On 'collective excitation' in the 5p spectra of barium and the lanthanides

Wendin's (1973, 1974) prediction of a collective resonance in the 5p spectrum of Ba I is explored for the lanthanide sequence. It is shown that a resonance does occur, and reasons why it is damped in La I and Ba I are advanced. The relevance of the conclusions to double photoionisation in Ba I and Eu I is explored.