Laser sideband spectroscopy: A macromolecular probe

Abstract

We have previously developed a model which includes energy and phasechanging collisional relaxations in a system of chemically reacting molecules absorbing light from a monochromatic field while immersed in an inert thermal bath. Some aspects of the model are presented here which relate to scattered light measurements on macromolecular systems. We predict that in the presence of low-intensity laser light the elastic component of the scattered light will be broadened by the rate constant for the chemical reaction. In the presence of high-intensity laser light, the scattered field may contain, in addition to the Rayleigh scattered light, two sidebands symmetrically displaced to either side of the Rayleigh band; the magnitude of the displacement is a function of the laser intensity. The Rayleigh band width is a direct measure of the phase relaxation time, and the sideband widths are a measure of the energy and phase relaxation times. We discuss several experimental systems in which sideband scattering data might be used to provide information related to relaxation and reaction mechanisms in macromolecular systems of biological interest; bound enzyme-substrate and enzyme-dye systems can be investigated as can vibrational energy transfer on membrane-bound systems. Some numerical computations are included for the magnitude of the sideband displacement and scattered intensities.