Electron-Impact Spectroscopy of Molecules

Abstract

Electrons and ions have been used for the excitation of matter ever since the Franck-Hertz experiment in 1914 (see [1]). Although it is not useful to speak of beam experiments in those early years, it soon became obvious that the use of a monochromatic electron or ion beam would open up new avenues for experimental and theoretical advancement. In 1925, Goudsmit and Uhlenbeck [2] discovered electron spin and were thereby able to explain the multiplet structures in their experimental spectra. (This is especially important for low-energy electron-impact experiments, where the spin also determines transition probabilities.) A third important development, the Bethe theory (1930), must be mentioned: This theory provided a basis for the analysis of the interaction between high-energy electrons and matter [3]. The initial formalism for explaining results was now complete; it became operational when, after World War II, technical improvements allowed for the study of single-particle collision processes. Better vacuum systems, electron optics and solid-state electronics permitted an enormous increase in experimental results. These results concerned not only the use of beams for gas-phase spectroscopic research but many other applications as well.