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

Abstract

The A -X transition of ArNO has been reinvestigated by laser induced fluorescence ͑LIF͒ 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 3s,A, 3p,C, and 3 p,D Rydberg states of NO. In order to quantitatively understand the observed structure of the A -X, C -X, and D -X excitation spectra, a global theoretical approach is proposed, based on ab initio calculations of the potential energy surfaces in the planar AЈ and AЉ 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 3 p,C state into two AЈ and AЉ components is very large, of the order of 4000 cm Ϫ1 . This effect is complicated by further mixing between the states of AЈ 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 oxygen͒. The C(AЉ) and the bound electronic component of the strongly mixed C ϩD(AЈ) 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 argon atom.

The dissociation of NO–Ar(Ã) from around threshold to 200 cm−1 above threshold

“…Indeed, this led to the bound-bound transitions remaining unobserved for some time. 1, 7 The poor Franck-Condon overlap is illustrated by the results of Shafizadah et al 5 who estimated the bound-bound transitions to be over two orders of magnitude weaker than the boundfree transitions. However, the overlap improves in vibrational modes that allow access to regions where the two states have common geometry.…”

“…4,9,15,25 Examination of the literature reveals that spectra of the bound region of the NO-Ar à ← X transition show unusual behavior. The reports by McQuaid et al, 9 Bush et al, 4 and Shafizadeh et al 5 are particularly revealing as they report spectra taken under various conditions. Certain experimental conditions produce the features that are now well established as belonging to NO-Ar.…”

“…Thuis et al 2 suggested that the X state of the complex is T-shaped, and this was subsequently confirmed by Mills et al 3 from radiofrequency and microwave spectra. In contrast, the à state is linear, 4,5 with the most recent results revealing that the argon is situated adjacent to the nitrogen atom. 6 Because of the large geometry change between the X and à states, transitions to the bound region near the origin are very weak and it was some time before they were observed.…”

“…While spectroscopic investigations have revealed broad features that have been attributed to larger complexes, 4,5,8,9 dissociation from such complexes following UV excitation has not been reported as problematic in past experimental explorations of NO-Ar à dissociation. None of the previous reports probing the NO͑A͒ products have reported interfer- ence from this source, including Parsons et al who first observed population in energetically inaccessible NO N states.…”

“…Nevertheless, it suggests that the changes in the magnitude of the Franck-Condon factors required are not unreasonable. Shafizadeh et al 5 have pointed out that the excitation moment decreases by almost three orders of magnitude from the bound-free to the boundbound transitions.…”