π* and σ* Shape resonances in the vibrational excitation of HCN, CH3CN, and CH3NC by low-energy electron impact

Broad shape resonances built on the electronic ground state of HCN, CH3CN, and CH3NC with an extra electron in one of the two lowest unoccupied molecular orbitals are observed in the vibrational inelastic electron scattering. In HCN, the shape resonances are of Π and Σ symmetry at respectively 2.3 and 6.7 eV; in CH3CN and CH3NC, they are of E and A1 symmetry at respectively 2.9 and 5.8 eV. Energy loss spectra, absolute differential cross sections, and angular distributions are presented for each molecule and each resonance. An isotope effect is observed for excitation of C-H (C-D) stretching modes in CH3CN and CD3CN through the A1 (σ*C-H) resonance. © 1990 American Chemical Society.