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 6s6p1P1 intermediate state of barium with concurrent one-photon and two-photon (ω-2ω) interactions. The laser interactions ionize the atoms in the vicinity of the series of autoionizing states converging upon the 5d2D5/2 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 2π, confirming the critical role of these resonances