Photoionization of the 5d6p3D1state of barium

We have carried out a relative measurement of the photoionization cross section of the 5d6p ${}_{}{}^3{}_{}{}^{}$ ${\mathit{D}}_1$ state of barium from the 6s ionization threshold to 875 ${\mathrm{cm}}^{\mathrm{-}1}$ (∼0.109 eV) above the 5${\mathit{d}}_{5/2}$ ionization threshold. The cross section was measured for two different final-state symmetries by varying the angle between the laser polarization vectors. Both photoionization cross sections contain ‘‘forbidden lines’’ which arise through a breakdown of the selection rules on J and ${\mathit{M}}_{\mathit{J}}$ due to the hyperfine interaction in the odd isotopes of barium. These experimental results are compared with jj-coupled eigenchannel R-matrix calculations and show good agreement. Approximate incorporation of the hyperfine effects into the calculations was necessary for a complete understanding of the measured photoionization cross sections. We found that for the odd isotopes (∼18% of natural barium) the excited 5d6p ${}_{}{}^3{}_{}{}^{}$ ${\mathit{D}}_1$ state was substantially depolarized by the effect of the hyperfine interaction.