mounted in a gas dosing chamber fitted with an optical window, a vacuum pump (nominal base pressure 10 ยฑ3 torr), and a Baratron pressure gauge. The particles were illuminated with a mechanically chopped 10 mW He/Ne laser at a distance of 20 m. Reflected light intensity was measured as the photocurrent from a Hammamatsu amplified Si photodiode fitted at the objective end of an 8-inch Celestron C-8 Schmidt-Cassegrain telescope, also positioned 20 m from the sample dosing chamber and next to the laser illuminator. The output of the photodiode was measured using a Stanford Instruments SRS-20 lock-in amplifier. No other optical or electronic filtering was used, and the measurements were made in the presence of normal room (fluorescent) lighting. Optical reflectivity spectra were obtained using a tungsten light source, optical microscope, and an Ocean Optics S2000 silicon CCD spectrometer fitted with a fiber optic input. The light collection end of the fiber optic was positioned at the focal plane of the optical microscope.
Vapor Dosing Experiments: All reagent-grade organic liquids were obtained from Aldrich Chemicals or Fisher Scientific and used without further purification. Distilled and deionized (18 MX) water was used. The liquids were freezepump-thaw degassed prior to introduction into the dosing chamber [36]. During a particular vapor exposure cycle, the sample chamber was exposed to a reservoir of the liquid analyte so that the equilibrium pressure in the chamber was equal to the published saturation vapor pressure of the analyte [37].