ORBITAL CONTRACTION IN CA AND ITS EFFECT ON COLLISIONS

Recent experiments on fluorescence from laser-excited Ca atoms in the presence of a buffer gas have revealed several anomalies, including a large increase in the apparent lifetime of one excited term. These ano malies are discussed in the framework of a simple model, and it is sho wn that centrifugal barrier effects, and in particular orbital collaps e, are responsible for the existence of compact excited states, which play a crucial role in the excitation mechanism. Based on Hartree-Fock calculations of the averaged orbital radii, some predictions are made as to which elements of the same homologous sequence may be expected to reveal a similar anomaly.