Recently demonstrated techniques for measuring collision-induced phase in macroscopic optical coherence are described and analyzed in detail. The time-dependent phase measured by this method is used to study imaginary collision kernels for optical radiators that are related to the phase by Fourier transformation. The results indicate that classical velocity changes may make an important contribution to the imaginary kernel, in contrast to the real kernel, which is dominated by diffractive velocity changes.

• Received 16 February 1989

DOI:https://doi.org/10.1103/PhysRevA.40.1941