A variety of complex spectral features occurs in multichannel Rydberg systems when the ionization thresholds vary in a simple manner with a tunable parameter such as the magnetic-field strength. We develop an analytical description of these features, identifying the key dynamical quantities controlling the appearance of ‘‘complex resonances’’ and their evolution with the parameter. The analysis predicts a ubiquitous occurrence of states with vanishing autoionization width, and of multiple q reversals associated with perturbing configurations. This description can be used to interpret some features of the diamagnetic spectrum of lithium near the zero-field ionization threshold.

• Received 13 March 1991

DOI:https://doi.org/10.1103/PhysRevA.44.1874