EXPERIMENTAL-OBSERVATION OF THE LOWEST (1)SIGMA(-2()(U) VALENCE STATEOF O)

Transitions into the lowest (1) Sigma(u)(+) valence state of O-2, whic h we shall name f'(1) Sigma(u)(+), are observed. A total of nine weak absorption bands is found from the system f'(1) Sigma(u)(+)<--X(3) Sig ma(g)(-) for the isotopes O-16(2) and O-18(2). The observed band origi ns and rotational constants are found to be significantly perturbed du e to an electrostatic interaction between the valence state f'(1) Sigm a(u)(+) and the Rydberg states 3p pi(u)f(1) Sigma(u)(+) and 4p pi(u)j( 1) Sigma(u)(+). In addition, rotational perturbations observed in cert ain vibrational levels of the Rydberg states f(1) Sigma(u)(+) and j(1) Sigma(u)(+) are found to be caused by the f'(1) Sigma(u)(+) state thr ough the same valence-Rydberg interaction. The f'<--X bands are found to be predissociating resonances that exhibit Beutler-Fano line shapes of widely varying asymmetry. In particular, the (13,0) band of O-18(2 ) is an example of a window resonance in molecular dissociation.