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The measurement of the time dependence of light scattered from a collection of moving particles, commonly referred to as quasi-elastic light scattering (QELS), provides information about the distribution of particle velocities. For monodisperse particles undergoing Brownian motion, the time dependence of the autocorrelation of the scattered light is a simple exponential function of the hydrodynamic radius of the particle. For polydisperse suspensions undergoing Brownian motion, the time dependence of the autocorrelation of the scattered light is a function of the particle-size distribution and can be expressed either as a Fredholm integral or a sum of exponentials. This paper describes improvements in the methods of data acquisition and analysis that significantly increase the resolution of the technique. These improvements enable accurate determinations and smooth representations of both bimodal and broad unimodal distributions without operator intervention or a priori information about the particle-size distribution.

The origin of spurious and irreproducible signals in the measurement of the hyper-Rayleigh scattering (HRS) of solutions is investigated. A new cell design, coupled with continuous solution filtration through a 20 nm filter, is shown to improve the reproducibility of the measurement to ±5%. A transverse cell dimension of ∼2 mm reduces absorption of the second harmonic. A simple monochromaticity test is used to identify luminescence, and we recommend that it is made routinely. Scattering from the [Ru(bipy)3]2+ cation, attributed to HRS by Zyss et al. [Chem. Phys. Lett. 206, 409 (1993)], appears to be largely due to two-photon excited luminescence. The reliability of calibration procedures is examined, and some revised hyperpolarizabilities are reported.

Polarized two-photon excitation (TPE) and emission spectra have been used to locate, and identify the symmetry of, more than 100 energy levels of terbium(II1) in the elpasolite Cs2NaTbC16. These are assigned with the aid of a one-electron crystal field Hamiltonian. In many cases the assignments were verified by modelling the Zeeman interaction. Systematic deviations from the predictions of the one-electron model provide evidence for electron correlation induced by the crystal field (CCF).

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