THE LOW-LYING STATES OF ALCU AND ALAG

The singlet and triplet states of AlCu and AlAg below about 32 000 cm( -1) are studied using the internally contracted multireference configu ration-interaction method. A more elaborate study of the X (1) Sigma() ground state of AlCu is undertaken using extended Gaussian basis set s, including the effect of inner-shell correlation and including a per turbational estimate of relativistic effects. Our best estimate of the spectroscopic constants (r(0), Delta G(1/2), and D-0) for the X (1) S igma(+) state with the experimental values in parentheses are: 4.416(4 .420) a0, 295(294) cm(-1), and 2.318(2.315) eV. The calculations defin itively assign the upper state in the observed transition at 14 892 cm (-1) to the lowest (1) Pi. state. The calculated spectroscopic constan ts and radiative lifetime for the (1) Pi state are in good agreement w ith experiment. The calculations support the tentative assignments of Behm et al. for three band systems observed in the visible region betw een 25 000 and 28 000 cm(-1). However, the computed spectroscopic cons tants are in very poor agreement with those deduced from an analysis o f the spectra. Analogous theoretical results for AlAg suggest that the (2)(3) Pi, (3)(3) Pi, and (3)(1) Sigma(+) states account for the band s observed, but not assigned, by Duncan and co-workers.