A-ELECTRONIC, C-ELECTRONIC, AND D-ELECTRONIC STATES OF THE AR-NO VAN-DER-WAALS MOLECULE REVISITED - EXPERIMENT AND THEORY

The A-X transition of ArNO has been reinvestigated by laser induced fl uorescence (LIF) both in the bound-free and bound-bound region. The di screte part of the spectrum is at least two orders of magnitude weaker than the continuum part, indicative of a large change in geometry fro m the ground state. This very different configuration, both from the g round state and from the C and D states, can only be explained by stro ng interactions, induced by the perturbing argon atom, between the exc ited states of the van der Waals complex converging to the 3s sigma,A, 3p pi,C, and 3p sigma,D Rydberg states of NO. In order to quantitativ ely understand the observed structure of the A-X, C-X, and D-X excitat ion spectra, a global theoretical approach is proposed, based on ab in itio calculations of the potential energy surfaces in the planar A' an d A '' symmetries, including a configuration interaction between the s tates of same symmetry. Small adjustments of the diabatic energy surfa ces lead to a satisfactory agreement between the observed and calculat ed spectra, In contrast to the ground state, the Renner-Teller splitti ng of the 3p pi,C state into two A' and A '' components is very large, of the order of 4000 cm(-1). This effect is complicated by further mi xing between the states of A' symmetry induced by the argon atom. The A state is anisotropic and weakly bound with a small potential well at the linear configuration (the argon atom being on the side of the oxy gen). The C(A '') and the bound electronic component of the strongly m ixed C + D(Al) states exhibit a vibrational structure close to that of the ion and, consequently, present some Rydberg character even if the Coulomb field central symmetry (s-p) is broken by the perturbing argo n atom. (C) 1998 American Institute of Physics.