Autoionization observed in the O+2(A 2Πu→X 2Πg) and O+2(b 4Σ−g→a 4Πu) fluorescence excitation spectra

Abstract

High resolution O⁺₂ b ⁴Σ⁻_g and A ²Π_u partial photoionization cross sections are obtained in the 450 to 750 Å range using the fluorescence excitation technique. The total photoionization cross section also is measured with the same photon resolution. Most of the fine structure observed in each individual channel is interpreted in terms of electronic and vibrational autoionization. The underlying continuum in the b ⁴Σ⁻_g cross section shows a strong enhancement at 650 Å (19 eV) photon energy (i.e., 0.8 eV above threshold) and a strong vibrationally resolved structure centered at 620 Å (20 eV). They are interpreted as 3σ_gεσ_u resonances associated with 3σ_g electron ejection, the 19 eV being a shape resonance in the b ⁴Σ⁻_g continuum, and the 20 eV a bound electronic state associated to B ²Σ⁻_g and strongly autoionized into the b ⁴Σ⁻_g continuum.