Excitation and ionization of atoms by simultaneous impact of two slow electrons

In an inelastic collision between an electron and an atom (or ion) the electron energy must be large enough so that part of it can be used to excite or ionize the atom. In a dense plasma the same inelastic processes may occur with two or more relatively slow electrons taking part simultaneously, where the energy of each of them is insufficient to excite (ionize) the atom. This process is analogous to the interaction processes after a collision or to the three-electron half Auger effect, in which two electrons (in the present case in the continuous spectrum) transmit their energy to a third electron. Until now multielectron collisions have not been treated in analyzing the different processes which occur in a dense plasma, although under some conditions they are the ones which determine the course of events. A similar phenomenon occurs in multiphoton excitation and ionization of atoms and has become accessible to study with the advent of intense radiation sources. An estimate is made for the excitation cross section of an atom colliding simultaneously with two electrons, each having an energy below the excitation threshold: σ2 ≅ 10-39 Ne (where Ne is the density of the incident electrons). The efficiency of this process is determined for a plasma with a Maxwellian electron velocity distribution. It is shown that for Ne ≥ 1022-1023 cm-3 the two-electron excitation process dominates