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

We present a multichannel reaction matrix theory for perturbed discret e and continuous spectra, using Hartree-Fock (HF) or MCHF-based config uration-interaction (CI) wavefunctions, in the Breit-Pauli approximati on. The function space is divided into two parts, P and Q, computed se parately for different configurations in a state-specific quasirelativ istic framework. The Q functions are correlated low-lying, inner-hole and valence-excited states, whilst the P functions are symmetry-adapte d products of correlated (N-1) electron cores with Rydberg and scatter ing HF or MCHF orbitals. Energy dependent, smooth reaction matrices ar e found, from which the total wavefunctions are constructed. Calculati ons 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 calc ulated and measured by Buurman and coworkers, who disagreed with earli er theoretical results of Weiss and others. Our results do not confirm their conclusion. Furthermore, the treatment of FS and the good agree ment with experiment show that, the inclusion of coupling of '3s3p(2)' D-2 and '3s3pnp' D-2 states is crucial. The contribution of other non relativistic terms is insignificant. Our results for oscillator streng ths below and above threshold, agree with available experimental value s and earlier theoretical results, but disagree with the most recent r esults and conclusions by Taylor and co-workers who treated this spect rum by a large scale, full CI calculation.