Observation of the I ' (1)Pi(g) outer well state in H-2 and D-2

We observed bound levels of the I' state in H-2 and D-2, confined in the ou ter well of the lowest (1)Pi(g) adiabatic potential close to its (1s + 2p) dissociation limit, with an equilibrium internuclear distance of approximat e to 8 a.u. Rovibronic levels (v = 0-2, J = 1-5 for H-2 and v = 0-5, J = 1- 6 for D-2) are populated with pulsed lasers in resonance enhanced XUV + IR (extreme ultraviolet + infrared) excitation, and probed by a third laser pu lse. Level energies are measured with an accuracy of approximate to 0.03 cm (-1), and are in reasonable agreement with predictions from ab initio calcu lations in adiabatic approximation; the smallness of Lambda-doublet splitti ng indicating that nonadiabatic interactions with (1)Sigma(g)(+) states are generally weak. Additional resonances are observed close to the n = 2 diss ociation limit, some of which can be assigned as high vibrational levels of the EF (1)Sigma(g)(+) state. (C) 1998 American Institute of Physics. [S002 1-9606(98)00246-3].