It is proposed to continue a program of experimental study and theoretical interpretation, of collision-induced phase in macroscopic optical coherence for two-level atoms. The experimental configuration is similar to that of an optical photon-echo measurement except that detection is based, not on a reradiated pulse, but on interrogation of the prepared system in terms of absorption from a weak c.w. probe laser beam. A phase is measured as a function of the partial pressure of perturber atoms. Measurement of phase, as opposed to intensity effects is, in effect, to study the "imaginary' rather than the "real" collision kernels. This investigation is unique in this respect, and surprising contrasts were found in studies of 174Yb, between the imaginary and the (well-studied) real collision kernel properties. The method will be extended to 154Sm, which may provide new surprises, and will, on the basis of a detailed comparison between Yb and Sm results, greatly facilitate physics interpretation of results.
