Basic electron-molecule interactions

At electron energies below the ionization threshold electron attachment can lead to efficient fragmentation of a molecule. These resonant processes take place within well-defined energy ranges. Electron detachment is a concurrent process that often leaves the molecule in a specific vibrationally excited state. This can be observed as enhancement of the corresponding signals in high-resolution electron-energy-loss (HREEL) spectra, which are obtained by analyzing the energy of electrons scattered inelastically from the sample. The interpretation of such data is complicated by diffraction phenomena that also depend on the energy of the electrons. Analyzis of the scattered intensities in HREEL spectra of solid SF₆ demonstrates nicely the interplay between resonant processes and non-resonant electron scattering. The resonant enhancement of vibration ν₁ within specific energy ranges leads also to a lower intensity of vibration ν₃ which is excited through long-range interaction as well as of elastically scattered electrons.

More information:
Competition of different scattering channels in electron impact vibrational excitation of thin solid films of sulfur hexafluoride;
B.Göötz, E.Burean, P.Swiderek, Surf.Sci. 598, 104 (2005).

HREEL spectra of solid samples can show a surprising broadening of the signals. Some of the vibrational bands of solid methane, for example, are much broader than expected on the basis of the instrumental resolution (coloured curves). Although the spectra were recorded at a temperature of only 20 K, the broadening is comparable to that observed in room temperature gas phase spectra which are subject to rotational broadening. The unexpected broadening of the condensed-phase HREEL spectra is explained by the fact that electrons in the condensed phase can undergo more than one inelastic collision with molecules. This makes the signals more diffuse.

More information:
Understanding HREEL spectra: The unusual broadening in vibrational spectra of methane;
P.Swiderek, E.Burean, J.C.P. 127, 214506 (2007).

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