High-resolution VUV laser spectroscopy of 18O2

Full text: Molecular oxygen plays an important role in the photochemistry of the terrestrial atmosphere. Near the second dissociation limit of O2, at 57136 cm-1, the spectroscopy is complicated by perturbations caused by the close approach, at large R, of the potential-energy curves for the many excited states converging on this limit. The state of knowledge of these perturbations is likely to be significantly improved by the analysis of isotopic spectra. We present the results of a high-resolution photoabsorption study of the B3Σu -> 3Σ-g transition of 18O2, using two-photon-resonant difference-frequency four-wave mixing of excimer-pumped dye-laser radiation to provide a narrow-bandwidth, tunable source of radiation, wavelength calibrated against a simultaneously acquired 16O2 spectrum. Example 16O2and 18O2spectra are shown in the figure, obtained with an effective instrumental resolving power of ∼ 6 x 105 . The results are analyzed to provide new spectroscopic parameters for the B state of 18O2, together with new data on level crossings which will help to establish the nature of the interactions between the B state and the other electronic states converging on the second dissociation limit of O2. This new information will be incorporated into a coupled-channel model of the O2 spectroscopy and dissociation dynamics which will be used in atmospheric photochemical models