Keldysh 1965 + Faisal 1973 + Reiss 1980 strong-field approximation: gamma_K = sqrt(I_p/2 U_p); tunneling regime gamma << 1 vs multiphoton gamma >> 1; basis of tunnel-ionization theory.
Keldysh 1965 + Faisal 1973 + Reiss 1980 strong-field approximation: gamma_K = sqrt(I_p/2 U_p); tunneling regime gamma << 1 vs multiphoton gamma >> 1; basis of tunnel-ionization theory.