Two-photon excitation of two Rydberg levels of O₂ above 95 130 cm^–1: Rotational-state dependence of predissociation linewidths

Two-photon absorption is employed to record sub-Doppler molecular-beam spectra of oxygen (¹⁶O₂), excited to the υ' = 1 level of its (X ²Π_g) 5dπ ¹Σ⁺_g Rydberg state above 95 130 cm^–1 and detected by photoionisation in a time-of-flight instrument. A nonlinear-optical source generates single-longitudinal-mode pulses of coherent ultraviolet light at ∼210 nm with sufficiently narrow optical bandwidth to enable more detailed lineshape measurements of rovibronic features than had been achieved in an early investigation [Pratt et al., J. Chem. Phys. 1990; 93: 3072–84]. A spectral width of 0.59 ± 0.02 cm^–1 is derived by Lorentzian and Beutler-Fano lineshape fits to the prominent ^PQ₁₃(0) feature, nominally at ∼95 133.7 cm^–1 in that spectrum; our estimated width is significantly narrower than the previously reported width of ∼1.0 cm^–1. Likewise, the adjacent ^RQ₁₁(2) feature, nominally at ∼95 143.8 cm^–1, is found to be markedly broader (2.2 ± 1.5 cm^–1), which is consistent with the supposed effects of J′-dependent predissociation arising from overlap between the (X ²Π_g) 5dπ ¹Σ⁺ _g Rydberg manifold and other dissociative states. The dependence of the O₂⁺ and O⁺ ion signals on acceleration voltages, O₂-jet stagnation pressure, ultraviolet pulse energy and optical polarisation are also examined.