Principles and methods for measurement of electron impact excitation cross sections for atoms and molecules by optical techniques

In a collision between an electron and an atom or an electron and a molecule, the internal energy (rotational, vibrational, and electronic) of the atom or molecule as well as the linear momentum of the colliding objects, may be altered. Two routes are open for the experimental study of such electron impact excitation of atoms and molecules. One can measure the flux and angular distribution of those scattered electrons that have lost an amount of energy corresponding to some particular internal excitation of the target atom or molecule. This energy loss method is used to study vibrational excitations of molecules and sometimes electronic state excitation in atoms and molecules. It is discussed more fully in this volume by Trajmar and McConkey. If, however, one is interested primarily in the excitation of the target rather than the angular distribution of the scattered electrons, one uses the other, complementary, approach and measures the density or number of atoms or molecules produced in the excited state of interest by the collision. For those cases in which the excited state can radiatively decay by means of short-lived (dipole-allowed) transitions, the optical method may be used to study the collision process and to determine the corresponding collision cross section. The optical method is so named because it relates the intensity of the decay radiation to the cross section for production of the excited state by the collision process. 111 refs., 8 figs