Observation of unexpected energy levels in molecular hydrogen and ortho-para energy transfer

Abstract

We report the observation of eight unexpected energy levels close to but clearly different from ${\mathrm{H}}_2$ ${\mathit{B}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ levels. In transitions involving these levels vacuum-ultraviolet transitions terminating in p-${\mathrm{H}}_2$ levels are observed subsequent to two-photon Q(1) excitation ${\mathit{X}}^1$ ${\mathrm{\Sigma }}_{\mathit{g}}^{+}$(v’ ’=0)→EF ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$(v’=6) of o-${\mathrm{H}}_2$. Several findings indicate that these strange levels may be a manifestation of electronically excited ${\mathrm{H}}_4$, which has been discussed in a number of theoretical contributions.