A, C, and D electronic states of the Ar–NO van der Waals molecule revisited: Experiment and theory

The $\mathrm{A–X}$ transition of ArNO has been reinvestigated by laser induced fluorescence (LIF) both in the bound–free and bound–bound region. The discrete part of the spectrum is at least two orders of magnitude weaker than the continuum part, indicative of a large change in geometry from the ground state. This very different configuration, both from the ground state and from the $C$ and $D$ states, can only be explained by strong interactions, induced by the perturbing argon atom, between the excited states of the van der Waals complex converging to the $\text{3sσ,A,}$ $\text{3pπ,C,}$ and $\text{3pσ,D}$ Rydberg states of NO. In order to quantitatively understand the observed structure of the $\mathrm{A–X,}$ $\mathrm{C–X,}$ and $\mathrm{D–X}$ excitation spectra, a global theoretical approach is proposed, based on ab initio calculations of the potential energy surfaces in the planar $A^{\prime }$ and $A^{″}$ symmetries, including a configuration interaction between the states of same symmetry. Small adjustments of the diabatic energy surfaces lead to a satisfactory agreement between the observed and calculated spectra. In contrast to the ground state, the Renner–Teller splitting of the $\text{3pπ,C}$ state into two $A^{\prime }$ and $A^{″}$ components is very large, of the order of $\text{4000\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}.$ This effect is complicated by further mixing between the states of $A^{\prime }$ 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 oxygen). The ${\mathrm{C(A}}^{″})$ and the bound electronic component of the strongly mixed ${\mathrm{C+D(A}}^{\prime })$ states exhibit a vibrational structure close to that of the ion and, consequently, present some Rydberg character even if the Coulomb field central symmetry $\mathrm{(s-p)}$ is broken by the perturbing argon atom.