Measurement of Scattering and Absorption Cross Sections of Microspheres for Wavelengths between 240 nm and 800 nm.

Abstract

A commercial spectrometer with a 150 mm integrating sphere (IS) detector was used to estimate the scattering and absorption cross sections of monodisperse polystyrene microspheres suspended in water. Absorbance measurements were performed with the sample placed inside the IS detector. The styrene absorption was non zero for wavelengths less than 300 nm. Correction for fluorescence emission by styrene was carried out and the imaginary part of the index of refraction, ni, was obtained. Absorbance measurements with the sample placed outside the IS detector were sensitive to the loss of photons from the incident beam due to scattering. The absorbance data was fitted with Lorenz-Mie scattering cross section and a correction for the finite acceptance aperture of the spectrometer. The fit parameters were the diameter, the suspension concentration, and the real part of the index of refraction. The real part of the index was parameterized using an expansion in terms of powers of the inverse wavelength. The fits were excellent from 300 nm to 800 nm. By including the imaginary part obtained from the absorbance measurements below 300 nm, it was possible to obtain a good fit to the observed absorbance data over the region 240 nm to 800 nm. The value of ni at 266 nm was about 0.0060±0.0016 for microspheres with diameters of 1.5 μm, 2.0 μm, and 3.0 μm. The scattering cross section, absorption cross section, and the quantum yield at 266 nm of microsphere with a diameter of 2.0 μm was 5.65±0.01 μm2, 1.54±0.03 μm2, and 0.027±0.002 respectively. The styrene absorption reduces the scattering cross section by 20 % at 266 nm.