ROTATIONAL FEATURES IN THE FLUORESCENCE EXCITATION SPECTRUM OF O(1D2) FROM VACUUM ULTRAVIOLET-LASER PHOTODISSOCIATION OF O-2

Seventh-order anti-Stokes Raman-shifted ultraviolet laser radiation is used to dissociate O2 in the 175-177 nm region of the Schumann-Runge band system, B3-SIGMA-u--X3-SIGMA-g-. A cross section for the production of O(1D2) is deduced from the 762 nm fluorescence of O2(b1-SIGMA-g+), a collisional de-excitation product of O(1D2) and O2(X3-SIGMA-g-). Step structure observed in the spectrum is attributed to rotational thresholds for absorption from X3-SIGMA-g- to energies above the B3-SIGMA-u- dissociation limit. The threshold energies define a limiting rotationless dissociation energy of 57 136.4 +/- 0.9 cm-1. Shape resonances, quasibound by the rotational barrier of B3-SIGMA-u-, are observed for the first time in O2. A theoretical calculation of the cross section is in agreement with the measured cross section when the near-dissociation outer limb of the B3-SIGMA-u- potential has an R-5 long range form. The shape of the adopted potential is consistent with an avoided crossing with another 3-SIGMA-u- state near R = 4.6 angstrom.