Photoelectron spectra of outer p shells of alkali metal atoms using He resonance radiation: multiplet and satellite structure

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Abstract
Atomic vapours of alkali metal atoms are studied with the technique of photoelectron spectroscopy, using He I and He II resonance lines. Multiplet structure due to np5(n + 1)s 3P~,1,0 and 'PI final states are well resolved for Na (n = 2), K (n = 3), Rb (n = 4) and Cs (n = 5). Additional strong satellites as a result of the interaction of np'nd configuration are observed for K, Rb and Cs. The intensity and the position of these satellites agree quite well with the existing empirically adjusted HF calculations for Rb and Cs. The observed 3P/'P intensity ratio for Na deviates strongly from the theoretical HF value for 40.8 eV radiation which indicates configuration interaction between the final continuum states. Alkali metal atoms have always received special attention due to their place in the periodic table of elements. Photoelectron spectra of the outermost s levels of these atoms have been measured using He I 21.2 eV radiation by Williams and Potts (1976), and of some core levels using A1 Ktu x-rays at 1453 eV by Banna er a1 (1978) and Martin er a1 (1978). The He I spectra showed strong inelastic scattering features and the x-ray spectra gave the core binding energies of these atoms in the gas phase as well as some strong shake-up satellites following the core ionisation, however the resolution in the x-ray spectra did not permit the observation of the multiplet structure of the photoelec- tron lines due to the interaction of the core vacancy with the outermost s electron. The np shells of alkali atoms just below the outermost (n + 1)s shells are beyond the ionisation reach of the He I 21.2 eV radiation for Na (n = 2), K (n = 3) and Rb (n = 4). For Cs (n = 5) Williams and Potts could not distinguish the peaks from the background (the background near the 1-2 eV kinetic energy part of the spectrum is high because of the inelastic scattering). These np levels are interesting candidates for excitation by He 11 40.8 eV radiation, in this case the photons are energetic enough to ionise, and also the resolution permits the multiplet structure to be studied. This multiplet structure is a simple example of the interaction of two open shells np5 (n + 1)s and we expect to see P (J = 2, 1,0) and 'P (J = 1) states.

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