Lewis, B. R.Gibson, S. T.England, J. P.Stark, G.West, J. B.2015-10-202015-10-200021-9606http://hdl.handle.net/1885/15981Using a tunable, narrow-bandwidth vacuum-ultraviolet source based on third-harmonic generation from excimer-pumped dye-laser radiation, the F³Πu←X³Σg-(1,0)photoabsorption cross sections of ¹⁶O₂ and ¹⁸O₂ have been recorded in high resolution. Rotational analyses have been performed and the resultant F(v=1) term values fitted to the ³Π Hamiltonian of Brown and Merer [J. Mol. Spectrosc. 74, 488 (1979)]. A large rotationless isotope effect is observed in the F(v=1)predissociation, wherein the Lorentzian linewidth component for ¹⁸O₂ is a factor of ∼50 smaller than the corresponding ¹⁶O₂linewidth. This effect, a consequence of the nonadiabatic rotationless predissociation mechanism, is described using a coupled-channel treatment of the strongly Rydberg-valence-mixed 3Πu states. Significant J, e/f-parity, and sublevel dependencies observed in the isotopic F(v=1) rotational widths are found to derive from an indirect predissociation mechanism involving an accidental degeneracy with the E³Σ−u(v=3) level, itself strongly predissociated by ³Σ−u Rydberg-valence interactions, together with L-uncoupling (rotational) interactions between the Rydberg components of the F and E states. Transitions into the E(v=3) level are observed directly for the first time, specifically in the ¹⁸O₂ spectrumPartial support was provided by an NSF International Opportunities for Scientists and Engineers Program Grant No. INT-9513350, and Visiting Fellowships for G.S. and J.B.W. at the Australian National University.http://www.sherpa.ac.uk/romeo/issn/0021-9606..."Publishers version/PDF may be used on author's personal website, institutional website or institutional repository" from SHERPA/RoMEO site (as at 20/10/15). Copyright 2002 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in The Journal of Chemical Physics and may be found at https://doi.org/10.1063/1.1436106Keywords: Bandwidth; Calibration; Dye lasers; Electron energy levels; Excimer lasers; Ground state; Hamiltonians; Harmonic generation; Light absorption; Monochromators; Photomultipliers; Potential energy; Pumping (laser); Signal to noise ratio; Ultraviolet radiatio2002-02-2210.1063/1.14361062015-12-11