Polarization Effects on Discrete Transitions in Light Alkali Atoms Calculated in the Linked Diagram Formalism

Abstract

A multi-configurational model space is used to construct effective operators for electromagnetic radiation. The operators are constructed by means of a Rayleigh-Schrödinger perturbation expansion of the wave operator to include the effect of the non-central part of the Coulomb interaction on the model wave functions. In this way polarization effects on Restricted Hartree-Fock (RHF) wave functions could be examined and effective radial integrals for electric dipole (E1) transitions could easily be calculated. Numerical results are given for the first members of the principal series ns → n'p and the first subsidiary series (diffuse series) np → n'''d of Li and Na. The calculation is performed in a non-relativistic scheme. Three different representations of the dipole operator are used: length, velocity and acceleration. When including polarization one could see a remarkably better agreement between the results obtained by the different representations as compared with the corresponding RHF calculation. The effect of polarization on RHF oscillator strengths increases from Li to Na, and for the higher members in the principal series of sodium the effect is large. A comparison of our calculated strengths with accurate ab initio calculations of the variational type, shows good agreement for the strengths of the sharp and diffuse series and for some intercombination lines of Li. Accurate experimental data being sparse, it is difficult to estimate the importance of correlation on the oscillator strengths. The set of values chosen indicates that correlation effects are large for the principal series (except for the resonance transition) and smaller for the sharp and diffuse series where a good agreement with experiment is obtained.