Observation of the I 8 1 P g outer well state in H 2 and D 2

We observed bound levels of the I8 state in H2 and D2 , confined in the outer well of the lowest Pg adiabatic potential close to its (1s12p) dissociation limit, with an equilibrium internuclear distance of '8 a.u. Rovibronic levels (v50 – 2, J51 – 5 for H2 and v50 – 5, J51 – 6 for D2) are populated with pulsed lasers in resonance enhanced XUV1IR ~extreme ultraviolet1infrared! excitation, and probed by a third laser pulse. Level energies are measured with an accuracy of '0.03 cm, and are in reasonable agreement with predictions from ab initio calculations in adiabatic approximation; the smallness of L-doublet splitting indicating that nonadiabatic interactions with Sg 1 states are generally weak. Additional resonances are observed close to the n52 dissociation limit, some of which can be assigned as high vibrational levels of the EF Sg 1 state. © 1998 American Institute of Physics. @S0021-9606~98!00246-3#