Fermi Method of Equivalent Photons and the Probabilities of Radiative-Collisional Transitions in Atoms

Abstract

The radiative-collisional processes appear to be a wide domain of the Kramers electrodynamics (KrED) application. These are the processes in which the electron participates while moving along the classical highly curved quasiparabolic orbits [4.1]. We shall demonstrate the applicability of the KrED method to the description of collisional (e.g., excitation of ions by electron impact) and radiative-eollisional (e.g., dielectronic recombination (DR) and polarizational radiation (PLR)) processes in plasmas. The most natural domain of the KrED application is the physics of multicharged ions (MCI). The description of these processes is obtained within the framework of the following assumptions: (1) According to the Fermi concept [4.2] of equivalent photons (EPh), the electromagnetic field produced by an external particle (e.g., an electron) at the MCI location may be interpreted as a flux of equivalent photons incident on the MCI. It may be shown that such a description is applicable provided the dipole approximation for the interaction is valid between the bound electron of the MCI and the incident electron of the plasma. The latter approximation universally treats all the processes of energy loss by the incident electron (either due to radiation emission during a collision with an ion or due to an inelastic non-radiative collision with an ion) as the processes of the emission of (real or equivalent, respectively) photons. The probability of both processes is determined by the dipole matrix element for the corresponding inelastic (radiative or non-radiative) transition of the incident electron. (2) The spectral intensity distribution of the EPh may be described on the basis of classical radiation theory (for a detailed discussion of the applicability of the classical approach for real photons, see Sect. 2.3.4). In this case the in-tensity of the EPh flux is simply determined by the Fourier transforms of the electron coordinates determined in turn by its classical trajectory.