INCOHERENT CROSS-CORRELATION SPECTROSCOPY

A new light scattering method is developed to measure scattering ampli tude fluctuations resulting from either internal motions of flexible m acromolecules or rotations of rigid particles. With a single-beam two- color cross-correlation scheme, the technique becomes insensitive to r apid phase fluctuations of the scattered light produced by translation al motions of the scattering particles. We frame the scattering theory so as to encompass rotations and number fluctuations of small particl es in a steady laminar flow. Experiment verifies the theory and demons trates its applications. The technique can be used to measure the magn itudes of the local velocity and the flow vorticity. It can also be us ed to measure internal motions and shape fluctuations of flexible macr omolecules. The advantages of the technique are its high spatial resol ution, fast temporal response, and ease of use.