Photothermal Deflection Spectroscopy Investigations of Uranium Electrochemistry
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Photothermal Deflection Spectroscopy Investigations of Uranium Electrochemistry JAMES D. RUDNICKI AND RICHARD E. RUSSO
Lawrence Berkeley Laboratory, Berkeley, California, 94720, USA
ABSTRACT Photothermal Deflection Spectroscopy (PDS) has been successfully applied to the study of uranium oxide electrochemistry. A brief description of PDS and preliminary results that demonstrate the technique are presented. Concentration gradients formed at the electrode surface are measured by this technique. The gradients give insight into the reaction mechanisms. There is some evidence of the initiation of non-electrochemical dissolution of the uranium oxide. Optical absorption by the uranium oxide is measured by PDS and the first results indicate that the absorption of the surface does not change during electrochemical experiments. This result is contrary to literature measurements of bulk samples that indicate that the optical absorption should be strongly changing.
INTRODUCTION
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Photothermal Deflection Spectroscopy (PDS) is an in situ technique used to study electrochemical %04 probe systems. The technique is capable of measuring the T optical absorption spectrum of the sample surface. The n technique can also measure concentration gradients formed near an electrode surface. These gradients are caused by the consumption and production of species at the electrode surface. The concentration gradients Figure 1. Basis of PDS. The thermal are useful in the determination of reaction mechanisms. gradient caused by the absorption of The technique has been used to study a wide variety of light deflects a probe beam. electrochemical systems [1-4] and has sufficient sensitivity to detect monolayer reactions. PDS is based on the measurement of the deflection of a probe laser as it passes adjacent to an electrode surface (Fig. 1). The beam deflection, 0, is directly proportional to the gradient of the refractive index
. W ..
(1)
where w is the width of the sample, and n. is the bulk refractive index. Both concentration and temperature affect the refractive index of the electrolyte adjacent to the electrode surface. Mat. Res. Soc. Symp. Proc. Vol. 294. C1993 Materials Research Society
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These measurements are performed in situ under dynamic conditions. The detection limit for beam deflections is on the order of 1 nrad. The original use of PDS was to measure the optical absorption spectrum of a surface. As shown in Figure 1, modulated excitation light is applied to the electrode surface. The absorbed excitation light is converted to heat and forms a thermal gradient. Because the excitation light is modulated, the thermal gradients cause periodic deflections of the probe beam. The periodic deflections are measured with a lockin amplifier. When PDS is applied to electrochemical interfaces, concentration gradients formed in the electrolyte can also deflect the probe beam. Because concentration gradients vary on a much slower time scale than the thermal gradients caused by the excitation light, they can be considered essentially DC.
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