MEASUREMENT AND ANALYSIS OF THE PHOTOABSORPTION SPECTRA OF LASER-PRODUCED AL AND SI-SUBSHELL EXCITATION( IN THE REGION OF 2P)

The 2p-subshell photoabsorption spectra of atomic aluminium and singly ionized silicon, measured using the dual-laser plasma technique, are presented. The spectra are dominated by 2p(6)3s(2)3p --> 2p(5)3s(2)3p[ nd+(n+1)s, n greater than or equal to 3] and 2p(6)3s(2)3p --> 2p(5)3s3 p(2)[np, n greater than or equal to 3] one-and two-electron excitation s, respectively. Relative photoabsorption cross sections, derived from the experimental spectra, are compared with computed theoretical cros s sections. An interpretation is provided within the framework of the Hartree-Fock configuration-interaction calculations. It is found that mixing between core singly and doubly excited configurations strengthe ns with increasing atomic number at the beginning of the aluminium iso electronic sequence. This observation is in contrast to our findings f or the Mg-like isoelectronic species Mg, Al+ and Si2+ (Sayyad M H et a l 1995 J. Phys. B: At. Mel. Opt. Phys. 28 1715) in which such mixing w eakens with increasing atomic number.