Relativistic and correlation effects in the 21.2-eV photoemission spectrum of atomic lead

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Abstract
Photoelectron spectra were obtained for atomic lead in the 700–800 °C range, using HeI radiation (21.2 eV) and a special, high‐temperature cell. Relativistic effects were obvious, most notably in the relative intensity of 1.000:0.071 in the peaks arising from Pb+(s 2 p; 2 P 1/2):Pb+(s 2 p; 2 P 3/2) states. This is a consequence of the Pb(s 2 p 2; 3 P 0) ground state being in fact mostly s 2 p 1/2 2. Correlation effects were also apparent. Several final states inaccessible from an s 2 p 2 initial‐state configuration were reached; this was explained by admixtures of the configurations s p 2 d and s 2 d 2 into the initial state. All transitions that would be allowed by the presence of these peaks were actually found. These ’’direct’’ observations of configuration interaction give an operational meaning to this concept that is not available to most spectroscopic methods. Energy‐loss peaks were observed at higher pressure and attributed to excitation of optical levels in neutral Pb by photoelectrons.