Resonance Scattering of Slow Electrons fromH2and CO Angular Distributions

Energy and angular dependences of the elastic and inelastic scattering of electrons from ${\mathrm{H}}_2$ and CO have been measured in the energy range from 0.5 eV to 10 eV and for scattering angles ranging from 5 to 110°. The elastic cross sections are composed of potential and resonance scattering and therefore are difficult to interpret. The excitation of molecular vibrations, i.e., scattering into inelastic channels, contains predominant contributions from short-lived negative ion compound states. The qualitative agreement of the measured angular dependence with that predicted from pure resonant scattering considerations is shown to be able to fix certain symmetry quantum numbers of the molecular state. Moreover, characteristic and similar $\sigma -, \pi -, \Delta -\dots$ type angular dependences throughout the whole energy range of the molecular resonance (independent of the final vibrational state) are presented as a means of verifying the presence of a resonance, even when the short lifetime masks the usually characteristic resonant peaks. The half-width of the ${\mathrm{CO}}^{-2\mathrm{}}\pi$ state is about 0.4 eV, that for ${{\mathrm{H}}_2}^{-}{}_{}{}^2{}_{}{}^{}{\Sigma }_u_{}^{+}{}_{}{}^{}$ between 2 and 4 eV. Absolute total cross sections for the different inelastic channels are given. The resonances of ${\mathrm{N}}_2$, CO, and ${\mathrm{H}}_2$ are compared with the predictions of the single-particle-shape resonance model and their physical properties are discussed using the different potential energy terms at large distances between the additional electron and the molecule. Throughout, stress is laid on the importance of choosing the proper experiment (i.e., exit channel) when one wishes to study a resonance. Predictably, in using angular dependence, certain channels hide and others exhibit the presence of the resonance.