We have systematically studied the line shapes and dynamics of several dissociative resonances near the second dissociation limits of ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD. Spectral overlap is eliminated using laser double resonance through the $\mathrm{EF}$ ${\mathrm{}}^1{\Sigma }_{\mathrm{g}}^{+}$state, and state-selective detection is used to distinguish dissociation to the $\mathrm{H}\mathrm{}\left(1s\right)+\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ and $\mathrm{H}\mathrm{}\left(1s\right)+\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ channels. The threshold region contains both dissociative Rydberg states belonging to the $3p$ complex and shape resonances from the $B$, $B^{\prime }$, and $C$ states. Line shapes range from nearly symmetric to highly asymmetric, and most fit well with Beutler-Fano profiles. The observed linewidths of near-threshold shape resonances are in strong disagreement with nonadiabatic theoretical calculations. In some instances, broad resonances coincide almost exactly with the dissociation thresholds, greatly affecting the cross sections and dissociation dynamics. An unusual resonance in HD is observed, in which the $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ cross section exhibits a strong interference line shape at the position of the threshold for $\mathrm{D}\mathrm{}\left(2s\right)\mathrm{}$ production.

• Received 11 September 1997

DOI:https://doi.org/10.1103/PhysRevA.57.949