Adiabaticity and diabaticity in strong-field ionization

TL;DR

This paper explores the concepts of adiabaticity and diabaticity in strong-field atomic ionization, clarifying their theoretical foundations and regimes through an analytic-continuation approach, and distinguishes different ionization regimes based on pulse characteristics.

Contribution

It introduces a rigorous classification of adiabatic states in strong-field ionization using analytic continuation, addressing the continuum nature of eigenstates and clarifying adiabatic versus diabatic dynamics.

Findings

Distinction between adiabatic and diabatic ionization regimes.

Application of analytic continuation to define adiabatic states.

Identification of two regimes within tunneling ionization.

Abstract

If the photon energy is much less than the electron binding energy, ionization of an atom by a strong optical field is often described in terms of electron tunneling through the potential barrier resulting from the superposition of the atomic potential and the potential associated with the instantaneous electric component of the optical field. In the strict tunneling regime, the electron response to the optical field is said to be adiabatic, and nonadiabatic effects are assumed to be negligible. Here, we investigate to what degree this terminology is consistent with a language based on the so-called adiabatic representation. This representation is commonly used in various fields of physics. For electronically bound states, the adiabatic representation yields discrete potential energy curves that are connected by nonadiabatic transitions. When applying the adiabatic representation to optical strong-field ionization, a conceptual challenge is that the eigenstates of the instantaneous Hamiltonian form a continuum; i.e., there are no discrete adiabatic states. This difficulty can be overcome by applying an analytic-continuation technique. In this way, we obtain a rigorous classification of adiabatic states and a clear characterization of (non)adiabatic and (non)diabatic ionization dynamics. Moreover, we distinguish two different regimes within tunneling ionization and explain the dependence of the ionization probability on the pulse envelope.