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 ($\omega \mathrm{\text{−}}2\omega$) interactions. The laser interactions ionize the atoms in the vicinity of the series of autoionizing states converging upon the $5d^2{D}_{5/2}$ 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.

• Received 13 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.053001