Experimental and theoretical autoionisation widths for the 5p23P levels of neutral cadmium

The 5p 2 3 P autoionising resonances of neutral cadmium were investigated using laser excitation from the 5s5p 3 P J levels populated in a discharge. The profiles were recorded using optogalvanic detection. The narrow autoionisation widths of the 5p 2 3 P 0 and 3 P 1 levels were measured for the first time by analysing the absorption profiles for the 5s5p/sup /3P J' -5p 2 3 P J transitions. The measured autoionisation widths for the 3 P 0 and 3 P 1 levels are 0.35 and 1.90 cm -1 respectively. A theoretical explanation for the different behaviours experimentally observed for the 5p 2 3 P J levels with respect to the autoionisation process is presented. The non-vanishing width for the 5p 2 3 P 1 level is ascribed to 'purely relativistic effects' which appear as a first-order effect in a relativistic treatment, or as a crossed second-order effect involving spin-orbit and electrostatic interactions in a non-relativistic formalism. Purely relativistic effects associated with breakdown of the Russell-Saunders coupling scheme induce strong cancellation effects which decrease the autoionisation width for the 5p 2 3 P 0 level. Similar phenomena do not appear for the 5p 2 3 P 2 level which possesses a broad width.