Absolute differential cross sections (DCSs) for elastic and vibrationally inelastic scattering of electrons from ${\mathrm{C}}_3{\mathrm{H}}_4$ isomers (allene and propyne) have been determined by a crossed-beam experiment in the energy range 1.5–100 eV and over the scattering angles between 15°–130°. The relative flow method was applied to normalize these cross sections, using helium as a standard. Clear evidence of the strong isomer effect has been observed in DCSs for both elastic and vibrational excitation processes, and the isomer effect still conspicuously persists in total elastic cross sections. Two resonance regions where the vibrational cross sections are strongly enhanced are observed in both allene and propyne below a few eV, while both the DCSs become identical at higher energies, hence weakening the isomer effect. More details of the observed mode selectivity are discussed based on the selection rules for vibrational excitation via a shape resonance as well as on an argument based on the angular correlation.

• Received 4 October 2001

DOI:https://doi.org/10.1103/PhysRevA.66.032714