Quantitative identification of different strong-field ionization channels in the transition regime.

We make a quantitative investigation on the tunneling and multi-photon channels in the transition regime from Keldysh parameter γ < 1 to γ > 1 by numerically solving the time-dependent Schrödinger equation (TDSE). A method is proposed to separate the contributions of those ionization channels based on the characteristics of the current. By analysing the dependence of the ionization rate on the Keldysh parameter γ, we identify a field independent transition point at γ ≈ 2, which is different from the well-accepted consensus of γ ≈ 1, from adiabatic to nonadiabatic regime.