Direct and Auto-Ionization ofH2near Threshold

Abstract

In our experiments involving the collision of electrons with ${\mathrm{H}}_2$ (${{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}, v=0, K$), the ionization-efficiency curve of ${\mathrm{H}}_2^{+}$ shows no structure which can be obviously related to the direct vibrational spectrum of ${\mathrm{H}}_2$. In the first-derivative spectrum, however, the major peaks correspond to the major peaks in the photo-ionization spectrum, which strongly suggests, for allowed transitions, a one-to-one correspondence between $\frac{d{\sigma }_e}{d{E}_e}$ and ${\sigma }_{h\nu }$. Other peaks no doubt are associated with auto-ionizing states of ${\mathrm{H}}_2$ which can only be reached through channels normally forbidden to photoexcitation. The major valleys can be related to the direct vibrational-excitation thresholds. From these experiments and other published results, we believe that the auto-ionizing lifetime ${\tau }_a$ for many states may be long.