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Кінематика процесу двофотонної іонізації атомарних систем

M. Ya. Agre

Published Jan 1, 2008• 0 citations
OpenAlex

Abstract

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.

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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.
The three-dimensional angular distribution of electrons in two-photon ionization of atoms by an elliptically polarized laser beam, from an initial state with arbitrary angular momentum J i , is expressed in terms of six frequency-dependent invariant atomic parameters. The specific effect of the field polarization on the angular distribution (the elliptic dichroism, ED) is discussed for the general case of elliptic polarization. The necessary conditions for a non-zero ED are established and its physical origin is explained. The general theory is illustrated by a simple analytical calculation for the -potential model. Numerical calculations of the angular distribution parameters for H and Cs atoms are also performed and the dependence of ED magnitude on the atomic quantum numbers and the photon frequency is discussed for different frequencies.
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.
In multiphoton ionization of atoms, elliptical dichroism may arise in the photoelectron angular distributions due to the interference of the possible ionization pathways. We here consider the interaction of atoms with an elliptically polarized biharmonic ($\ensuremath{\omega} + 2\ensuremath{\omega}$) field which simultaneously allows one- and two-photon ionization of the atoms. The interference between these two ionization pathways introduces contributions to the elliptical dichroism in addition to the dichroism that arises from the two-photon ionization alone. We show that these additional dichroism contributions can lead to a stronger dichroism in comparison to the one arising from two-photon ionization only. We present a relativistic analysis of the corresponding photoelectron angular distributions and discuss individual contributions to the dichroic phenomena. Detailed computations have been performed for biharmonic ionization of neutral helium atoms.
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.
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.