Electronic structure and angular momentum coupling as reflected in electron excitation out of rare-gas metastable levels: Excitation cross sections of krypton

We present measurements of electron-impact excitation cross sections into levels of the 4p⁵5p configuration from the J=0 and J=2 metastable levels of krypton. Metastable-atom targets were generated using two different sources, a hollow-cathode discharge and via charge-exchange collisions between a fast Kr⁺ beam and Cs atoms. The metastable atoms are excited to 4p⁵5p levels by a monoenergetic electron beam and the fluorescence from the levels are used to determine the excitation cross sections. Laser quenching of the hollow-cathode target is used to separate the signal contributions from excitation of the two metastable levels. Like excitation from the metastable levels of Ar, cross sections for dipole-allowed excitations are generally larger than ones for dipole-forbidden excitations. Krypton differs from Ar and Ne, however, in having a larger spin-orbit coupling for the 4p⁵ core so that the energy levels of each excited configuration segregate into two tiers based on the value of the core angular momentum. Cross sections for dipole-allowed excitation with a change in the core angular momentum are not only much smaller than their core-preserving counterparts, but also have different energy dependence. The measured cross sections are compared with recent theoretical calculations and with previous experimental work.