Core polarization effects in the decay of 1s -> np (n=2-6) resonantly excited beryllium

The photoelectron angular distribution parameter of the relevant decay chan nels following the resonant 1s --> np (n = 2-6) excitations in beryllium wa s measured. The 2s and 3s photoelectrons present at the 2p and 3p resonance s, respectively, have beta = 2 as predicted by theory. For the mp' spectato r channels, small but distinct deviations from the expected complete isotro py were found. An explanation of these deviations is proposed in terms of t he polarization of the 2s(2) subshell due to the excited np electron.