No abstract

The localized-hole model of the V center in the alhdine earth oxides is represented as a positive hole trapped on one oxygen atom adjacent to a positive-ion vacancy (in the ground state and on the time scale of an ESR experiment). Upon trapping the hole, the oxygen atom is presumed to relax away from the vacancy, thus lowering the symmetry from octahedral to tetragonal. The vahdity of this model has recently been challenged on the basis that a proper description of the ground state is a molecular orbital representing a linear combination of states with the hole on either of the two oxygen atoms opposite one another about the vacancy. We present the analysis of some ESR spectra in which we observed the resolved hyperfine splitting dne to a single 'Mg + ion adjacent to the trapped hole and on the axis of the defect. From the natural abundance of the~Mg isotope one can calculate the expected hyperfine line intensities relative to that of the central line arising from V centers having only "Mg or ' Mg atoms in the axial positions, for the localized and for the delocalized models. The measured relative intensities agree with those calculated for the localized model. An analysis of the axial spectrum indicates the presence of an unusually intense forbidden line at X band. This becomes relatively more intense at the Xu band, as expected. Shifts of line position and intensity relative to the lines of an allowed hyperfine sextet agree with those predicted. The values of the hyperfine parameters are found to be A =+2.84 MHz and B = -0.86 MHz. The quadrupole parameter is found to be +0.18 MHz. The sign of A is anomalous in that a contact interaction is expected to give an isotropic parameter with the sign of the nuclear magnetic moment. This may result from exchange polarization. The magnitude o B is interpreted as indicating a compact unpaired charge distribution. The V centers studied here were generated by fast-neutron irradiation, and there is no confusion with impurity-associated hole centers; the former have a long-term stability in MgO, while the latter decay rapidly to negligible levels at room temperature. I

Three paramagnetic Fe defect complexes have been observed by electron-spin resonance in chemically reduced crystals of SrTi03. These are identified as (1) in Fe + ion substituted on a Ti site with a nearby charge-compensating interstitial impurity, and (2) and (3) two charge states of substitutional Fe with a near-neighbor oxygen v icancy, Fe +-Vp and Fe'+-&p. These three defects are related in the sense that photoexcitation can reversibly switch electrons between the vacancy-associated and interstitial-associatedsites. An analysis of the response to pol prized light, and of the kinetics of charge transfer, shows that in both the forward and reverse directions of optical pumping the mobile charge carrier is a valence-band hole. Four distinct charge states of the Fe-&p system have now been identified, ind the rel itive instability of SrTi03 toward formation of multiple-impurity charge states is briefly discussed.

We report a simple analytic form for the convolution integral in transmission Mossbauer spectroscopy allowing accurate representation of the line shape even for very thick absorbers {t=10), and permitting easy fitting to the true line-shape function. This representation permits the accurate determination of all Mossbauer-effect (ME} parameters, including position, width, cross section, and interference. This analytic method can be applied to deconvolute accurately information contained in either source or absorber, and an explicit analytic form for the emission and absorption Fourier transforms is given. We show that from the asymptotics of the line shape, it is possible to determine all line-shape parameters, and that line-shape asymptotics can circumvent short-ranged hyperfine or instrumental broadening contributions to the observed spectrum. A formula for the correction to the line shape caused by source self-absorption is given, and it is shown that when there is significant source resonance self-absorption a "good" fit to data, judged by a chi-squared analysis, can yield completely wrong ME line-shape parameters. We find an equation for the dependence of the area under the absorption curve and the resonance peak height, and give its explicit dependence on the interference parameter and source broadening parameters. Although these effects have been neglected in earlier work, their contribution may be of order 10% in many cases of interest.

ESR spin trapping technique was used to detect and analyze free radical formation. When 6-hydroxydopamine (6-OHDA) was incubated alone or in the presence of a free radical generating system (H2O2 and FeSO4), hydroxyl free radicals were observed in a concentration-dependent manner. Glutathione was found to be the most effective scavenger of the ESR signal when compared with vitamin E or Mannitol. The addition of ethanol resulted in the formation of the pure hydroxyethyl free radicals. The amount of hydroxyethyl free radicals in the system was dependent upon the concentration of ethanol and the formation of hydroxyethyl free radicals correlated well with the extent of lipid peroxidation and the loss of enzymic activity of the membrane-bound (Na+,K+)-ATPase. We suggest that in the biological system ethanol may potentiate the neurotoxicity of 6-OHDA with the formation of hydroxyethyl free radicals, which are longer-lived and far more damaging to membranes than the hydroxyl radicals. These data lead us to further hypothesize that the neuronal degeneration caused by 6-OHDA and other compounds that generate free radicals could be potentiated in the presence of ethanol.