A C. Pipino, John T. Woodward IV, Curtis W. Meuse, Vitalii I. Silin
The cavity ring-down technique is used to probe the absolute optical response of the localized surface plasmon resonance (SPR) of a gold nanoparticle distribution to adsorption of trichloroethylene (TCE) and perchloroethylene (PCE) from the gas phase. Extended Mie theory for a coated sphere with a particle-size-dependent dielectric function is used to elucidate size-dispersion effects, the size-dependence of the SPR sensitivity to adsorption, and the kinetics of adsorption. An approximately Gaussian distribution of nanospheres with a mean diameter of 4.5 nm and a standard deviation of 1.1 nm, as determined by atomic force microscopy, is provided by the intrinsic granularity of an ultra-thin, gold film, having a nominal thickness of . 0.18 nm. The cavity ring-down measurements employ a linear resonator with an intra-cavity flow cell, which is formed by a pair of ultra-smooth, fused-silica optical flats at Brewster s angle, where the Au film is present on a single flat. The total system intrinsic loss is dominated by the film extinction, while the angled flats alone contribute only . 5x10-5/flat to the total loss. Based on a relative decay time precision of 0.1 % for ensembles averages of 25 laser shots from a pulsed optical parametric oscillator (OPO), the minimum detectable concentrations of PCE and TCE are found to be 2x10-8 mol/L and 7x10-8 mol/L, respectively, based on a 30 s integration time.