Collision of Electrons with Molecules

Abstract

Collision of electrons and molecules represents a more complex problem than electron-atom collisions. The reason for this complexity is easy to understand. A molecule is usually defined as a group of atoms held together by valence forces. We may regard even the unstable systems of atomic nuclei and electrons as molecules. Thus, whereas atoms have only one center (nucleus), molecules are essentially multicenter objects. Due to the motion of the nuclei, molecules possess additional degrees of freedom. The motion of the electrons gives rise to quantized electronic states and the motion of the nuclei produces vibrational and rotational states. These two motions are not independent of each other but to a good approximation they can be separated from one another (see Sec. 9.2). Each electronic state has a number of vibrational states and each vibrational state contains a number of rotational states. Thus the energy spectra of molecules are much more complex than the atomic spectra.