Prism-Cell Laser Light-Scattering Spectrometer

A prism-cell laser light-scattering (PCLLS) spectrometer capable of measuring the angular distribution of time-averaged scattered intensity at small scattering angles (2°≲ θ ≲ 13° at λo = 632.8 nm and n = 1.33, corresponding to 4600 cm" ≲ K ≲ 30000 cm-1, with K, λo and n being the magnitude of the scattering vector, the wavelength of light in vacuo and the refractive index of the scattering medium, respectively) by means of a linear intensified photodiode array (PDA) detector and simultaneous determination of intensity-intensity time correlation functions at two different scattering angles using two photomultiplier tube (PMT) detectors and associated digital photon correlators is described. The unique prism light-scattering cell also permits measurements of the refractive index of the scattering medium to better than one part in 106. The use of high quality optical glass in the entrance window and in the exit prism of the light-scattering cell for the incident laser beam, together with an aperture after the entrance window and in the scattering medium, results in a substantial reduction of the parasitic scattering along the light path. With the PDA and one of the PMTs mounted on a rotating arm, the angular ranges of our PCLLS spectrometer are limited mainly by optical geometry. For typical dilute aqueous solutions or suspensions, self-beating light-scattering measurements down to a scattering angle of 2° have been achieved. Measurements of self-beating linewidths for a polymer solution undergoing coil-to-globule transition are reviewed and some signal-to-noise ratio considerations for particle sizing in polymer solutions are presented.