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Abstract:
The triplet character of doubly excited 2lnl' resonances close to the He+ (N = 2) ionization threshold has been seen to affect the angular distribution of photoelectrons associated with the He+ (1s) state: a peak has been observed at a photon energy just below the N = 2 threshold when the photoelectrons are detected at 90° with respect to the photon polarization direction. This is forbidden in pure LS-coupling and therefore the observation immediately indicates the presence of triplet configurations due to spin–orbit interactions. Such weak interaction effects are usually overwhelmed by LS-allowed processes, particularly in such a low-Z atom, but the choice of observation angle in this case desensitizes the measurements to the dominant ionization processes. The results have been reproduced by R-matrix MQDT calculations which also indicate that all seven relativistically allowed Rydberg series are present.
