Stark-effect studies in xenon autoionizing Rydberg states using a tunable extreme-ultraviolet laser source

The Stark spectra of autoionizing Rydberg states of Xe converging to the second ionization limit 5$p^5$ ${}_{}{}^2{}_{}{}^{}P_{1/2}^{}{}_{}{}^{}$ have been investigated as a function of applied electric field over the range 26–2362 V/cm. Single-photon excitation from the Xe ground state was achieved using coherent radiation near 92.5 nm, generated by frequency tripling the pulsed output of a frequency-doubled dye laser in a free jet of rare-gas atoms. For the lowest electric field only the ns’ and nd’ series were observed. As the electric field was increased the np’ series appeared because of the mixing of wave functions with different l values. Further increase of the electric field caused the appearance of hydrogenlike Stark manifolds. The Stark spectrum was simulated in a two-step procedure with no adjustable parameters. Using a jl-coupling basis set the Hamiltonian matrix was diagonalized to determine the energy levels and corresponding eigenfunctions for a given electric field strength. The intensities and line shapes were then calculated using the formalism developed by Fano for a many-discrete-level–single-continuum configuration interaction. This treatment was sufficient to reproduce the positions, widths, and shapes of most observed features. At field strengths between 1000 and 2000 V/cm, however, an additional modulation structure with a spacing of about 2.7 ${\mathrm{cm}}^{\mathrm{-}1}$ appeared on top of the ‖M‖=1 autoionizing resonances, which could not be explained by our simple theoretical model.