Effect of high-n and continuum eigenstates on the Stark effect of resonance lines of atoms and ions

We investigate the Stark effect for spectral transitions between low-lying levels of atoms and ions. We find that often a strong cancellation of the shifts of the lower and upper levels occurs, resulting in a significant reduction in the shift. For accurate calculations, therefore, one has to account for the influence of highprincipal-quantum-number and continuum levels. As examples, the shifts, splitting, and intensities of the components of the Li I 2s-2p , Na I 3s-3p , Ba II 5d-6p , Ba II 6s-6p , and the quadrupole Ba II 6s-5d ~shifts only! transitions are calculated for electric fields up to '20 MV/cm. Comparison of the calculated shift parameters to experimental data for the Li I 2s-2p , Na I 3s-3p , and Ba II 6s-5d lines, available as yet only for low fields for which the effect is quadratic, showed an excellent agreement for most of these lines. @S1050-2947~97!02610-3#