Optical studies of electrode-electrolyte solution interphase

Modulated specular reflectance spectroscopy has been used to investigate the electrode-electrolyte solution interphase. This in situ technique enables small reflectivity changes, arising from structural and electronic changes at the interface, to be measured with a high sensitivity (AR/12 =± l x 10-6 ).Three systems have been studied, lead and mercury electrodes in aqueous sodium fluoride, and a platinum electrode in. aqueous sulphuric acid. A liquid mercury electrode is unsuitable for study by this technique due to electro-mechanical oscillations which cause spurious reflectance charges. It is shown that a thin mercury film on an electrolytically amalgamated platinum substrate gives excellent electrochemical and optical behavior. For both the lead and mercury systems the major components of the observed reflectivity changes were found to arise from the electroreflectance effect in the surface of the metal and changes in the layer of adsorbed water. These effects were separated, and from the letter component the' change in surface water concentration was computed as a function of charge. For lead this concentration appeared to be constant between-13 and -6 x 10 6 C cm 2 and to increase by about .6 x 1013 molecules cm2 between -6 and +5 x 10-6 C cm 2. The values for mercury were similar, being constant between -10 and -1a. x 10 6 C cm 2 and increasing by 5 x 1013 molecules cm -2 on increasing the charge to +10 x 10-6 b CM-2. The electroreflectance spectrum of lead is shown to arise from both the modulation of the free electron density and the modulation of the interband transition energies. Computed spectra were fitted to the experimental data by considering the modulation of three interband transitions using the modulations of their energies as empirical fitting parameters. It was found necessary to include these interband effects at all surface charges. The mercury electrode was shown not to exhibit the simple free electron behavior anticipated. The optical behavior of the platinum electrode in the double layer region was found to be very similar, the major components being from the electroreflectance effect and changes in the adsorbed water layer. In the hydrogen adsorption region complex optical behavior due to three different forms of hydrogen adatom was observed and the results are discussed in the light of present theories concerning these species.