John E. Wertz

Biological applications of electron spin resonance

The first publication on the use of ESR techniques in connection with biological systems appeared only nine years after the first demonstration of paramagnetic resonant absorption.† In that first report, a variety of biological systems, such as leaves, seeds, and tissue preparations, were shown to contain free radicals. A definite correlation was found between the concentration of the radicals and the metabolic activity of the material. This work appears to confirm earlier ideas that free radicals are involved as intermediates in metabolic processes. However, the question as to which free radicals are involved in a given metabolic process proved to be a much more difficult question to answer.

A HYDROGEN-CONTAINING TRAPPED HOLE CENTER IN MAGNESIUM OXIDE

Abstract Earlier ESR studies of X-irradiated magnesium oxide had shown the existence of centers having a positive hole trapped on an oxygen atom adjacent to a positive ion vacancy ( V 1 -centers). Some MgO samples show a qualitatively similar ESR spectrum with each line a doublet. Electronnuclear double resonance studies show unambiguously that the splitting arises from a hydrogen atom collinear with the positive hole and the vacancy. Infrared studies of unirradiated MgO had earlier shown a sharp band at 3296 cm −1 , which we observe to be shifted to 3323 cm −1 on irradiation. Decay of the 3323 cm −1 band occurs with corresponding growth of the 3296 cm −1 band and loss of the ESR signal. The 3296 cm −1 band is attributed to an OH− ion with hydrogen adjacent to the vacancy. The shift occurs when a hole is trapped on an oxygen opposite the vacancy from the hydrogen atom of OH − . We refer to the new center as the V OH − center.

Spin Resonance of Point Defects in Magnesium Oxide

Transition metal impurity ions and intrinsic defects such as trapped electron and trapped hole centers are readily detected in magnesium oxide crystals by electron spin resonance. The valence state of impurity ions may readily be changed by heat treatment or by irradiation. For nickel and cobalt one may observe both the 2+ and 1+ states, while for iron, one additionally observes the 3+ state. The extra positive charge of trivalent ions is usually compensated by positive ion vacancies. Vacancies, either single or in clusters, alter the electric field symmetry about an impurity ion or intrinsic defect. Hence they may indicate their presence by altering the symmetry from cubic to tetragonal, rhombic or lower type. The impurities are generally substitutional, but one interstitial type has been found. Isolated positive ion vacancies can trap either one or two holes, in each case producing centers of axial electric field symmetry. Other hole centers are observed after extensive atomic displacements have been pr...

Electron spin resonance studies of radiation effects in inorganic solids

Defects induced in magnesium oxide by neutron irradiation are largely the result of atom displacements. The vacancies which are left behind are sites at which either electrons or holes may be trapped, depending upon the sign of the displaced ion. Such bound electrons or holes are paramagnetic entities which may give rise to ESR absorption. Thus, negative-ion vacancies in three different environments are trapping sites for electrons, and positive-ion vacancies of two types are trapping sites for holes. These two hole centres are in addition to those types found in crystals which were not neutron-irradiated. Some of these centres may be induced by grinding and x irradiation of MgO or other alkaline earth oxides, sulfides or selenides which have the MgO structure. Mechanical working is believed to distribute negativeion vacancies by dissociation from dislocations which are swept through the crystals. In the neutron-irradiated crystals, there is evidence that some local regions are so disrupted that they rearrange to body-centred instead of face-centred packing. Extended heating largely anneals out the neutron-produced structural defects. One is then enabled to see ESR spectra of defects which are highly sensitive to distortions of cubic symmetry. (auth)

III. Spin resonance studies of defects in magnesium oxide

Abstract : Excluding impurities, trapped holes are the most important defect centres which are observable by electronspin resonance in presently-available magnesium oxide crystals. Models are suggested for two types of trapped hole centres. The first, involving one hole, is believed to involve the following linear array: hole trapped on an oxygen atom-positive ion vacancy-normal O(-2) iontrivalent impurity ion. The second centre, which is far less stable, involves two holes opposite one another about a positive-ion vacancy. It is suggested that another defect is associated with this pair to cause localization of the axis of symmetry. These conclusions are drawn from electronspin resonance spectra showing simple axial symmetry for single-hole centres lying along principal crystal axes. For hole-pair centres one observes pairs of lines approximately centred upon the positions of the previously-mentioned axial lines. More complex hole centre spectra have also been observed. (Author)

Doubly-associated cation-vacancy centers in magnesium oxide☆

Abstract Some Cr 3+ ions in MgO are known to be associated with a vacancy in either the nnn or nn cation positions, as inferred from their well-established ESR spectra showing tetragonal ( T ) and rhombic ( R ) symmetry respectively. We examine here some “satellite” spectra designated as T ′ and R ′, which show the same angular dependence and principal g -values respectively as the T and R spectra. They thus reflect similar tetragonal or rhombic symmetries of the centers responsible. These T ′ and R ′ centers are the focus of interest in this paper. The zero-field splitting parameter D for the T ′ center is −864.4 G , while that of the T center is −887.1 G at 295°K. The proposed model for the T ′ center is: Cr + -O- -O- M + , lying along 〈001〉, 〈010〉 or 〈100〉 axes. The model for the R ′ center is: Cr + - - M + lying along a [110] or equivalent direction. (Only deviations from normal site charge are shown.) A sample doped with aluminum gave an ESR line intensity ratio T′ T = 90 1 after heating at 400°. This behavior is interpreted as implicating Al 3+ as M + ion in this case.

Trivalent titanium in tetragonal symmetry in MgO and CaO

Abstract Observation of 47,49Ti hyperfine structure in a tetragonal EPR spectrum in magnesium oxide has established conclusively that the impurity involved is titanium. The spin-Hamiltonian parameters are g∥ = 1.95304 ± 0.00010, g⊥ = 1.89878 ± 0.00010, |47,49A∥| = (26.8 ± 0.2) × 10−4 cm−1 and |47,49A ⊥ ⋎ = (10.0 ± 0.5) × 10 −4 cm −1 . These indicate that the titanium is trivalent and is situated in a field of predominantly octahedral symmetry, with a smaller tetragonal component. The spectrum is ascribed to the structure Ti+  O  []. Here deviations from the normal lattice charge are shown; the brackets represent a magnesium vacancy. The parameters for the corresponding center in calcium oxide are found to be g∥ = 1.9427 ± 0.0001, g⊥ = 1.9380 ± 0.0001, |47,49A∥| = (27.4 ± 0.1) × 10−4 cm−1 and |47.49A.⊥| = (9.9 ±1.5) × 10−4 cm−1. Two further tetragonal centers are reported for CaO. The parameters are g∥(A) = 1.9425 ± 0.0001, g⊥(A) = 1.9365 ± 0.0001 and g∥(B) = 1.9424 ± 0.0001, g⊥(B) = 1.9359 ± 0.0001. It is tentatively suggested that these centers have the form Ti+  O  []  O  M, where M is a diamagnetic ion with positive charge greater than two.

Thermoluminescent Formation of Cr2+ in Magnesium Oxide

In undoped crystals of MgO, x irradiation converts a large fraction of Cr3+ in octahedral symmetry to Cr2+. Heating such crystals until a blue thermoluminescence has been observed causes further reduction. Usually 95% or more of the Cr3+ is converted by the combined treatments. However, in Cr‐doped MgO one is able to reduce only a small fraction of the octahedral chromium to Cr2+. The Cr2+ is reconverted to Cr3+ by heating. ESR measurements show that in the same temperature interval nearly all of the Fe1+ formed on x irradiation has disappeared. The source of electrons acquired by Cr3+ thus appears to be the Fe1+ ions. The thermoluminescence is ascribed to the Cr3+‐electron interaction, as suggested by Hansler and Segelken.

Electron spin resonance and optical studies of the double-hole (V0) centre in MgO

Isolated cation vacancies are created by neutron irradiation of MgO crystals maintained at approximately 32 degrees C; these vacancies trap one positive hole to form stable V- centres (Mg O+(=)O Mg). Subsequent X-irradiation at 77K causes some V- centres to trap an additional hole to form V0 centres (Mg O+(=)O+ Mg). In these representations, only derivations from normal site charge are shown. Independently, some unstable VM centres (Mg O+(=)O M+) are formed. Here M+ represents a substitutional trivalent ion. ESR spectra of V- and VM centres are indistinguishable at 77K; however, at room temperature, the V- centres give an isotropic line which permits the unambiguous determination of their number. Decay of V0 and VM centres at room temperature regenerates the number of V- centres originally present before X-irradiation. Analysis of the optical absorption of V0 centres shows that at 77K their absorption band in MgO is centred at 2.37 eV and has a half-width of 0.95 eV. Measurements of the oscillator strength of the V0 centre give f=0.24, approximately 1.7 times that of the V- centre.

The VOH center in CaO

In MgO; an OH- impurity ion substituting for O2- may associate with a cation vacancy to give the array O [ = ] HO+. (Here only effective charges are shown.) A positive hole may then be trapped on the oxygen atom to give the VOH center O+ [ = ] HO+. ENDOR studies verify the geometry of this defect in CaO.