Spectroscopy of barium6p1/2nkautoionizing states in a weak electric field

Abstract

We have studied the Ba $6{p}_{1/2}nk$, $n=17$, $5\ensuremath{\leqslant}k\ensuremath{\leqslant}n\ensuremath{-}1$ Stark autoionizing states by isolated core excitation (ICE) of the Ba $6snk$ Stark bound states in very weak electric fields, an unexplored regime. In zero field the broad $\ensuremath{\ell}=5$ state overlaps all the higher $\ensuremath{\ell}$ states, and in fields less than $\ensuremath{\sim}$12 V/cm the Stark states composed of the $\ensuremath{\ell}>5$ states are within the zero-field width of the $nh$ state. In the ICE spectra the strongest transition from the $6snk$ Stark state is to the $6{p}_{1/2}nk$ Stark state, and shake-up satellite transitions are observed to higher-lying Stark states but not to lower-lying Stark states, a phenomenon which we attribute to the fact that the two Stark manifolds are both incomplete and different. The observed spectra can be described by treating the $6{p}_{1/2}nk$ Stark states, composed of the $\ensuremath{\ell}>5$ states, as discrete states embedded in a continuum, the broad $6{p}_{1/2}nh$ state.