Inelastic scattering of low-energy electrons by molecular oxygen.

Abstract

By means of a crossed-beam technique, we have measured electron energy loss spectra for gaseous phase molecular oxygen. The scattering angles of the measurements were from 12 through 156$\sp\circ$, in 12$\sp\circ$ increments. A variety of electron impact energies in the range 5 through 50 eV were employed. The results of our measurements are differential cross sections for vibrational excitation, excitation of the states comprising the Schumann-Runge continuum, and excitation of the longest and second bands. Using these, we computed integrated cross sections. For the Schumann-Runge continuum, Franck-Condon region potential-energy curves were also obtained. The vibrational-excitation cross sections exhibit a resonance just below 10 eV impact. Additionally, their angular distributions possess pronounced P-wave character near this impact energy. Based on the available information regarding the O$\sbsp{2}{-}$ ion, it seems the only candidate states of this ion which could be responsible are the $A\sp2\Pi\sb{u}(v)$ and $a\sp4\sum\sbsp{u}{-}(v)$ states; the latter seems more likely. Via computerized least-square analysis, the Schumann-Runge continuum spectra were decomposed into contributions from the 1$\sp3\Pi\sb{g}(v)$ and $B\sp3\sum\sbsp{u}{-}(v)$ states, and one unidentified excitation. Cross sections and linear Franck-Condon region potential-energy curves were obtained for the 1$\sp3\Pi\sb{g}(v)$ and $B\sp3\sum\sbsp{u}{-}(v)$ states were obtained. While the origin of the unidentified excitation is unclear, it seems most likely a computational artifact resulting from some combination of a pertubation to the $B\sp3\sum\sbsp{u}{-}(v)$ state's potential-energy curve and a nonconstant moment for the $X\sp3\sum\sbsp{g}{-}\gets B\sp3\sum\sbsp{u}{-}(v)$ transition. Finally, we have measured cross sections for excitation of the longest and second bands. Substantial D-wave character was apparent in the angular distributions for these cross sections. Additionally, the angular distributions indicate that both excitations are composite in nature, arising from the excitation of at least two electronic states. It is not possible to conclusively identify these states, but theoretical predictions, coupled with the notion of perturbations to the nearby $B\sp3\sum\sbsp{u}{-}(v)$ suggest that at least one of the states has $\sp3\sum\sbsp{u}{-}$ symmetry.