Evanescent wave cavity ring-down spectroscopy for probing surface processes

Sub-monolayer detection of adsorbed I2 is demonstrated with the cavity ring-down technique by using intra-cavity total-internal reflection to generate an evanescent field that probes the adsorption process. A precision, fused-silica Pellin-Broca prism with ultra-smooth facets (surface roughness ∼0.05 nm r.m.s) is employed to provide the intra-cavity TIR. The known cross-section for the 1Σg+→3Πu transition of I2, which is largely invariant between pressure-broadened gaseous, weakly chemisorbed, and liquid states, provides quantification of sensitivity. A minimum detectable coverage of ∼0.04 monolayer is determined at a weakly absorbed probe wavelength.