The cross-over from tunnelling to multiphoton ionization of atoms

TL;DR

This paper develops a theoretical framework that describes the transition from tunnelling to multiphoton ionization in atoms exposed to strong laser fields, unifying two ionization regimes.

Contribution

A simple formula is derived that separates the ionization process into initial polarization and tunnelling, bridging tunnelling and multiphoton ionization regimes.

Findings

The formula captures the transition between ionization regimes.

Tunnelling from the ground state is naturally included.

Multiphoton absorption without tunnelling is also described.

Abstract

We present a theory illuminating the cross-over from strong-field tunnelling ionization to weak-field multiphoton ionization in the interaction of a classical laser field with a hydrogen atom. A simple formula is derived in which the ionization amplitude appears as a product of two separate amplitudes. The first describes the initial polarization of the atom by virtual multiphoton absorption and the second the subsequent tunnelling out of the polarized atom. Tunnelling directly from the ground state and multiphoton absorption without tunnelling appear naturally as the limits of the theory.