Electron Paramagentic Resonance Research Articles

Elucidating the Structural Composition of a Fe-N-C Catalyst By Nuclear and Electron Resonance Techniques

Today, state of the catalysts in proton exchange fuel cells are platinum-based. Cost estimates show that basically the catalyst is one of the most expensive components in the FC stack. While platinum is very active for the hydrogen oxidation reaction, large quantities (approx. 80 % of the overall Pt content) are required on the cathode for the oxygen reduction reaction (ORR). Thus, alternative catalysts are required. Fe-N-C catalysts achieve very promising ORR activities in PEFC applications. The most active ones are prepared from zinc immidazole frameworks in combination with an iron source (and nitrogen precursor), that are then pyrolysed at temperatures of 900 °C or even higher. This makes it very likely that inorganic spectator species are formed. In order to get some more fundamental insight, in this work we focused on the characterization of a Fe-N-C model catalyst. The material was prepared from iron porphyrin supported on carbon with low iron loading at 600 °C followed by acid leaching. For this condition, previous results indicate that still the overall amount of FeN4 centers remain intact, but transforms to slightly different local environments. Herein, we will show by the use of nuclear inelastic scattering (also known as Nuclear Resonance Vibrational Spectroscopy), low temperature Mössbauer spectroscopy and Electron Paramagentic Resonance spectroscopy that the above given conclusion was wrong: even for these mild preparation conditions large amounts of spectator species are found, that overlay in room temperature Mössbauer spectroscopy with the so-called D1 doublet, previously assigned as ORR active site by us and others. Nevertheless, still the catalyst contains about 50 % FeN4 centers, thereof a partial fraction that is coordinated by oxygen molecules. These results underline the importance of in-depth analysis of Fe-N-C catalysts as the amount of spectator species should be decreased in best case to zero, in order to avoid side reactions.

Distribution of vanadyl complexes and free radicals in asphaltenes fractions from electron paramagnetic resonance

Structural characterization of asphaltenes in complex systems is in the focus of scientific and industrial interests since many years. Asphaltenes isolated from the high-viscous Ashal’cha oil (2500 mPa·s) were studied. Five asphaltene fractions were obtained by stepwise extraction with an alcohol-benzene solvent with different alcohol contents after precipitating the asphaltenes from the oil with hot isooctane. Redistribution of paramagnetic centers between the fractions of asphaltenes is revealed by electron paramagentic resonance (EPR) while no significant changes in EPR spectral parameters are found. The results can be used for tracking the asphatenes formation as well as following the changes of their supramolecular structure.

In Situ Spectroelectrochemical Investigations of the Redox-Active Tris[4-(pyridin-4-yl)phenyl]amine Ligand and a Zn(2+) Coordination Framework.

An investigation of the redox-active tris[4-(pyridin-4-yl)phenyl]amine (NPy3) ligand in the solution state and upon its incorporation into the solid-state metal-organic framework (MOF) [Zn(NPy3)(NO2)2·xMeOH·xDMF]n (MeOH = methanol and DMF = N,N-dimethylformamide) was conducted using in situ UV/vis/near-IR, electron paramagentic resonance (EPR), and fluorescence spectroelectrochemical experiments. Through this multifaceted approach, the properties of the ligand and framework were elucidated and quantified as a function of the redox state of the triarylamine core, which can undergo a one-electron oxidation to its radical cation. The use of pulsed EPR experiments revealed that the radical generated was highly delocalized throughout the entire ligand backbone. This combination of techniques provides comprehensive insight into electronic delocalization in a framework system and demonstrates the utility of in situ spectroelectrochemical methods in assessing electroactive MOFs.

Generation of a RuII–Semiquinone–Anilino‐Radical Complex through the Deprotonation of a RuIII–Semiquinone–Anilido Complex

Calming the radicals: Aminyl radicals may one day be useful oxidation catalysts if their thermodynamic instability can be overcome, for example, through complexation with a metal. The semiquinone–anilino-radical and catechol–anilino-radical complexes 1 and 2 have now been prepared and the anilino-radical character of the tetradentate amine ligand proved by electron paramagentic resonance and resonance Raman spectroscopy, as well as DFT calculations.

Electron Traps on Oxide Surfaces: (H+)(e−) Pairs Stabilized on the Surface of 17O Enriched CaO

(H+)(e-) pairs generated at the surface of polycrystalline CaO are analyzed for the first time in terms of the interaction of the unpaired electron spin with the nuclear spin of the 17O anions of the surface. CaO crystals enriched in the 17O isotope are prepared and the corresponding hyperfine coupling constants are measured in electron paramagentic resonance (EPR) spectra. The results are analyzed on the basis of cluster model density functional theory calculations. The computed hyperfine coupling constants for (H+)(e-) pairs formed on the edge, corner, and reverse corner sites of the CaO surface allow a tentative assignment of two observed spectral features to specific morphological surface sites.

Ferromagnetic Mn-Mn Interactions in Low Doped Zn1-xMnxO Thin Films

ABSTRACTWe have studied by electron paramagentic resonance spectroscopy the Mn2+ EPR spectra in low doped (x=0.03) Zn1-xMnxO films of n-type conductivity prepared by metalorganic chemical vapour deposition. Contrary to higher doped films with x=0.07, the x=0.03 film shows a weak ferromagnetic Mn-Mn interaction which we attribute to the presence of moderate deep donors leading to magnetic polaron formation.

Image reconstruction in regions of interest from truncated Radon transforms of even dimensions

It is well known that an exact image within a region of interest (ROI) can be reconstructed from truncated Radon transforms of odd dimensions. It remains, however, unclear whether exact ROI images can be reconstructed from the truncated n-dimensional (nD) Radon transform, where n ⩾ 4 is an even number. In this work, we show that it is possible to reconstruct ROI images exactly from truncated Radon transforms of even dimensions. In particular, based upon our derived formula, two algorithms can be devised for exact reconstruction of ROI images from truncated Radon transforms of even dimensions. This work may find applications in high-dimensional tomographic imaging such as magnetic resonance spectroscopic imaging and electron paramagentic resonance imaging in which the data function can be interpreted as the 4D Radon transform.

Photooxidative Degradation of Beer Bittering Principles: A Key Step on the Route to Lightstruck Flavor Formation in Beer

Isohumulones, dihydroisohumulones, tetrahydroisohumulones, and humulinones, important hop-derived bittering compounds in beer, were shown to give rise to reactive triacylmethyl radicals on interaction with triplet-excited riboflavin after spin trapping by 5,5-dimethyl-1-pyrroline N-oxide or 2-methyl-2-nitrosopropane, followed by electron paramagentic resonance spectroscopy combined with spectral simulation. Electron abstraction from the ionized beta-tricarbonyl chromophore, which is common to all five-membered ring hop derivatives, is the initial event on photoinduced degradation. Radicaloid decomposition of isohumulones leads to precursors for 3-methylbut-2-ene-1-thiol, the lightstruck constituent in beer. Interaction of reduced derivatives of isohumulones with triplet-excited riboflavin furnished radical precursors of volatile aldehydes, which may lead to the development of unpleasant stale or cardboard flavors.

Antigenotoxic properties of Cassia tea (Cassia tora L.): Mechanism of action and the influence of roasting process

Antigenotoxic properties and the possible mechanisms of water extracts from Cassia tora L. (WECT) treated with different degrees of roasting (unroasted and roasted at 150 and 250°C) were evaluated by the Ames Salmonella/microsome test and the Comet assay. Results indicated that WECT, especially unroasted C. tora (WEUCT), markedly suppressed the mutagenicity of 2-amino-6-methyldipyrido(1,2-a:3′:2′-d)imidazole (Glu-P-1) and 3-amino-1,4-dimethyl-5H-pyrido(4,3-b)indole (Trp-P-1). In the Comet assay performed on human lymphocytes, WECT exhibited significant protective effect on Trp-P-1-mediated DNA damage followed the order of unroasted (55%) > roasted at 150°C (42% ) > roasted at 250°C (29%). Pre-treatment of the lymphocytes with WEUCT resulted in 30% repression of DNA damage. However, no significant effect on excision-repair system was found during DNA damage expression time in post-treatment scheme (p>0.05). WEUCT showed 84% scavenging effect on oxygen free radicals generated in the activation process of mutagen detected by electron paramagentic resonance system. Two possible mechanisms were considered: (1) neutralization the reactive intermediate of Trp-P-1; and (2) protecting cells directly as an antioxidant that scavenge the oxygen radicals from the activation process of mutagen. The individual anthraquinone content in extracts of C. tora was measured by HPLC. Three anthraquinones, chrysophanol, emodin and rhein, have been detected under experimental conditions. The anthraquinone content decreased with increased roasting temperature. Each of these anthraquinones demonstrated significant antigenotoxicity against Trp-P-1 in the Comet assay. In conclusion, our data suggest that the decrease in antigenotoxic potency of roasted C. tora was related to the reduction in their anthraquinones.

Nickel superoxide dismutase structure and mechanism.

The 1.30 A resolution crystal structure of nickel superoxide dismutase (NiSOD) identifies a novel SOD fold, assembly, and Ni active site. NiSOD is a hexameric assembly of right-handed 4-helix bundles of up-down-up-down topology with N-terminal hooks chelating the active site Ni ions. This newly identified nine-residue Ni-hook structural motif (His-Cys-X-X-Pro-Cys-Gly-X-Tyr) provides almost all interactions critical for metal binding and catalysis, and thus will likely be diagnostic of NiSODs. Conserved lysine residues are positioned for electrostatic guidance of the superoxide anion to the narrow active site channel. Apo structures show that the Ni-hook motif is unfolded prior to metal binding. The active site Ni geometry cycles from square planar Ni(II), with thiolate (Cys2 and Cys6) and backbone nitrogen (His1 and Cys2) ligands, to square pyramidal Ni(III) with an added axial His1 side chain ligand, consistent with electron paramagentic resonance spectroscopy. Analyses of the three NiSOD structures and comparisons to the Cu,Zn and Mn/Fe SODs support specific molecular mechanisms for NiSOD maturation and catalysis, and identify important structure-function relationships conserved among SODs.

Significant Pulmonary Response to a Brief High-Level, Nose-Only Nitrogen Dioxide Exposure: An Interspecies Dosimetry Perspective

Brief, high-level nitrogen dioxide (NO 2) exposures are major hazards during fires and heat-generating explosions. To characterize the lung response to a brief high-level NO 2 exposure, we exposed two groups ( n = 5) of 325–375 g, male, Sprague–Dawley rats to either 200 ± 5 ppm (376 ± 9 mg/m 3) NO 2 or room air for 15 min. The rats were nose-only exposed in a multiport exposure chamber fitted with pressure transducers to monitor their respiration during exposure. One hour after exposure, we euthanized the rats, collected blood samples, lavaged the lungs with warm saline, and then excised them. One lung lobe was cooled to −196°C and used for low-temperature electron paramagentic resonance (EPR) analysis. The remainder was homogenized and used for biochemical analyses. Inspired minute ventilation ( V i) during exposure decreased 59% ( p < 0.05). Calculated total inspired dose was 0.880 mg NO 2. In lung lavage, both total and alveolar macrophage cell counts declined (∼75%, p < 0.05), but epithelial cell count increased 8.5-fold. Lung weight increased 40% ( p < 0.05) after exposure. In the blood, potassium and methemoglobin increased 45 and 18% ( p < 0.05), respectively; glucose, lactate, and total hemoglobin were not altered significantly. EPR analysis of lung tissue revealed hemoglobin oxidation and carbon-centered radical formation. Vitamins E and C and uric acid were depleted, and lipid peroxidation measured by three different methods (TBARS, conjugated dienes, and fluorescent peroxidation end products) was elevated, but total protein, DNA, and lipid contents were unchanged. These observations combined demonstrate that a brief (15 min) high-level (200 ppm) NO 2 exposure of rats was sufficient to cause significant damage. However, comparison of the exposure dose normalized to rat body weight with previously reported sheep and estimated human values revealed significant differences. This raises a question about interspecies dosimetry and species-specific responses when animal data are extrapolated to humans and used for safety standard setting, particularly with high-level brief exposures.

Significant Pulmonary Response to a Brief High-Level, Nose-Only Nitrogen Dioxide Exposure: An Interspecies Dosimetry Perspective

Brief, high-level nitrogen dioxide (NO 2 ) exposures are major hazards during fires and heat-generating explosions. To characterize the lung response to a brief high-level NO 2 exposure, we exposed two groups ( n = 5) of 325–375 g, male, Sprague–Dawley rats to either 200 ± 5 ppm (376 ± 9 mg/m 3 ) NO 2 or room air for 15 min. The rats were nose-only exposed in a multiport exposure chamber fitted with pressure transducers to monitor their respiration during exposure. One hour after exposure, we euthanized the rats, collected blood samples, lavaged the lungs with warm saline, and then excised them. One lung lobe was cooled to −196°C and used for low-temperature electron paramagentic resonance (EPR) analysis. The remainder was homogenized and used for biochemical analyses. Inspired minute ventilation ( V i ) during exposure decreased 59% ( p < 0.05). Calculated total inspired dose was 0.880 mg NO 2 . In lung lavage, both total and alveolar macrophage cell counts declined (∼75%, p < 0.05), but epithelial cell count increased 8.5-fold. Lung weight increased 40% ( p < 0.05) after exposure. In the blood, potassium and methemoglobin increased 45 and 18% ( p < 0.05), respectively; glucose, lactate, and total hemoglobin were not altered significantly. EPR analysis of lung tissue revealed hemoglobin oxidation and carbon-centered radical formation. Vitamins E and C and uric acid were depleted, and lipid peroxidation measured by three different methods (TBARS, conjugated dienes, and fluorescent peroxidation end products) was elevated, but total protein, DNA, and lipid contents were unchanged. These observations combined demonstrate that a brief (15 min) high-level (200 ppm) NO 2 exposure of rats was sufficient to cause significant damage. However, comparison of the exposure dose normalized to rat body weight with previously reported sheep and estimated human values revealed significant differences. This raises a question about interspecies dosimetry and species-specific responses when animal data are extrapolated to humans and used for safety standard setting, particularly with high-level brief exposures.

NO Monomers on MgO Powders and Thin Films

The interaction of nitric oxide with the surface of MgO at low temperature has been studied experimentally and theoretically. High-surface-area polycrystalline MgO prepared by chemical vapor deposition has been exposed to both 14NO and 15NO and the corresponding adsorption complexes have been monitored by means of electron paramagentic resonance (EPR) spectra. MgO(100) thin films grown on Mo(100) in UHV conditions have been exposed to 15NO and the adsorption products have been investigated by thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. The structure and properties of NO/MgO(100) have been studied by cluster model density functional theory (DFT) calculations. The EPR data show that only 0.5% of the total NO deposited is in a paramagnetic state; out of the paramagnetic species, 98% are physisorbed and only 2% are chemisorbed. This is explained with the fact that on the terrace sites NO monomers interact very weakly and prefer to form dimers, (NO)2. Only at defect sites (low-coordinated cations) the interaction of NO monomers with the MgO surface is stronger and prevents the formation of the diamagnetic dimers. A small minority of chemisorbed species is formed only at low coordinated anions (steps, edges, or corners) or oxygen vacancies.

Ordered arrays of II/VI diluted magnetic semiconductor quantum wires: formation within mesoporous MCM-41 silica.

We present a novel way of synthesising highly ordered arrays of hollow Cd(1-x)Mn(x)S quantum wires with lateral dimensions of 3-4 nm separated by 1-2 nm SiO2 barriers by forming Cd(1-x)Mn(x)S (0 < or = x < or = 1) semiconductors inside the pore system of mesoporous MCM-41 SiO2 host structures. X-ray diffraction and transmission electron microscopy (TEM) studies reveal the hexagonal symmetry of these arrays (space group p6m) and confirm the high degree of order. Physisorption measurements show the filling of the pores of the MCM-41 SiO2. The X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), electron paramagentic resonance (EPR), and Raman studies confirm the good crystalline quality of the incorporated (Cd,Mn)S guest. The effects of reducing the lateral dimensions on the magnetic and electronic properties of the diluted magnetic semiconductor were studied by photoluminescence (PL) and PL excitation spectroscopy and by SQUID and EPR measurements in the temperature range 2-400 K. Due to the quantum confinement of the excitons in the wires, an increase of about 200 meV in the direct band gap was observed. In addition, the p-d hybridisation-related bowing of the band gap as a function of Mn concentration in the wires is much stronger than in the bulk. This effect is related to the increase in the band gap due to quantum confinement, which shifts the p-like valence band edge closer to the 3d-related states of Mn in the valence band. Thus, the p-d hybridisation and the strength of the band gap bowing are increased. Compared to bulk (II,Mn)VI compounds, antiferromagnetic coupling between the magnetic moments of the Mn2+ ions is weaker. For the samples with high Mn concentrations (x > 0.8) this leads to a suppression of the phase transition of the Mn system from paramagnetic to antiferromagnetic. This effect can be explained by the fact that the lateral dimensions of the wires are smaller than the magnetic length scale of the antiferromagnetic ordering.

Effects of RuO2 dispersion on photocatalytic activity for water decomposition of BaTi4O9 with a pentagonal prism tunnel and K2Ti4O9 with a zigzag layer structure

Effects of RuO2 dispersion on the photocatalytic activity for water decomposition of BaTi4O9 (BTO) with a pentagonal prism tunnel structure and K2Ti4O9 (KTO) with a zigzag layer structure were studied. These titanates were impregnated with either a RuCl3 aqueous solution or a Ru3(CO)12 complex in THF and were subjected to oxidation at high temperatures. In comparison with the chloride, the employment of Ru3(CO)12 complex gave rise to the higher photocatalytic activity of BTO and a significant improvement of photocatalytic performance of KTO. High resolution transmission electron microscopic observation showed that the carbonyl complex led to the formation of uniformly distributed small RuO2 particles (average particle size, ρav, was 1.8 nm for BTO and 5.1 nm for KTO) than did the chloride (ρav was 2.7 for BTO and 15 nm for KTO). The tunnel structure of BTO is shown to have the high capability to disperse RuO2 as small particles, compared to the zigzag layer. The photoexcited charge formation of KTO with UV irradiation was examined by electron paramagentic resonance (EPR) measurements. A correlation between the photocatalytic activity and the ability of surface lattice O− formation is established for these titanates with different structures. The size effects of RuO2 particles on photocatalysis are discussed.

Microencapsulation of paramagnetic particles by pyrroxylin to preserve their responsiveness to oxygen when used as sensors for in vivo EPR oximetry.

Using the broadening of the electron paramagentic resonance (EPR) linewidth of paramagnetic particles by oxygen, it is possible to make measurements of the partial pressure of oxygen in vivo. While the results obtained so far with EPR oximetry are very encouraging, several paramagnetic materials may lose their responsiveness to oxygen in tissues. This aim of this study was to provide evidence that an appropriate coating can preserve the oxygen sensitivity of paramagnetic materials in vivo. Two charcoals that have the oxygen-sensing properties required for EPR oximetry (combined with a tendency to lose responsiveness to oxygen when placed in tissues) were coated using pyroxylin. Sensitivity to variations in pO2 was checked by inducing hypoxia in the muscles of mice injected with charcoal. While the uncoated material lost responsiveness to oxygen within few days, the particles coated with 20-30% of pyroxylin did not lose their responsiveness for more than 2 months.

Structure of the Molybdenum Site of Dimethyl Sulfoxide Reductase

Molybdenum K-edge X-ray absorption and Mo(V) electron paramagentic resonance (EPR) spectroscopies have been used to probe the metal coordination in oxidized and reduced forms of both wild-type and a site-directed mutant of Rhodobacter sphaeroides dimethyl sulfoxide (DMSO) reductase. We confirm our earlier findings (George, G. N.; Hilton, J.; Rajagopalan, K. V. J. Am. Chem. Soc. 1996, 118, 1113−1117) that the molybdenum site of the oxidized Mo(VI) enzyme possesses one terminal oxygen ligand (MoO) at 1.68 Å, four thiolate ligands at 2.44 Å, and one oxygen at 1.92 Å and that the dithionite-reduced Mo(IV) enzyme possesses a desoxo species with three or four Mo−S at 2.33 Å and two different Mo−O ligands at 2.16 and 1.92 Å. Mo(V) EPR indicates the presence of one exchangeable oxygen ligand, most likely an Mo−OH, in the signal-giving species, probably originating from the MoO of the oxidized enzyme (Em8.5(IV/V) = +37 mV, Em8.5(V/VI) = +83 mV). The addition of dimethyl sulfide, in the reverse of the physiological reaction, reduces the enzyme. In this form, the enzyme contains a desoxo active site with four Mo−S at 2.36 Å and two different Mo−O ligands at 1.94 and 2.14 Å. Recombinant wild-type R. sphaeroides DMSO reductase expressed in Escherichia coli initially has a dioxo structure (two MoO at 1.72 Å and four Mo−S at 2.48 Å) but assumes the wild-type Mo(VI) structure after a cycle of reduction and reoxidation. The site-directed Ser147→Cys mutant possesses a monooxo active site in the oxidized state (MoO at 1.70 Å) with five sulfur ligands (at 2.40 Å), consistent with cysteine 147 coordination to Mo. The dithionite reduced form of the mutant possesses a desoxo site also with five Mo−S ligands (at 2.37 Å) and one Mo−O at 2.12 Å. The variant has substantially different Mo(V) EPR and electrochemistry (Em8.5(IV/V) = −43 mV, Em8.5(V/VI) = +106 mV). The active-site structure and catalytic mechanism of DMSO reductase are discussed in the light of these results.

Influence of iron-removal procedures on sequential electron transfer in photosynthetic bacterial reaction centers studied by transient EPR spectroscopy.

Electron spin polarized electron paramagentic resonance (ESP EPR) spectra were obtained with deuterated iron-removed photosynthetic bacterial reaction centers (RCs) to specifically investigate the effect of the rate of primary charge separation, metal-site occupancy, and H-subunit content on the observed P865+QA- charge-separated state. Fe-removed and Zn-substituted RCs from Rb. sphaeroides R-26 were prepared by refined procedures, and specific electron transfer rates (kQ) from the intermediate acceptor H- to the primary acceptor QA of (200 ps)-1 vs (3-6 ns)-1 were observed. Correlation of the transient EPR and optical results shows that the observed slow kQ rate in Fe-removed RCs is H-subunit-independent, and, in some cases, independent of Fe-site occupancy as Zn2+ substitution does not ensure retention of the native kQ. In addition, shifts in the optical spectrum of P865 and differences in the high-field region of the Q-band ESP spectrum for Fe-removed RCs with slow kQ indicate possible structural changes near P865. The experimental X-band and Q-band spin-polarized EPR spectra for deuterated Fe-removed RCs where kQ is at least 15-fold slower at room temperature than the (200 ps)-1 rate observed for native Fe-containing RCs have different relative amplitudes and small g-value shifts compared to the spectra of Zn-RCs which have a kQ unchanged from native RCs. These differences reflect the trends in polarization predicted from the sequential electron transfer polarization (SETP) model [Morris et al. (1995) J. Phys. Chem. 99, 3854-3866; Tang et al. (1996) Chem. Phys. Lett. 253, 293-298]. Thus, SETP modeling of these highly resolved ESP spectra obtained with well-characterized proteins will provide definitive information about any light-induced structural changes of P865, H, and QA that occur upon formation of the P865+QA- charge-separated state.

Direct simulation of continuous wave electron paramagnetic resonance spectra from Brownian dynamics trajectories

A method is developed for the direct simulation of continuous wave electron paramagentic resonance (CW-EPR) line shapes from molecular trajectories. The method is applied to the cases of an axially symmetric nitroxide spin label undergoing two different types of motion (i) two-site jumping motion and (ii) isotropic rotational Brownian motion. Probabilistic models are developed that generate trajectories stable for large numbers of evolution time steps. Spectra at both 10 (X band) and 35 (Q band) GHz of satisfactory quality are generated from 40 trajectories of 214 steps sampled at 0.05 ns intervals. The resulting calculations illustrate that this method can be used as a basis of CW-EPR spectral simulation for systems undergoing arbitrarily complex dynamics using trajectories generated by classical Brownian dynamics.

Some effects of pulsed CO 2 laser heating of silica

Silicon dioxide glass (silica) has been irradiated with pulsed multi-mode CO 2 laser light at wavelengths at 9.3 and 10.6 μm. These wavelengths fall within the restrahlen bands of silica. Electron paramagentic resonance spectra of irradiated samples contain the E′ 1 component (singly charged oxygen vacancies) which was not observed prior to irradiation. Dissolving the surfaces of irradiated samples with HF decreases the intensity of the E′ 1 component. The concentration of E′ 1 centers decreases with distance from the irradiated surfaces. The profile of concentration versus depth obtained by successive HF etching treatments shows a decrease by a factory of 3 between the surface and a depth of 0.2 μm. a constant concentration to a depth of 1 μm and then a decrease below detectable limits at 2 μm. An approximate calculation of temperature produced by the laser pulse at 9.3 and 10.6 μm shows that temperatures > 2000 K were produced at depths ∼ 0.5 μm. The high concentration of E′ 1 centers is attributed to the production of oxygen vacancies by high temperatures produced by the laser pulses and the subsequent rapid cooling.