Differential cross section for photodissociation of ${\mathrm{H}}_2$ by tunneling through the barrier of the ${\mathit{i}}^3$${\mathrm{\Pi }}_{\mathit{g}}$ state

Metastable molecular hydrogen in the ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{\mathrm{-}}{}_{}{}^{}$(v=5, N=1) state is excited by absorption of a photon to the ${\mathit{i}}^3$ ${\mathrm{\Pi }}_{\mathit{g}}$(v’=5, N’=1, N’=2) quasibound levels. Dissociation of the excited molecules occurs by tunneling through the barrier in the electronic potential of the ${\mathit{i}}^3$ ${\mathrm{\Pi }}_{\mathit{g}}$ state. The quasibound levels appear as partially overlapping resonances in the photodissociation cross section. The cross section is determined as function of both photon energy and dissociation angle. The coherence of the excitation process is observed experimentally as a change of the photofragment angular distribution as a function of the photon energy. Ab initio calculations qualitatively reproduce the findings. Possible causes of quantative differences are discussed.