Ionization and Excitation by Photon Impact at Higher Energies

Abstract

Since the nonrelativistic photoionization cross section decreases rapidly with an increase of the photon energy, the higher-order processes dominate in the formation of ions at larger values of photon energy. We carry out relativistic analysis of the second- and third-order processes. If the photon energy is large enough, the Compton scattering becomes the dominant mechanism for creation of ions. We show that the seagull term of the nonrelativistic Compton scattering amplitude can be viewed as the contribution of the negative-energy intermediate states in the relativistic amplitude. We obtain the general equations for the characteristics of the Compton scattering on the Bethe ridge and show their connection with equations of the impulse approximation. Employing the results obtained in Chap. 4, we obtain the differential distributions outside the Bethe ridge. We demonstrate the infrared stability of the sum of the contribution to the Compton scattering cross section coming from the soft scattered photons and the photoionization cross section that includes the radiative corrections. At still larger photon energies exceeding certain value ω0, the ions are produced mainly accompanied by the creation of electron–positron pairs. We determine the energy distribution of the electrons ejected due to this mechanism. We calculate the dependence of ω0 on the value of the nuclear charge Z for the single-electron ions and for the atoms containing Z electrons. We find also the photon energy region where this mechanism dominates in the creation of excited atoms.