Observing thermal single-particle dynamics of ions and molecules in water with light

We observe thermal single-particle dynamics of ions and molecules in water through light extinction. The motions of particles appear as time-dependent intensity variations, characterized through their averaged spectra. The theoretical spectrum derived from random motions of one particle is found to describe the measured spectra. Since such spectra have not been observed previously and the corresponding theoretical formula is derived in this work, we obtain various physical properties from the observed spectra to corroborate our understanding of the phenomena. The diffusion constants of liquid mixtures and solutions, which correspond to binary diffusion and thermal diffusion, are extracted from the spectral measurements, and are found to be consistent with previous macroscopic measurements. We also estimate the sizes of the particles.