Xe ₂ gerade Rydberg states observed in the afterglow of a microplasma by laser spectroscopy of a³ Σ_u ⁺ (1_u, O_u^-) absorption in the green (545-555 nm) and near-infrared (675-800 nm)

Bound ← bound transitions of the Xe dimer at small internuclear separation (R < 4.0 Å) have been observed in the 545-555 nm and 675-800 nm spectral regions by laser spectroscopy in the afterglow of a pulsed Xe microplasma with a volume of ∼ 160 nl. Transient suppression of Xe ₂ A¹ Σ_u⁺(O_u ⁺)→ X¹Σ_g⁺(O _g⁺) emission in the vacuum ultraviolet (∼ 172 nm), induced by laser excitation of Ω_g ←a³Σ _u⁺(1_u,O_u^- ) [Rydberg ←Rydberg] transitions of the molecule, has confirmed the existence of structure between 720 and 770 nm (reported by Killeen and Eden [J. Chem. Phys. 84, 6048 (1986)]) but also reveals red-degraded vibrational bands extending to wavelengths beyond 800 nm. Spectral simulations based on calculations of Franck-Condon factors for assumed Ω_g ←a3Σ _u⁺ transitions involving Ω = 0^±,1 gerade Rydberg states suggest that the upper level primarily responsible for the observed spectrum is an Ω = 1 state correlated, in the separated atom limit, with Xe(5p^{6 1}S₀) + Xe(5p⁵ 6p) and built on a predominantly A²Π _3/2g molecular ion core. Specifically, the spectroscopic constants for the upper state of the 1_g ←1u,Ou± absorptive transitions are determined to be T_e = 13 000 ± 150 cm^{- 1}, ω e′ =120±10 cm^-1, = 1.1 pm 0.4,rm cm- 1 ω e′ xe′ =1.1±0.4 cm - 1, D_e = 3300 ± 300 cm^{- 1}, and Δ Re=Re′- Re″ =0.3±0.1Å which are in general agreement with the theoretical predictions of the pseudopotential hole-particle formalism, developed by Jonin and Spiegelmann [J. Chem. Phys. 117, 3059 (2002)], for both the (5)1_g and (3)O_g⁺ states of Xe₂. These spectra exhibit the most extensive vibrational development, and provide evidence for the first molecular core-switching transition, observed to date for any of the rare gas dimers at small R (<4 Å). Experiments in the green (545-555 nm) also provide improved absorption spectra, relative to data reported in 1986 and 1999, associated with Xe ₂ Rydberg states derived from the Xe(7p) orbital.