Near-threshold electronic excitation by electron impact of multilayer physisorbedN2and CO

Electronic excitation of ${\mathrm{N}}_2$ and CO condensed (i.e., physisorbed) on a metallic substrate has been investigated by high-resolution electron-energy-loss (HREEL) and low-energy electron-transmission (LEET) spectroscopies in the incident energy range 0–19 eV. The HREEL results yield a more complete picture of the spin-forbidden vibronic bands of the solids than previously available from photon spectroscopies. All levels are found to be shifted down by a few tens of meV and broadened with respect to the gas-phase values. As expected, Rydberg states are absent from such spectra. The LEET experiments provide a measurement of inelastic transitions near the electronic excitation threshold. By recording the second energy derivative of LEET spectra, transitions having a strong cross section near threshold are identified by the sharp structure they produce. These are the a ${}^3$Π state in CO and, in ${\mathrm{N}}_2$, the A ${}^3$ ${\Sigma }_u^{+}$ and B ${\mathrm{}}^3$ ${\Pi }_g$ states and a new previously unobserved vibronic band near 12 eV. The magnitudes of thresholds cross sections in the solid phase are discussed in terms of single-electron–molecule scattering mechanisms.