Research - ELS Group

Electrons meet molecules in Fribourg: in nature and in our laboratory.

Collisions of free electrons and molecules (or atoms) at low energies play a key role in a number of natural and man-made systems, such as in the upper layers of atmosphere (or in the lightning in the picture above), in plasmas used for semiconductor manufacture, in plasma-assisted chemical vapor deposition, in the glowing gas in front of space vehicules during re-entry, or even in specialized waste treatment.

Understanding and systematic optimization of such systems requires the knowledge of the different excitation and dissociation processes induced by electron impact and their quantitative characterization by measurement of the corresponding absolute cross sections.

The objective of our research is to measure these absolute cross sections and to study all aspects of the electron-molecule collisions for a wide range of targets.

Electron-Impact Spectroscopy is also a useful service tool for chemistry: It can determine triplet energies and (in certain cases) the electron affinities.

The objective of our research on electron-molecule scattering is to characterize with the greatest possible detail all aspects of the electron-molecule collisions for a wide range of targets.

Greatest possible detail means:

(i) experimental characterization of all the final channels, that is elastic scattering, vibrationally and electronically inelastic scattering, and dissociative electron attachment, and

(ii) measurement of high-quality, quantitative, absolute DCSs whenever possible and required.

The techniques:

Electron Transmission Spectroscopy (ETS)
- is a simple method to find temporary negative ion states (shape resonances) of a given molecule
- We use the Trochoidal Electron Spectrometer (TES) (click for pictures) to record ET spectra.
Electron Energy Loss Spectroscopy (EELS)
- We have two instruments: the TES (see above) and the Spectrometer with Electrostatic Analysers (EAS) (click for pictures)
- Depending on the mode of operation this method can be used to record three types of spectra:
  - The Energy-Loss Spectrum, where signal is plotted against the Electron Energy-Loss (for a given incident or residual electron energy).
    The spectrum shows excited states of the neutral molecule, often including spin and dipole forbidden transitions.
  - The Excitation Function, where signal is plotted against the Incident Electron Energy (for a given Energy-Loss).
    The spectrum shows states of the negative ion. This method is more versatile than ETS:
    - it shows even the 'weak' bands (e.g., s* resonances or core excited resonances)
    - the selectivity of excitation yields additional information on the nature of the anionic state
  - The Angular Distribution
Dissociative Electron Attachment Spectroscopy (DAS)

measures the yield of stable fragment anions in function of the incident electron energy

Examples of problems to which these techniques are applied:

Dissociation of aromatic halogenated hydrocarbons, illuminating the role of orbital symmetry on intramolecular electron transfer.
Electronic excitation and dissociation of molecules relevant for atmospheric chemistry (nitrogen, oxygen, ozone), with emphasis on the experimentally demanding but decisive aspects such as absolute DCSs, the full angular range 0° - 180°, and the near-threshold region.
High quality, absolute DCSs for inelastic electron-helium collisions, interesting from the theoretical point of view.
Measurement of (negative) electron affinities and triplet excitation energies, useful in other areas of chemistry.
and more ...

Examples of development of new instrumentation:

A method to measure inelastic DCSs at 180° scattering angle, using an ëelectron mirrorí in a magnetically focused spectrometer.
An innovative implementation of the "Manchester solenoid" magnetic angle changer.
Improvements of resolution of resolution of gas-phase electron spectrometers (currently 7 meV).
Improvements of the capacity to measure reliably the shape of cross section for slow electrons (starting at about 30 meV). (Important for the study of threshold effects.)

May 23, 2001 (MA)