QUANTITATIVE SURFACE RAMAN-SPECTROSCOPY OF PHYSISORBED MONOLAYERS ON GLASSY-CARBON

Efficient spectrometer design and low-noise CCD detectors permit acquisition of Raman spectra from monolayers of adsorbates on carbon surfaces, particularly glassy carbon(GC). Raman spectra of monolayers of beta-carotene, rhodamine 6G, bis(4-methylstyryl)benzene (BMB), and the laser dye BPBD on GC were acquired for the first time. By using GC Raman scattering as an internal standard, scattering from the adsorbate was quantitatively related to surface coverage. Adsorption from solutions of varying concentration demonstrated Langmuirian adsorption for all four compounds, thus allowing unambiguous surface coverage determinations, The Raman signal and signal to noise ratio (SNR) depended on the product of the cross section and the surface number density (beta(ads) D-ads), and a detection limit (SNR = 3) for this product was determined to be 1.1 x 10-14 sr(-1) far current instrumental conditions. This value is comparable to that of a monolayer of an unenhanced absorbate such as benzene (6.5 x 10(-15) sr(-1)). The weakest scatterer studied here (BPBD) is not considered to be resonance enhanced at 514.5 nm, and its observable surface Raman features have cross sections 5-10 times that of benzene. The results represent the first observation of Raman scattering from monolayers on carbon surfaces without the benefit of electromagnetic or strong resonance enhancement.