Dynamic Coupling Phenomena in Molecular Excited States. II. Autoionization and Predissociation inH2, Hd, andD2

Vibrationally induced autoionization and one class of predissociation of electronically excited ${\mathrm{H}}_2$ and its isotopes are treated by a perturbed-stationary-state theory. Autoionization and predissociation rates are given for a number of states of ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$. In addition to direct bound-continuum coupling, consideration is given to effects of higher-order coupling; these effects cause order-of-magnitude changes in isolated cases. The auto-ionization rates vary with principal quantum number $n$ as $n^{-3\mathrm{}}$, directly with vibrational energy $\mathrm{vh}\nu$, and decrease sharply with vibrational quantum change $\Delta v$. As the principal quantum number increases, transitions of successively smaller $\Delta v$ become possible; the net effect of this is to override the $n^{-3\mathrm{}}$ dependence in total autoionization rates. Competition between predissociation and autoionization is examined; the two processes show very different dependence on $n$ and $v$, with the consequence that decay in most regions of ($n, v$) space is dominated by one or the other process; but the two mechanisms are competitive for some ($n, v$) states. The isotope effect also is rather different for the two decay processes, enough so that, in effect, the isotope effect amounts to a qualitative change from one mechanism of decay to the other, with mass change.