OBSERVATION OF THE 2ND (3)PI(U) VALENCE STATE OF O-2

Rotational perturbations in the B (3) Sigma(u)(-) state of molecular o xygen are studied by measuring high-resolution vacuum ultraviolet lase r photoabsorption cross sections of the (16,0)-(18,0) B (3) Sigma(u)(- ) <-- X (3) Sigma(g)(-) Schumann-Runge bands of O-16(2). The observati on of numerous extra lines in the spectrum, together with ab initio ca lculations of relevant potential-energy curves, enables the perturbing state to be assigned as the second valence state of (3) Pi(u), symmet ry. This weakly bound state, which we name C'(3) Pi(u), is also likely to be principally responsible for the well-known rapid increase in th e magnitudes of the B-state triplet-splitting constants lambda(upsilon ) and gamma(upsilon) as the dissociation limit is approached.