Observing individual thermal motions of ions, and molecules in water with light

We observe thermal motions of ions, and molecules in water through light extinction, at the individual particle level. The motions appear as time dependent intensity variations, characterized through their averaged spectra. Theoretical spectrum derived from random motions of one particle describes these measured spectra. This theory is used to extract diffusion constants of liquid mixtures and solutions, that correspond to binary diffusion, and thermal diffusion, which are consistent with previous macroscopic measurements. We also estimate the sizes of the particles.