Ultrahigh-resolution (1+1) photoionization spectroscopy of Kr i: Hyperfine structures, isotope shifts, and lifetimes for then=5,6,74p5ns Rydberg levels

High-resolution measurements of the hyperfine structures and isotope shifts are reported for Kr i n=5,6,7 4$p^5$ns Rydberg levels, obtained using an extreme-ultraviolet laser with a bandwidth of 210 MHz in a resonant two-photon-ionization scheme. Use of known ${\mathrm{I}}_2$ frequencies yields an improved absolute calibration of the Kr energy levels by more than one order of magnitude. The nuclear quadrupole hyperfine structure indicates that the 4$p^5$6s and 4$p^5$7s states are described by a pure jj-coupling scheme, whereas the 4$p^5$5s states depart from a pure jj-coupling scheme by 0.37(6)%. The magnetic hyperfine structure shows that the 4$p^5$ns states are mixed with 4$p^5$n’d states. The isotope shifts can be described as pure mass effects within the precision of our experiment. For the 4$p^5$6s and 4$p^5$7s states, lifetimes were determined that differ markedly from theoretical literature values.