Observation of the I′ 1Πg outer well state in H2 and D2

We observed bound levels of the I' state in H-2 and D-2, confined in the outer well of the lowest (1)Pi(g) adiabatic potential close to its (1s + 2p) dissociation limit, with an equilibrium internuclear distance of approximate 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 pulse. Level energies are measured with an accuracy of approximate to 0.03 cm(-1), and are in reasonable agreement with predictions from ab initio calculations in adiabatic approximation; the smallness of Lambda-doublet splitting indicating that nonadiabatic interactions with (1)Sigma(g)(+) states are generally weak. Additional resonances are observed close to the n = 2 dissociation limit, some of which can be assigned as high vibrational levels of the EF (1)Sigma(g)(+) state. (C) 1998 American Institute of Physics. [S0021-9606(98)00246-3].