High-energy electron spectra of atoms undergoing direct tunnelling ionization by linearly polarized laser radiation

Simple analytic expressions have been obtained for the electron energy spectra of atoms and atomic ions undergoing direct tunnelling ionization by linearly polarized laser radiation. The new results are analytic expressions for the high-energy part of the energy spectrum for when the electron kinetic energy is of the order of or greater than the ponderomotive electron energy. the contribution from direct tunnelling ionization in the hot part of the energy spectrum can be comparable with the contribution from the tunnelling ionization occurring in the rescattering processes. The high-energy part of the energy spectrum is independent of the ionization potential of the atom considered.