THEORY AND COMPUTATION OF PERTURBED SPECTRA - APPLICATION TO THE (ALD)-D-2 RELATIVISTIC SPECTRUM

We present a multichannel reaction matrix theory for perturbed discrete and continuous spectra, using Hartree-Fock (HF) or MCHF-based configuration-interaction (CI) wavefunctions, in the Breit-Pauli approximation. The function space is divided into two parts, P and Q, computed separately for different configurations in a state-specific quasirelativistic framework. The Q functions are correlated low-lying, inner-hole and valence-excited states, whilst the P functions are symmetry-adapted products of correlated (N-1) electron cores with Rydberg and scattering HF or MCHF orbitals. Energy dependent, smooth reaction matrices are found, from which the total wavefunctions are constructed. Calculations very close to and on threshold present no problems. Application is made to the (AlD)-D-2 spectrum, for which quantum defects, oscillator strengths and fine structure (FS) are calculated. The latter was calculated and measured by Buurman and coworkers, who disagreed with earlier theoretical results of Weiss and others. Our results do not confirm their conclusion. Furthermore, the treatment of FS and the good agreement with experiment show that, the inclusion of coupling of '3s3p(2)' D-2 and '3s3pnp' D-2 states is crucial. The contribution of other nonrelativistic terms is insignificant. Our results for oscillator strengths below and above threshold, agree with available experimental values and earlier theoretical results, but disagree with the most recent results and conclusions by Taylor and co-workers who treated this spectrum by a large scale, full CI calculation.