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OBSERVATION OF A HIGHLY EXCITED LONG-LIVED VALENCE STATE IN H2

We present the first observation of a long-lived valence state of H{sub 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{ovr H}{sup 1}{Sigma}{sup +}{sub g} potential at a large internuclear distance of 11a.u., have been determined with an accuracy of 0.04cm{sup -1}. Observed weak autoionization, increasing with vibrational energy, is explained by tunneling through the potential barrier. {copyright} {ital 1997} {ital The American Physical Society}