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 (O-16(2)), excited to the v'= 1 level of its (X-2 Pi(g)) 5d pi( 1)Sigma(-)(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 similar to 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 (P)Q(13)(0) feature, nominally at similar to 95 133.7 cm(-1) in that spectrum; our estimated width is significantly narrower than the previously reported width of similar to 1.0 cm(-1). Likewise, the adjacent (R)Q(11)(2) feature, nominally at similar to 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- 2 Pi(g)) 5d pi (1)Sigma(+)(g) Eg Rydberg manifold and other dissociative states. The dependence of the O2+ and O+ ion signals on acceleration voltages, O-2-jet stagnation pressure, ultraviolet pulse energy and optical polarisation are also examined. (C) 2020 Elsevier Ltd. All rights reserved.