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

ABSTRACT

In 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.

KEYWORDS:

• Multiphoton processes
• beyond-dipole approximation
• photon momentum intense lasers

PACS NUMBERS:

• 03.65.Ud
• 82.53.Kp
• 33.80.Gj
• 33.80.Eh

Acknowledgments

The authors thank Compute Canada for access to massively parallel computer clusters and NSERC for financial support. We also thank Paul Corkum (Ottawa) for stimulating discussions.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

grant No. 025836 and computer time from Compute Canada.