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 2Πg) 5dπ 1 Σ 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 2Πg) 5dπ 1 Σ 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.