Scattering of a focused beam by moving particles

The scattering of a focused laser beam by moving particles is studied. A solution is given for a spherical scatterer that is small compared to the wavelength. Several numerical examples are given. It is concluded that for particles that have a short time of flight within the laser beam, the bandwidth and frequency spectrum of the scattered field is critically dependent on the laser beam shape. This fact is particularly important when information on the motion of small particles or macromolecules is to be inferred from the measured bandwidth and spectrum of the scattered field due to an incident wave that is not a plane wave.