Strong correlation effects in atomic photoelectron angular distributions far above thresholds

We scrutinize individual interchannel coupling effects on atomic dipole and nondipole (E1-E2) photoelectron angular distribution parameters of valence electrons far above thresholds, choosing 2p photoionization of N and Ne in the photon energy range from 1 to 10 keV as case studies. It is found that individual correlation effects are strong far above thresholds. However, a cancellation effect is also discovered that largely obviates the net correlation effect on photoelectron angular distributions. It is shown that the cancellation can be removed, i.e., strong correlation effects can be observed, by considering core-ionized (core-excited) initial states; this is expected to be quite general. The importance of this work is that it shows that the tacit belief in the insignificance of correlation in nondipole parameters far above thresholds is quite misleading. In addition, it suggests studies of core-ionized or core-excited atoms as a means of exploring these large correlation effects in nondipole photoelectron angular distributions far above thresholds.