Stark effect for the Rydberg states of the krypton atom near the ionization threshold

We have measured the Stark effect on absorption spectra of highly excited Rydberg levels near the ionization threshold of Kr. The spectra were recorded against the continuum from a synchrotron-radiation (SR) light source using a high-resolution vacuum-ultraviolet spectrometer and applying electric fields that were parallel or perpendicular to the polarization of the SR. The main features are hydrogenlike Stark manifolds that arise from mixing of high-orbital-angular-momentum states with negligible quantum defects, and a number of lines belonging to the np and nd (J=2) series, the appearance of the latter being strongly dependent on the direction of the external field with respect to the SR polarization. Based on the diagonalization of the Stark energy matrix including more than 450 configurations, the np and nd (J=2) series are correlated with the zero-field states. The term values of the zero-field states are obtained by extrapolation of observed energy values to the zero electric field. A multichannel-quantum-defect-theory analysis has been carried out for the J=1 even-parity energy levels to derive the eigenchannel quantum defects and transformation matrix elements.