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 las er excitation scheme with extreme ultraviolet radiation in the first s tep. Level energies of rovibrational states, confined in the outer par t of the double-well <H(H)over bar (1)>Sigma(g)(+) potential at a larg e internuclear distance of 11 a.u., have been determined with an accur acy of 0.04 cm(-1). Observed weak autoionization, increasing with vibr ational energy, is explained by tunneling through the potential barrie r.