Photon momentum transfer in photoionisation: unexpected breakdown of the dipole approximation

Abstract

ABSTRACTIn most models and theoretical calculations describing multiphoton or tunnelling ionisation by infrared light, and in most part of the spectroscopy, the dipole approximation is used. This is equivalent to setting the very small photon momentum to zero. We review and complete our recent theoretical investigations which showed the importance of photon momentum in photoionisation of atoms in various regimes. We report signatures of the photon momentum in one photon, multiphoton and tunnelling regimes and compare them to the predictions of a new variant a simple standard tunnelling three-step model in which the Lorentz force is included in the final (classical) ionisation step. Importance of the magnetic quiver motion is underlined. In each regime, the photon momentum effect shows up in a specific way, in particular, the dependence of the photon effect on the laser polarisation is reviewed in detail. We also outline how this effect shows up in molecular photoionisation.