Evanescent Wave Cavity Ring-down Spectroscopy as a Probe of Surface Processes

Sub-monolayer detection is demonstrated with the cavity ring-down technique by using intra-cavity total-internal reflection to probe the adsorption of I₂ on fused silica. A minimum detectable coverage of 0.04 monolayers of I₂ is achieved at a weakly resonant probe wavelength with a prototype cavity. The known cross-section for the ³P_u ¹S_g⁺ transition of I₂, which is largely invariant between high-pressure gaseous, adsorbed, and liquid phases, provides quantification of sensitivity. Adsorption of I₂ on fused silica is found to approximately follow Langmuir kinetics with a large activation barrier which may arise from an electron transfer interaction. General cavity design concepts and an estimate of optimized performance are discussed.

*National Research Council postdoctoral associate/NIST.