Intracavity, laser absorption spectroscopy of Ar₂Aπ _u5p and Aσ_u5p Rydberg states

Inter-Rydberg transitions of Ar₂ in the green (∼460-530 nm) have been studied by intracavity, laser absorption spectroscopy. All five Hund's case (c) members of Aπ_u5p manifold, predicted by the guidelines established by Mulliken [J. Chem. Phys. 52, 5170 (1970)] for Xe₂ and later adapted by Chang and Setser [J. Chem. Phys. 69, 3885 (1978)] for Ar ₂, have been observed. The molecular transitions originate from Aσ_g4s(1_u,0_u^-) and terminate on the A ² Σ_1/2u^- ion core, ³Π_g-derived (5p Rydberg electron) states (ranked in ascending energy): 0_g^-, 0_g⁺, 2 _g, 1_g, and 1_g. Transitions involving three of these are strongly red degraded [revealing that R_e4s(1_u, 0_u^-) <R_e5p(0_g^-) ≲ R_e (0_g⁺) <R_e (1_g upper)] whereas the bands associated with the 2_g and lowest 1 _g levels are not. Analysis of newly observed hot bands (Δυ≡υ′ - υ″ <0) yields the following vibrational constants for Aσ_g4s(1_u, 0 _u^-):ω″_e = 297.2 ± 1.0 cm^-1, ω″_e χ″_e = 3.3 ± 0.4 cm^-1, and ω″_e y″_e = 0.08 ± 0.05 cm^-1. Several clear vibrational sequences are observed and tentative assignments for the/iir.,5/7 separated atom limits are reported. The 0_g, 0_g ⁺, (2_g, 1_g), and 1_g (upper) states apparently correlate (in the separated atom limit) with Ar(¹S ₀) + Ar 5p[1/2]₁, [5/2]₂, [5/2]₃, and [3/2]₁, respectively, which is consistent with the known 4p, 5p→4s oscillator strengths, the Ar 3p⁵ 5p fine structure splittings, and the measured energy defects between the Aπ_u5p substrates. Rather than being degenerate at small R, the 0_g ^-, and 0_g⁺ states of the π_u5p manifold are split by 247 cm^-1 near R_e. Bandheads arising from the Aσ_u5p(0_g) ←Aσ_g4s(1_u,0_u^-) transition have also been observed. Calculations show that the Aσ _u5p(0_g^-) state dissociation energy (D ₀≃1.0 eV) is noticeably smaller than that for Aπ_u5p levels (D₀ = 1.26 ± 0.01 eV).