The energy spectrum of aluminium monoxide

SCF calculations using an extended double-zeta basis set have been performed in order to obtain information about the energy spectrum of AlO. The variational calculations concerned six states of AlO and three states of AlO+. Correlation energy differences were estimated semiempirically to correct the single-configuration approximation. The predominant configurations of the observed states of AlO have been identified by using a comparison between the calculated and observed spectroscopic elements: term values, rotational, vibrational and spin-orbit constants, compatibility of observed systems. This has led us to deduce the probable locations of as yet unobserved states and to predict the wavelength of a number of transitions in which they are involved. The ground state of AlO+ has been found to be stable but its symmetry ((3)Pi or (1)Sigma(+)) could not be ascertained. Particular emphasis has been laid on a study of the (2 pi(3) 6 sigma(2) 7o3 pi) configuration or the highest-lying observed states of AlO. D-2 Sigma(+) and E-2 Delta(j). It is suggested that the quartet levels of this configuration are responsible for the perturbations experimentally found in C-2 Pi(I). The predissociation observed for E-2 Delta(i) is finally discussed in terms of avoided crossings of potential curves.