Photoabsorption of the 4d electrons in barium

Abstract

The absorption spectrum of gaseous barium was obtained between 120 AA and 140 AA, the region of the N_IV,V thresholds. The similarity to lanthanum of some of the features observed in barium suggests that the 4f orbital contracts and overlaps the 4d orbit. This contraction produces two terms, ³P and ³D, of the 4d⁹4f configuration below the 4d ionization limits, while the large electrostatic exchange interaction drives the ¹P term of this configuration some 10 eV above the limit. Furthermore, extensive mixing of the 4d⁹(6s²)6p configuration with 4d⁹(5d²)6p and 4d⁹(5d6s)6p produces many weak resonances in addition to the resonances associated with the 4d⁹4f configuration. A suggested classification of these features is given with the aid of known features of the La I spectrum. Finally, from the known X-ray splitting of the N_IV,V threshold and the energy interval between the 6s²6p ²P and 6s^{2 1}S levels in La I and La II, the ionization thresholds of the 4d electron were determined to be 814800 (1000) cm^-1 and 792500 (1000) cm^-1.