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Atoms

Circular Dichroism in the Photoionization of Unpolarized Atoms by Two Crossing Photon Beams

Alexei V. Meremianin, Nikolai L. Manakov

Published Dec 3, 2021• 0 citations
Publisher

Abstract

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.

DOI
10.3390/atoms9040108

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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.
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 (CP). Circular dichroism (CD) in the angular distribution of the photoelectrons appears if the helicity of the CP field is reversed. Numerical results reveal the influence of the photon frequencies chosen on the CD in the azimuthal angular distribution of the ejected photoelectrons.
The circular dichroism (CD) effect is predicted in a specific case of a two-color ionization of atoms, due to interfering a dc field-induced resonance two-photon ionization by electromagnetic radiation with frequency (omega) and the one- photon ionization by the wave with frequency 2(omega) . The maximum degree of CD may be observed in the range of fields where the probability for the dc field-induced two-photon ionization becomes comparable with that of the single-photon process and exceeds essentially the field-free two-photon ionization probability. The effect is sensitive to the relative phase between the two ac field components. The atomic parameters for this effect in alkali atoms are calculated. The range of fields is determined, for which the dichroism becomes observable. The use of the effect to control the branching ratio and the relative phase of the two radiations is discussed.
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.
A compact invariant expression for the angular distribution of photoelectrons escaping from unpolarized atoms and molecules in the process of two-photon ionization is derived on the basis of the general symmetry considerations. The dependence on all geometric parameters is completely separated in the angular distribution. It is shown that the circular dichroism effect can be observed in the angular distribution of photoelectrons escaping from the optically active molecules, while the specific elliptical dichroism effect only remains for the optically inactive molecules and atoms. The invariant expression for the total probability of two-photon ionization of axisymmetrically polarized (oriented and aligned) atomic systems is also derived, and caused by the orientation and alignment effects of circular and elliptical dichroism are analysed.
Using the electric dipole approximation, we present, in invariant form, the cross section of an arbitrary three-photon transition between the discrete states of an atom with total angular momenta J i and J f . The cross section contains scalar and mixed products of the photon polarization vectors, and invariant atomic parameters dependent only on the photon frequencies. We determine the number of independent atomic parameters at fixed values of J i and J f and obtain their explicit expressions in terms of the reduced composite dipole matrix elements. The polarization dependence of the cross sections is expressed in terms of the degrees l and ξ of linear and circular photon polarizations. We analyze the phenomenon of dissipation-induced circular dichroism in three-photon processes, i.e., the difference Δ of the cross sections for opposite signs of the degree of circular polarization of all the photons. We study in detail the case of two identical photons and the phenomenon of elliptical dichroism, when Δ∼ lξ holds and dichroism occurs only when the photons are elliptically polarized, with 0<| ξ |<1. Finally, we discuss the dissipation-induced effects of atom polarization in three-photon processes involving linearly polarized or unpolarized photons.
Circular dichroism has been detected in the polarization of the fluorescence resulting from the decay of excited calcium ions. Synchrotron radiation was used to both ionize and excite calcium atoms from their ground state, and the photoejected electron was detected in coincidence with the polarized fluorescent photon. By introducing a degree of left, then right circularly polarized light into the incident photon beam the circular dichroism could be measured. The analysis of the results was carried out using recent developments of the statistical tensor method, and it was possible to determine the sign of the phase difference between the outgoing waves.