12) Schmldt, D. E.; Giese, R. W.; Conron, D.; Karger, 8. L. Anal. Chem. lQB0. 52. 177-182. ----, --. (13) Pinkerton, T. C.; Hagestam, I. H. US. Patent, 6646 153, 1984. (14) Bethell, G. S.; Ayerer, J. S.; Hancock, W. S.; Hearn, M. T. J . 8/01. Chem. 1979, 254, 2572-2574. (15) Lasson, P. 0.; Glad, M.; Hasson, L.; Mansson, M. 0.; Ohlson, S.; Mosbach, U. (21) Haroon, V.; Keith, D. A. J . Chromatogr. 1983, 276, 445-450. (22) Lunde, P. K. M.; Rane, A.; Yaffe, S. J.; et al. C/h. Pharm. Ther. 1970, (23) Miller, T. D.; Plnkerton, T. C. Anal. Chim. Acta, In press. Sir:We report here a spectrophotometric technique for determining concentration vs. distance profiles of absorbing solution species generated at an electrode surface. The technique can accurately describe diffusion layers as thin as 8 pm, after only 50 ms of electrolysis. In this paper, we describe the apparatus and its performance and discuss its potential for providing new information about mass transport and reaction mechanisms. The fundamental importance of mass transfer to electrochemistry has spawned a variety of theoretical and experimental examinations into diffusion (1, 2), convection (3-51, and migration (6) as mechanisms for the transport of redox species to an electrode. For several well-defined mass transport situations, the Faradaic current may be predicted from theories of diffusion and hydrodynamics, but in many cases solutions are not readily available. Diffusion to microelectrode arrays (7,8), mass transport in flowing streams (9, IO), and mixed convection/migration conditions are examples of cases where concentration vs. distance profiles are not available, and the Faradaic current is not accurately predictable from theory. Several approaches have been pursued to experimentally observe concentration-vs. distance profiles near an electrode, including interferometric methods (10-15) based on refractive index gradients near an electrode and on UV-vis absorption by electrogenerated species (16-18). The present approach is a spatidy resolved UV-vis absorption measurement which permits concentration vs. distance profiles to be obtained with better resolution and shorter time scales than those from previous methods. The cross section of a beam passing parallel to a planar electrode surface is magnified and imaged onto a photodiode array detector. Each diode samples a discrete distance from the electrode, and Beer's law may be used to directly determine a spatially resolved concentration profile.The cell, electrodes, and chemical systems were identical with those described previously for diffractive spectroelectrochemistry (19). Trianisylamine (TAA) in acetonitrile was oxidized at a platinum electrode to TAA+. at +0.8 V vs. aqueous SCE. The diffusion coefficient of TAA (1.25 X 10" cm2/s) and molar absorptivity for TAA+. (11OOO & 200 M-' cm-' , at 633 nm) have been reported previously (19). The optical apparatus shown in Figure 1 consists of a 2Ox magnifier producing an image of the beam cross section on the face of a 1024 element photodiode array. L1 ...

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Three prominent paramagnetic absorption systems have been detected in single crystals of a aluminum oxide (AI2O3) following reactor irradiation with and without subsequent low-temperature gamma-ray irradiation. The most prominent system has an asymmetric, slightly anisotropic absorption with g«2.00, which appears to result from the superposition of components from several paramagnetic centers. A second system consisting of twelve equally intense lines corresponds to a spin-one center in six inequivalent sites whose z axes are perpendicular to the crystal c axis and nearly parallel to rows of 0 2~ ions. The spin-Hamiltonian parameters for this defect are: g z = 2.0105± 0.0020, ^ = g y = 2.0190±0.0020, 5=1, \D\ =0.0710 ±0.0002 cm" 1 , and \E\ =0.0025±0.0005 cm -1 , where D and E have the same sign. Its spectrum is attributed to an A10 3 "" molecular ion in a pair of oxygen sites, formed by a replacement collision. The third system is obtained when a reactor-irradiated crystal of AI2O3 is gamma-ray irradiated at 77°K and measured prior to warming up. It is a three-line system which saturates with gamma-ray dose but whose saturation level increases with reactor dose. This system corresponds to a spin-| center with a symmetry axis parallel to thee axis; its spin-Hamiltonian parameters are:g n = 2.018=b0.002,g i = 2.01 l±0.002,S = f, and |Z>| =0.0373 ±0.0002 cm -1 . It is attributed to interstitial 0 + formed from interstitial 0° by ionization or hole trapping.

Single crystals of NaCIO a were subjected to gamma-ray irradiation at room temperature. Analysis of the resulting electron spin resonance spectrum indicates the production of three paramagnetic centers, initially, each with an effective spin of l. The measured parameters of a center having resolved hyperfine structure and identified as CI03 were gn =2.0085±0.00l0, g.L= 2.0090±0.OOlO, A = 14.8±0.1 G, and B=21.2±0.1 G. No hyperfine interaction was detected for the remaining two centers having gO =2.007± 0.001 and g.L=2.020±0.001, and g~2.008, respectively. The centers are reasonably stable at room temperature for several months.