A THEORETICAL-STUDY OF THE ELECTRONIC-STRUCTURE AND SPECTROSCOPIC PROPERTIES OF THE LOW-LYING ELECTRONIC STATES OF THE MOLECULE ALSI

Twenty-one lowest-lying electronic states of the species AISI are desc ribed theoretically using the internally contracted multireference con figuration interaction approach and natural orbitals generated from a state-averaged density matrix. Correlated consistent valence quadruple -zeta (cc-pVQZ) atomic functions are used in the expansion of the one- electron basis. Potential energy curves are presented for all the stat es as well as a description of the electronic structure characterizing the most relevant ones. Dissociation and excitation energies, and dip ole moment functions complete the electronic structure description. So lution of the radial nuclear equation allowed the determination of vib rational energies, and vibrational and rotational constants. For the g round state (X(4)Sigma(-)), R-e = 2.424 Angstrom and D-e = 2.53 eV. Th e first excited quartet is a A (4)Delta located 2.29 eV (T-e) higher i n energy, with a longer equilibrium distance (2.887 Angstrom), and a s mall D-e, 0.24 eV. The first quartet directly accessible by a one-phot on transition is the (BII)-I-4 (T-e = 2.31 eV, R-e = 2.520 Angstrom, D -e = 0.22 eV); the transitions X(4)Sigma(-)-B (II)-I-4 are expected to fall in the green region of the visible spectrum. Higher-lying (II)-I -4 states show very noticeable changes in the potential function due t o avoided crossings. Within similar to 1.2 eV from the ground state th ere are located five doublet states; the lowest one (2)Sigma(-), with T-e = 0.72 eV, R-e = 2.415 Angstrom, and D-e = 1.81 eV. For selected s tates, transition dipole moments, transition probabilities and radiati ve lifetimes are also presented. Crossings of various states and the e nergetic closeness of others are expected to play non-negligible pertu rbative effects in the spectra The global picture of the electronic st ates presented will certainly be an important aid to experimentalists in the spectroscopic investigation of this species. (C) 1998 Elsevier Science B.V. All rights reserved.