GENERAL SCIENTIFIC SUMMARY
Introduction and background. In an intense laser field, an atom may absorb multiple photons, more than required for ionization, and then ejects an electron with very high energy. Sometimes the ionized electron may be driven back to the parent ion by the laser field and recombine into the ground state, emitting a photon. This process provides a source of coherent XUV radiation. In order to understand such phenomena intuitively, some semiclassical approaches have been successfully applied, although there is no definite evidence for the key assumption that the motion of the ionized electron can be treated by classical mechanics.

Main results. Bohmian mechanics is a different version of quantum theory from that of Copenhagen. In Bohmian mechanics, the quantum trajectory concept is used to describe the motion of particles with the Bohm–Newton equation. The difference between this equation and the classical Newton equation is that there is an extra term in the Bohm–Newton equation, called the quantum potential. When the quantum potential is negligible, the Bohm–Newton equation will reduce to the Newton equation. In our work, Bohmian mechanics is introduced to intense laser-atom physics. We find quantum potentials of electrons trend to be small and then become negligible soon after the electrons were ionized. Then their motion can be described by classical mechanics. Our result thus presents an evidence for the treatment of the ionized electrons in the semiclassical approaches.

Wider implications. Bohmian mechanics can also describe the motion of an electron in the form of a trajectory before the ionization. It may be applied to the study of quantum chaos of an atom in an external field.

Figure. The time-dependent values of the quantum potential and the ordinary potential of electrons with corresponding initial positions r₀. The dotted lines represent the ionization time of the corresponding electrons. We find that the quantum potentials of the electrons tend to be small and then become negligible soon after the electrons have been ionized.

32.80.Fb Photoionization of atoms and ions

42.50.-p Quantum optics

31.15.xg Semiclassical methods

Atomic and molecular physics

Computational physics

Optics, quantum optics and lasers

Issue 11 (November 2009)

Received 11 August 2009

Published 20 November 2009