,California

The dating of the San Andreas Fault, California based on a qualitative analysis of EPR spectra of mylonite has been carried out.     

Optical and ESR studies on Fe doped ZnS nanocrystals

Zn_1 − xFe_xS (x=0.0, 0.1, 0.2, 0.4 and 0.6) nanocrystals have been obtained by chemical co-precipitation from homogeneous solutions of zinc and iron salt compounds, with S²⁻ as precipitating anion formed by decomposition of thiophenol. The TEM micrographs show a spherical shape for ZnS nanocrystals and their average size is around 7 nm. The optical absorption spectra indicate a blue shift of the absorption edge with increasing Fe-content. The luminescence of nanoparticles excite at about 370 nm with an emission peak at around 490 nm. At room temperature, ESR signal characteristic of Fe³⁺ was observed in samples of all concentrations.     

Time-resolved Fourier-transform and continuous-wave ESR studies on photo-initiated polymerization

Radical polymerization of vinyl monomers as initiated by the diphenylphosphinoyl (DPO) radical which is formed by the photo-cleavage of 2,4,6-trimethylbenzoyl diphenylphosphine oxide (TMDPO) was investigated by continuous-wave electron spin resonance (cw ESR) and Fourier-transform (FT) ESR. Well-resolved hyperfine structures (hfs’s) of the starting radicals were observed by the time-resolved cw ESR for vinyl acetate, ethyl vinyl ether, styrene, methyl methacrylate (MMA), and isoprene. The rates of formation and the spin-lattice relaxation were determined by time-resolved FT ESR for these starting radicals. In the polymerization of MMA and isoprene the primary propagating radicals were found for the first time by the observation of their well-resolved hfs’s with timeresolved cw ESR. On the basis of the kinetic analysis including the spin-lattice relaxation, the rates of formation and the spin-lattice relaxation were determined by simulation of the time profile of FT ESR for the primary propagating radicals of MMA and isoprene. The rate of the primary propagating step was found to be by two orders greater than an average value of whole propagating steps as obtained by a steady-state measurement.     

Single-crystal ESR studies of a nitroxide radical

Summary ESR spectra are reported for a stable nitroxide radical 4-hydroxy-2, 2, 6, 6-tetramethyl-piperidine-1-oxyl (TANOL) introduced as impurity in a matrix of known structure, 4-hydroxy-2, 2, 6, 6-tetramethylpiperidine atX- andQ-band. The use of the perdeuterated nitroxide showed a considerable improvement in ESR spectra resolution for most orientations in the crystal. The interactions of protons in the neighbourhood of the paramagnetic group N−O were studied and the superhyperfine tensors for the two strongly coupled protons were determined: one of these protons is hydrogen-bonded to the N−O at a distance of 2.09 ?. The principal values of the tensor are −5.2; +5.4; −5.1 G. The second proton is further away from the electron and is certainly not involved in hydrogen bond. The principal values are in this case 2.0, −1.7, −2.7 G. Thea crystal axis is a principal direction for all magnetic tensors for TANOL in this system.

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Riassunto Si riportano gli spettri ESR per un radicale nitrossido stabile 4-idrossi-2, 2, 6, 6-tetrametil-piperidina-1-ossile (TANOL) introdotto come impurità in una matrice di struttura nota, 4-idrossi-2, 2, 6, 6-tetrametilpiperinadi (1) nelle bandeX eQ. L'uso del nitrossido predeuterato ha mostrato un considerabile miglioramento nella risoluzione degli spettri ESR per moltissime orientazioni nel cristallo. Si studiano le interazioni dei protoni nei pressi del gruppo paramagnetico N−O e si determinano i tensori superiperfini per due protoni con accoppiamento forte: uno di questi protoni è legato con l'idrogeno al N−O ad una distanza di 2.09 ?. I valori principali del tensore sono −5.2; +5.4; e −5.1 G. Il secondo protone è molto spostato dall'elettrone ed è certamente non coinvolto in un legame a idrogeno. I valori principali sono in questo caso 2.0, −1.7, −2.7 G. L'assea del cristallo à una direzione principale per tutti i tensori magnetici per il TANOL in questo sistema.

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Резюме Приводятся ESR спектры для устойчивого радикала окиси сзота TANOL, введенного, как примесь, в матрицу известной структуры вX- иQ-зону. Использование передейтерированной окиси азота обнаруживает значительное улучшение разрешения в ESR спектрах для большинства ориентаций в кристалле. Исследуется взаимодействие протонов в окрестности параметрической группы N−O. Определяются сверхтонкие тензоры для двух сильно связанных протонов: один из этих протонов представляет водород, связанный с N−O на расстоянии 2.09 ?. Главные значения этого тензора представляют −5.2, +5.4, −5.1 гаусс. Второй протон находится довольно далеко от электрона и не включается в водородную связь. В этом случае главные значения тензора равны 2.0, −1.7, −2.7 гаусс. Кристаллическая осьa представляет главное направление для всех магнитных тензоров для TANOL в этой системе.

     

ESR studies of thermally oxidized silicon wafers

We have performed ESR measurements on oxidized (111) silicon wafers and found two ESR centers. One, the previously known P_b center,was found to be a two level interface trap by corona bias studies. The other one, previously known as “damaged silicon” center was found to be a silicon center, probably related to the oxide growth process.     

ESR and conductivity studies of doped polyacetylene

Low temperature ESR spectra of cis-rich and trans-rich polyacetylene, lightly doped with pentafluorides (AsF₅, SbF₅) exhibit two clearly distinguishable ESR lines. Detailed studies of these two lines as a function of temperature, doping concentration and microwave power indicate that one of these lines originates from localized spins, the other from conduction electrons. The concentration of the localized spins, generally speaking, decreases with doping. The Pauli-like susceptibility associated with the conduction electrons changes from 1.5×10^–7 to 15×10^–7 emu·cm³/mole for various doping levels and correlates with the conductivity of the same samples. The longitudinal relaxation time of the localized spins in doped trans-rich was found to beT ₁100 s at low temperatures, consistent with values of others in undoped trans rich. The temperature dependence of the homogeneous transverse relaxationT ₂ of the localized spins in doped trans-rich polyacetylene is similar to that of undoped trans polyacetylene. These facts show that the localized spins originate from undoped regions of the doped polyacetylene, perhaps due to inhomogenous doping. Our results are consistent with either formation of metallic islands, or, probably more, with impurity band conduction in strongly disordered semiconductors.     

ESR studies op irradiated aqueous solutions of hydroxypyrimidines

The irradiated aqueous solutions of 4-hydroxypyrimidine and 2-hydroxypyrimidine hydrochloride were examined by means of the In situ radiolysis steady-etate ESR method. One and three short-lived radicals respectively were detected in alkaline solutions. The struotures and reaction mechanisms of radicals were derived. The spin density distributions were calculated by the improved McConnell's relationships and the value of coupling factor Q^H _NC was determined.     

IN SITU optical and ESR studies of polyazulene

The neutral polyazulene shows an extraordinary large ESR spin density ≈ 10²⁰ spins/g and two optical absorption peaks corresponding to the interband transitions. As the doping proceeds, the spin density as a function of applied potential charges anomalously and new optical absorption peaks characteristic to each doping stage appear successively. These results are explained tentatively by taking a unique band structure of polyazulene into account.     

Fluorescence and ESR studies on membrane oxidative damage by gamma radiation

Oxidative damage to cellular membranes critically controls the manifestation of cellular response to ionizing radiation. To gain further insight into the damaging mechanisms, we have investigated the effects of γ-radiation-generated free-radical-mediated peroxidative damage in egg yolk lecithin unilamellar liposomal membranes by employing 1,6-diphenyl-1,3,5-hexatriene (DPH). Alterations in lipid bilayer fluidity and malondialdehyde (MDA) formation were measured in irradiated liposomal membranes as a function of radiation dose (0.1-1 kGy). A relationship seems to exist between the degree of radiation-induced peroxidative damage and the magnitude of DPH fluorescence decay in irradiated membranes. Radiation-induced membrane rigidization and MDA formation were significantly reduced when α-tocopherol, a natural membrane antioxidant, was present in the liposomes suggesting an involvement of lipid free radicals in the mechanism of the damage process. The results of the present study have been compared with those obtained by the electron spin resonance (ESR) technique on human erythrocyte ghost membranes with spin-labeled phospholipids having the unique capability to sensitively report on the dynamic state of the lipid environment inside the bilayer membrane. Iodoacetamide and N-ethylmaleimide spin labels were used to investigate alterations in membrane proteins. These results have contributed to our understanding of mechanisms involved in radiation membrane oxidative damage in terms of lipid peroxidation, fluidity changes and involvement of -SH groups of membrane proteins. Combined use of fluorescence and ESR spin-label techniques is of potential interest in probing the deeper molecular mechanisms of radiation injury in cellular membranes for developing strategies to modify the radiation damage to cells.     

ESR studies on Mn(II) ion in Chinese limnetic pearls

The X-band and Q-band ESR spectra of Mn(II) ion in Chinese limnetic pearls have been studied. No ESR signal of melanin-like radical has been detected. A method of theoretical treatment on ESR spectrum of Mn(II) ion has been modified. Using it, the parameters of zero field splitting D=224 Gs, E=74 Gs and E/D=0.33 for Chinese limnetic pearls with bright pearlite colour have been calculated. The computer simulation has been done using experimental ESR spectra parameters, and satisfactory results have been obtained.     

ESR studies on sputtered amorphous SiC,SiGe and GeC films

We have investigated how the ESR signals change with compositions of the binary alloys. Two signals originating from two components are interacting rather strongly with each other and narrowed somewhat by exchange interaction. The origin of the ESR signals has been made clear by comparing those at X-band and Q-band.     

ESR spectroscopic studies of alkylammonium,pyridinium and arsonium solid salts of C 60 −

The synthesis of a series of new solid salts of the monoanion of C₆₀ using a variety of countercations, e.g., tetraalkylammonium, pyridinium and tetraalkylarsonium cations, and their initial characterization by ESR spectroscopy are reported. Both the size of the countercation and the solvent used in the synthesis of the C₆₀ monoanion salts have been found to affect the ESR spectral characteristics of the resulting salts. For example, for countercations less than 10 Å in diameter, the ESR spectral linewidths decrease as the countercation diameter decreases. In a similar manner, those salts believed to be solvent free display narrower ESR spectral linewidths and somewhat largerg-values than those in which solvent is believed to be incorporated in the crystal structure of the salt in question. Finally, ESR spectroscopy was used to show that exposure of the salts reported herein to ambient atmosphere leads to a decrease in the original ESR signal and an increase of a sharp (i.e., narrow line) signal which is superimposed on the original broad line.     

Optical and ESR studies in pure and Mn-doped LiF crystals

Growth of F absorption band with the time of X-irradiation at RT and the changes caused in the F absorption as a function of F light and thermal bleaching along with the simultaneous ESR studies in pure and Mn-doped LiF single crystals are reported.     

Spin probe ESR studies of PEGxLiClO4 polymer electrolyte systems

The technique of Electron Spin Resonance (ESR) is shown to be useful in the study of dynamics of solid polymer electrolytes (SPE). Through the ESR of the nitroxide radical (2,2,6,6-tetramethyl-1-piperidine-1-oxyl; TEMPO) dispersed in the SPE PEG₄₆LiClO₄ temperature dependence of correlation time is found. The glass transition temperature T_g is estimated to be −51 °C from the measurement of T_50G, the temperature at which the extrema separation 2A_ZZ becomes 50G and is found to be close to that measured using DSC (−51.7 °C). T_g for pure PEG-2000, which could not be measured from DSC because of its high crystallinity, is determined to be −72 °C by spin probe ESR. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

ESR studies of europium ions in alkali chlorides and bromides

ESR studies of europium ions in alkali chlorides and bromides were performed at room and liquid nitrogen temperatures. There were first reported spin Hamiltonian parameters for NaBr: Eu²⁺ and RbBr: Eu²⁺ crystals and also for coagulated europium ions in NaCl, KCl, KBr and RbBr crystals. The dependence of b₂⁰ and b₂² on the lattice constant, a, of host crystals was discussed; a single relation b₂⁰αa^?2.6 was obtained, while relations for b₂² were treated seperately for Na, K and Rb halide series, respectively. From the relative values of b₂² and b₂⁰, displacements of the nearest neighbor anions surrounding the Eu²⁺-vacancy pair were estimated to be 0.083a for NaCl and NaBr, 0.081a for KC1, 0.080a for KBr, 0.074a for RbCl and 0.078a for RbBr.