Absolute surface coverage measurement demonstrated using vibrational overtones.

Using evanescent-wave cavity ring-down spectroscopy (EW-CRDS), a technique pioneered at NIST, measurement of absolute surface coverage with submonolayer sensitivity has been demonstrated. The first C-H stretching overtones of trichloroethylene (TCE), cis-dichloroethylene, and trans-dichloroethylene were probed for the gas- and adsorbed-phase species using a seeded optical parametric amplifier. Polarized absolute adsorbate spectra were obtained by EW-CRDS using a high-finesse fused-silica monolithic optical resonator, while absolute absorption cross sections for the gasphase species were determined by conventional CRDS. A measure of the average transition moment orientation on the surface, which is required for monolayer coverage determination, was derived from the polarization anisotropy of the surface spectra. As the vector-character of absorption yields only an average tilt angle to characterize the orientation distribution, the method is accurate for highly oriented, unimodal surface distributions. For such well-ordered systems, coverage uncertainties of [+ or -]0.1 monolayer are possible. The EW-CRDS technique also was shown to provide a significant advance in sensitivity for chemical detection of TCE, which is a major environmental contaminant.

This work, performed collaboratively by NIST and the Eindhoven University of Technology in the Netherlands, has been supported by the Environmental Management Science Program of the Department of Energy.

CONTACT: Andrew Pipino, [phone]; [e-mail].