Above-threshold-ionization structures in photoelectron momentum distributions for single ionization of He by a strong electromagnetic field

Abstract

Our numerical calculations support a recently advanced hypothesis that an individual photon absorption is still a valid concept in the tunneling regime of atomic ionization in a strong electromagnetic field. We observe characteristic structures in the photoelectron momentum distribution resulting from single ionization of the helium atom in the tunneling regime which are normally associated with above threshold ionization (ATI). The nodal structure of these ATI-like rings varies with the photon energy which can be attributed to altering parity of the photon numbers producing the corresponding rings.