Observation of the phase lag in the asymmetric photoelectron angular distributions of atomic barium

We have observed and measured the phase lag in the phase-dependent variation of the asymmetric photoelectron angular distribution of atomic barium. For these measurements, we photoionize the 6s6p(1)P(1) intermediate state of barium with concurrent one-photon and two-photon (omega-2 omega) interactions. The laser interactions ionize the atoms in the vicinity of the series of autoionizing states converging upon the 5d 2d(2)D(5/2) threshold. We study the variation of the phase lag as a function of the laser frequency. The variation shows strong correlation to the location of the autoionizing resonances, with full range exceeding 2 pi, confirming the critical role of these resonances.