Energy Loss Spectra of Low-Energy Electrons Scattered from Thin Solid Molecular Films

An electron impact spectrometer designed for measurement of energy loss spectra of low‐energy electrons reflected from thin solid films is described. The spectrometer operates at impact energies as low as 1 eV and is capable of monitoring dynamic changes in electronic excitation cross‐sections in the important threshold region. Spectra of benzene demonstrate a progression from vibrational excitation through electronic excitation to ionization as the most prominent energy loss mode as incident electron energy is increased. Excitations of benzene to the optically forbidden ${}^3{B}_{\text{1u}}$ or ${}^1{B}_{\text{2u}}$ states, while highly probable near threshold, show a rapid falloff in excitation probability at higher impact energies as compared to the optically allowed ${}^1{E}_{\text{1u}}{\mathrm{\hspace{0.17em}\leftarrow \hspace{0.17em}}}^1{A}_{\text{1g}}$ transition. In studies on 1‐hexene, losses involving excitation to the lowest triplet state are also prominent near threshold.