High-Rydberg atomic fragments from electron-impact dissociation of molecules

The production of high-Rydberg (HR) atomic fragments by electron-impact dissociation of 13 molecules has been compared to the results of previous work on the production of HR rare gas atoms. Measurements have been made of principal quantum number distributions, effective radiative lifetimes, and excitation cross sections including both the shapes as a function of electron-impact energy and the absolute magnitudes. Principal quantum number (n) distributions peak at lower n values than those for the rare gases and are consistent with the shorter times of flight of dissociation fragments. The HR atomic fragments appear to form in high angular momentum (l) states as a direct result of the dissociation process, whereas rare gas atoms form initially in low-l HR states and require subsequent electron collisions to reach high-l HR states. The energy dependence of the excitation cross sections, a slow rise from threshold with a peak near 100 eV, resembles that of other dissociative excitation processes and does not display the step function threshold characteristic of the rare gases. Magnitudes of the excitation cross sections are expressed in a form which separates the apparatus-dependent radiative decay factor from the initial excitation cross section. The result permits calculation of HR densities under a variety of electron-impact dominated conditions.