High-resolution spectroscopy of molecular hydrogen in the extreme ultraviolet region

We report high-resolution spectroscopy of the $D{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$, $D^{\prime \prime }{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$, and $B^{\prime \prime }{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ states in ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$, performed in absorption in the vicinity of 83 nm. Predissociative widths as narrow as 0.3 ${\mathrm{cm}}^{-1\mathrm{}}$ have been measured, and profile analyses based on the theory of Fano are performed on these predissociated states. The importance of overlap integrals in the predissociation of $B^{\prime \prime }$ is shown, and the absorption cross section $\sigma =(7.2\mathrm{}\pm 0.7)\times {10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ for the $B^{\prime \prime }\leftarrow X(2,0)\mathrm{}$ is determined. Anomalous behavior observed in the $D\mathrm{}(v=5\mathrm{})\mathrm{}$ states of ${\mathrm{D}}_2$ is analyzed in terms of an accidental perturbation by the $D^{\prime \prime }\mathrm{}(v=0\mathrm{})\mathrm{}$ states.