A theory of momentum distribution of electrons from intense-field ionization of atoms in the ‘tunnel regime’

Abstract

In this report we present a theory of low energy ionization in intense fields in the so-called tunnel regime (Keldysh γ < 0.5) and derive explicit analytical formulas for the energy and momentum distributions of the emitted electrons. The theory is applied to obtain the probability distributions of the parallel momentum as well as the simultaneous two-dimensional distribution of the parallel versus the perpendicular momentum of electrons. The results are compared with the corresponding distributions recently measured in high resolution momentum measurements, in the case of He atoms. In stark contrast to the smooth distributions predicted by the field-ionization mechanism, the present analysis shows that the observed unexpected structures can arise from a discrete photon absorption mechanism, in the tunnel regime.