Observation of a highly excited long-lived valence state in H-2

We present the first observation of a long-lived valence state of H-2 close to the ionization threshold, populated in a double resonance laser excitation scheme with extreme ultraviolet radiation in the first step. Level energies of rovibrational states, confined in the outer part of the double-well <H(H)over bar (1)>Sigma(g)(+) potential at a large internuclear distance of 11 a.u., have been determined with an accuracy of 0.04 cm(-1). Observed weak autoionization, increasing with vibrational energy, is explained by tunneling through the potential barrier.