Density effect on newly identified high-nRydberg series of krypton by a resonantly enhanced multiphoton ionization method

Four even-parity atomic series corresponding to transitions from the 5s’[1/2${]}_1^0$ level to the ${(}^2$ ${\mathit{P}}_{3/2}^0$)np,nf levels of krypton have been observed, using a resonant multistep two-color photoionization method. The energies of 7p’[3/2${]}_1$, 7p’[1/2${]}_1$, 7p’[3/2${]}_2$, 7p’[1/2${]}_0$, np[1/2${]}_1$, np[3/2${]}_2$, np[1/2${]}_0$, and nf[3/2${]}_{1,2}$ levels are determined and the principal quantum numbers are calculated for n up to 45, 32, 49, and 44, respectively. Strong spectral irregularities in intensity and position are analyzed with the help of Lu-Fano diagrams and multichannel quantum defect theory in the case of the most intense 5s’[1/2${]}_1^0$→np[1/2${]}_0$ series. Density shift rates have the constant value of -7.4(2)×${10}^{\mathrm{-}19}$ ${\mathrm{cm}}^{\mathrm{-}1}$/${\mathrm{cm}}^{\mathrm{-}3}$ for the s’→p high-n series members, outside regions where irregularities are observed, in agreement with existing doped krypton data. A close agreement with the Fermi model is obtained for these high-n members of regular series. Departure from this agreement is found for members whose excited levels are mixed with 7p’ states. A clear relationship between the p∼p’ admixture and shift rate decrease, in regions of irregularities, is demonstrated.