Simultaneous Electron-Photon Excitation of Atoms

Abstract

Excitations of atoms by simultaneous collisions with electrons and photons are considered theoretically. Their characteristic behavior as functions of laser frequency and polarization direction are discussed in connection with excitations in H atoms. The excitation cross sections show a linear dependence on laser intensity I and are comparable with the ordinary electron excitation cross sections for I≈1012 W/cm2. Such processes are related to but different from the more familiar “on-shell” (or resonant) excitation-collision processes and can permit experimental study of inelastic electron-atom scattering cross sections at “off-shell” electron energies. The differential cross sections show strongly distinguishable dependence on the electron scattering angle as well as on the laser polarization angle. Frequency dependence of the cross sections exhibits both the usual as well as certain “postcollision” resonant excitations (or ionization) at characteristic frequencies less than the first atomic resonance frequency; they are mediated by the simultaneous presence of the electron collision. Besides their basic theoretical interest, off-resonant electron-photon excitations and ionization of H atoms (and their inverse processes) could affect the electron temperature in laser heating of fusion plasma at ultrahigh intensities (I≫ 1012 W/cm2), currently in use.