Rydberg states of the Ar₂ molecule

Extensive spectra attributable to transitions from the 4sσ a ³Σ⁺ metastable state of Ar₂ to excited Rydberg states have been observed by intracavity absorption spectroscopy and by laser excitation spectroscopy in the afterglow of a pulsed corona discharge. Of these the most extensive and best resolved were the laser induced fluorescence spectra. Most of the spectra can be assigned to vibronic transitions in the nfλ (³Π_g, ³Σ_g ⁺) and npλ(³Π_g,³Σ _g⁺) ← a ³Σ_u⁺ series. The 5pπ ³Π_g ← a ³Σ _u⁺ and 7pσ ³Σ_g ⁺ ← a ³Σ_u⁺ transitions observed near 19 823 and 19 529 cm^-1, respectively, exhibit many bands with v′ ≠ 0 in their (v′-v″) vibrational band system developments. The higher n value members of these and other observed series are dominated by (0-0) transitions. Rotational structure is partially resolved in a few bands of the 7pσ ³Σ_g⁺ -a system, but most of the spectra observed appear to be either rotationally unresolved or made up of blended collections of rotational lines. Above the 7pσ, 5pπ pair, the (n + 2)pσ ³Σ_g ⁺ and npπ ³Π_g members of the npλ series rapidly coalesce, indicating a rapid onset of decoupling of the electronic orbital angular momentum, L, from the internuclear axis. Such decoupling leads to np-complex formation at n values much lower than observed in the npλ series of the lighter dimers: He₂ and Ne₂. From the data for the observed series, the lowest ionization limit of Ar ₂ (relative to a 3Σ_u⁺,v = 0) was determined to be 29 373 ± 3 cm^-1. Vibrational intervals ΔG(v + 1/2) for a ³Σ_u⁺ (v≤4) and 7pσ ³Σ_g⁺ (v≤2) yield the vibrational constants ω_e = 296 and 282 cm^-1 and xω_e = 2.5 and 17 cm^-1, respectively.