A method Is herein described for the In situ detection of species adsorbed on electrode surfaces which employs a vibrating grating to modulate the wavelength of the Incident light. This technique denoted as reflection absorption wavelength mod ulation spectroscopy (RAWMS) has made It possible to obtain at a fixed electrode potential normalized, differential UV-vislble spectra of a single, Irreversibly adsorbed monolayer of cobalt tetrasulfonated phthalocyanine (CoIITsPc) on the basal plane of highly oriented pyrolytic graphite (HOPG(bp)) and of methylene blue (MB) on graphite. The (wavelength) Integrated difference RAWMS spectra for these adsorbed species were remarkably similar to those observed for the same compounds in aqueous solutions when present In the monomeric form. Complementary wavelength modulation experiments Involving a conventional transmission geometry have shown that the Instrument Involved In the In situ RAWMS measurements Is capable of resolving absorbance changes on the order of 0.002 units.
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