Absolute Surface Coverage Measurement Using a Vibrational Overtone
A C. Pipino, J P. Hoefnagels, N Watanabe
Sub-monolayer absolute surface number densities are obtained by cavity ring-down spectroscopy (CRDS) for three haloethylenes with differing molecular symmetries on silica using the first C-H stretching overtones. Gas-phase CRDS measurements provide absolute absorption cross sections, while evanescent wave CRDS (EW-CRDS) measurements provide polarized absolute adsorbate spectra. The absolute surface number densities are found from conservation of the integrated band intensities with adsorption along with a measure of surface orientation as derived from the polarization anisotropy. The first stretching C-H overtones of trichloroethylene (TCE), cis-dichloroethylene (c-DCE), and trans-dichloroethylene (t-DCE) occurring around 6050 cm-1 are probed with the idler of a seeded optical parametric amplifier having a 0.075 cm-1 linewidth. The EW-CRDS measurements employ a fused-silica monolithic folded resonator with a peak finesse of .28,500. An independent determination of the absolute surface coverage of TCE on a SiO2 surface is also obtained by mass-spectrometer-based uptake measurements. The sensitivity of EW-CRDS for TCE detection with an unclad resonator is found to be comparable to that obtained with a long-effective-path-length waveguide having a TCE-enriching polysiloxane coating.