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Physical Review A

Circular dichroism from unpolarized atoms in multiphoton multicolor ionization

Richard Taïeb, Valérie Vëniard, A. Maquet, Nikolai L. Manakov, S. I. Marmo

Published Jun 7, 2000• 41 citations
Publisher

Abstract

The angular distribution of the photoelectrons produced from a ground-state atom ionized under the combined action of two (or several) radiation fields can exhibit circular dichroism effects, when reversing the polarization state (right versus left, for instance) of one of the fields. We address the question of such dichroic effects in a currently explored scenario in which the atom is ionized in the presence of a strong laser field and of one (or several) of its higher harmonics.

DOI
10.1103/physreva.62.013402

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The angular distribution of the photoelectrons produced from a ground-state atom ionized under the combined action of two (or several) radiation fields can exhibit circular dichroism effects, when reversing the polarization state (right versus left, for instance) of one of the fields. We address the question of such dichroic effects in a currently explored scenario in which the atom is ionized in the presence of a strong laser field and of one (or several) of its higher harmonics.
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We study dichroic effects in the two-photon ionization of hydrogen from its ground state due to the interaction with a bichromatic field of commensurate frequencies. The two field components have different polarization: one is linearly polarized and the other is circularly polarized. Circular dichroism in the angular distribution of the photoelectrons appears if the helicity of the circularly polarized field is reversed. Numerical results reveal the influence of the photon frequencies chosen on the circular dichroism in the azimuthal angular distribution of the ejected photoelectrons.
The dichroic effects in the multiphoton ionization of the ground state hydrogen atom by a coherent superposition of a laser beam and its third harmonic, are studied via perturbative calculations. The final state of the photoelectrons, which has the energy E = E 1 + 3ℏω ( E 1 the ground state energy and ω the laser frequency), is reached by two interfering quantum paths: (a) absorption of one harmonic photon and (b) absorption of three laser photons. In the chosen regime of field intensities, the radiative corrections to the absorption of one harmonic photon may be disregarded. Our numerical results illustrate the influence of the laser frequency, of the relative intensity and of the harmonic phase upon the dichroic signal.
The theory of multiphoton ionization for an atomic system of arbitrary complexity is developed using a density matrix formalism. An expression is obtained which determines the differential N-photon ionization cross section as a function of the polarization states of the target atom and the incident radiation. The parameters which characterize the photo-electron angular distribution are related to the general reduced matrix elements for the N-photon transition. Two-photon ionization of unpolarized atoms is treated as an illustration of the use of the theory. The dependence of the multiphoton ionization cross section on the polarization state of the incident radiation, which has been observed in two- and three-photon ionization of Cs, is accounted for by the theory. Finally, the photoelectron spin polarization produced by the multiphoton ionization of unpolarized atoms, like the analogous polarization resulting from single-photon ionization, is found to depend on the circular polarization of the incident radiation.
The polarization dependence of the photoionization probability was analyzed in the case when a randomly oriented atom is irradiated by two crossing polarized monochromatic photon beams with the same frequency. It was found that the angular distributions of photoelectrons exhibit the effect of circular dichroism (CD), which consists of the dependence of the photoionization probability on the sign of the circular polarization degree of each beam. We demonstrate that the CD effect exists only for coherent crossing photon beams. It was shown that CD effects are strongly dependent on the phase difference between the electric field vectors of the photon beams and have a quite large magnitude. The possibilities of the experimental observation of CD effects are discussed.