Techniques for External Reflectance Spectroscopy

FIGURE 15. Schematic diagram of a LPSIRS experiment (a) Three-dimensional diagram giving the absorbance versus the potential E during a sweep between E and for different wavenumbers V held constant (b) reconstructed absorption spectra for two different potentials, E and 2- 

FIGURE 16. Block diagram of the complete EMIRS spectrometer. After Bewick et 

The advantage of LPSIRS is that repetitive potential sweeps continuously control the electrode surface by means of LSCV. This technique is particularly useful when unstable systems are considered. However, the successive spectral accumulations may require a rather long time (several hours), and the method is therefore restricted to investigations in narrow wavelength ranges. The equipment is the same as for EMIRS and need not be described further. 

This technique is similar to one which was first applied to studies at the solid/gas interface. It is based on Greenler s theory and on the surface selection rules which have been discussed in Section 2.3, and accordingly, adsorbed molecules (more precisely those with a nonzero dipole moment perpendicular to the electrode surface) are able only to absorb p-polarized radiation and not the 5-polarized component. In contrast, due to their random orientation, molecules in the bulk absorb both the p and s components. Thus, in principle, it is possible to take advantage of this different behavior to discriminate between molecules at the surface and those in the bulk solution. 

FIGURE 17. Schematic diagram of the IRRAS spectrometer designed by Golden et 

---