Perturbations in the barium I spectrum

Quantum mechanics predicts that mutual interactions occur between adjacent spectral terms of the same j value arising from similar (odd or even) electron configurations. These interactions affect both the series term values and the intensities of transitions from the involved levels, and the normal multiplet intensity formula; no longer hold. Theoretically, one expects that when such perturbations occur, the intensity sum of each enlarged j group of the transition array for all the interacting configurations should be invariant, i. e ., equal to the theoretical sum which is obtained if the intensities in each multiplet be calculated separately for isolated LS coupling. These theoretical sums depend on uncalculated parameters (concerned with the radial functions) which connect the intensities in one multiplet with those of another arising from a different electron configuration, and which must be determined from the experimental data. The adjustment is easily made, however, if there exist transitions in each multiplet which are perturbed very slightly or not at all. It has been possible to identify the interacting terms which influence the intensities of the first three multiplets of the diffuse and of the fundamental series in the Ba I spectrum, and to show that for the most part, the experimental results confirm the theoretical prediction that each enlarged j group intensity sum should be invariant. Three striking exceptions occur, but they have an explanation other than a failure of the theory. This agreement is highly encouraging in view of the fact that the perturbations are in some instances several times the normal line intensity. For the present, no attempt has been made to calculate the perturbation to be expected in individual lines. The spin-orbit interaction matrices, for example, can be determined, but owing to the fact that the parent electron configurations are not known for all the perturbing terms, calculations which would give a more detailed test of theory, are at present impracticable.