Predissociation and spectroscopy of the 3A2(0 0 0) state of 18O3 from CRDS spectra of the 3A2(0 0 0)←X1A1(1 1 0) hot band near 7900 cm-1

The spectroscopy and predissociation of the 3A2(0 0 0) state of 18O3 are investigated from the analysis of the absorption spectrum of the 3A2(0 0 0)←XA1(110) hot band recorded by high sensitivity cw-CRDS between 7600 and 7920 cm-1. The noise equivalent absorption of the recordings is on the order of 1×10-10 cm-1. Forty transitions could be rotationally assigned and their line positions were used to derive approximate spectroscopic parameters of the 3A2(0 0 0) upper state. Most of the transitions exhibit a clear broadening due to the predissociation of the 3A2(0 0 0) upper state at 9573.61 cm-1. The width of the Lorentzian component (from 0.02 up to more than 0.5 cm-1) was retrieved for some of the assigned transitions. The variation of the corresponding predissociation rates is due to the dependence of the spin-orbit coupling between the 3A2(0 0 0) level and high lying dissociative vibrational levels of the XA1 electronic ground state, with (i) the J and Ka rotational quantum numbers and (ii) the energy gap between the coupled pair of rotational levels. From the predicted energies of the XA1 vibrational states and their vibrational overlap with the 3A2(0 0 0) state, a few of these XA1 vibrational levels are identified as possibly responsible for the 3A2(0 0 0) predissociation. The predissociative mechanism in 18O3 is compared to that taking place in 16O3.