Multiplet Separations and Perturbed Terms

It is well known that the multiplets in spectral series show an anomalous widening in regions where they are strongly perturbed by a vagrant term of large multiplet structure, which tends to impress its characteristics upon the series. Formulae are derived in this paper for the multiplet widths of perturbed terms, which enable one to calculate the separation of the multiplet levels providing the total shift of the "centers of gravity" of the multiplets from their unperturbed values is known. If a Ritz correction, n*=n-0.03629+0.0399n², is assumed to give the positions of the terms exclusive of interaction with the perturbing 3p3d³F term, one obtains excellent agreement between calculated and observed multiplet widths for the 3snf³F series of Al II. Other examples treated are the ²P series of Cu I and the 6snf³F series of Ba I.