A. V. Meremianin

A new technique in the theory of angular distributions in atomic processes: the angular distribution of photoelectrons in single and double photoionization

Special reduction formulae for bipolar harmonics with higher ranks of internal spherical functions are derived, which will be useful in problems involving multiple expansions in spherical functions. Together with irreducible tensor operator techniques these results provide a new and effective approach, which enables one to extract the geometrical and dynamical factors from the cross sections of atomic processes with polarized particles with an accurate account of all the polarization effects. The angular distribution of polarized electrons and the circular dichroism in photoionization of polarized atoms with an arbitrary angular momentum are presented in an invariant vector form. A specific circular dichroism, which is caused by the correlation of electron and atom orientations, is discussed. The angular distribution of escaping electrons in double photoionization of unpolarized atom is presented in a simple form. A convenient parametrization is proposed for describing the dependence of the photoprocess cross sections on the polarization state of the photon beam.

Photon-polarization effects and their angular dependence in relativistic two-photon bound-bound transitions

The quantum-electrodynamic theory of photon-polarization effects in two-photon bound-bound atomic transitions is developed, including relativistic and retardation effects. The differential cross section for photon scattering from a randomly oriented target is expressed in terms of eight polarization correlation parameters ai which are associated with vector products involving the photon polarizations. We restrict our detailed discussions to the case of photon scattering from atoms, though the results are easily generalized to other situations. Symmetry arguments are used to establish the necessary conditions for these polarization effects to occur. One of the eight parameters ai describes circular dichroism in the case that the scattered photon has a suitable linear polarization, and another describes the appearance of elliptically polarized scattered photons for the case of a suitable linearly polarized incident beam. The particular cases of atomic scattering with Ji = Jf = 0 and 1/2 (where Ji and Jf are the total angular momenta of the initial and final states of the target, respectively) are analysed in detail. Numerical estimates of the parameters are given for ground-state atoms in the Ji = Jf = 0 approach, using the independent-particle approximation, and for 1s1/2→1s1/2 and 2p3/2 transitions in hydrogen-like ions, demonstrating the feasibility of experimental observation of the above-mentioned circular dichroic effect in high-Z atoms and ions.

The three-body rigid rotator and multipole expansions of the three-body continuum

Two approaches in the derivation of multipole expansions of the three-body continuum wavefunction are considered. These approaches are determined by the basis of angular functions used. The two angular bases considered are bipolar harmonics and Wigner D-functions. In the asymptotic region of large inter-particle separations the continuum wavefunction tends to be a product of two plane waves. It is shown that the coefficients in the multipole expansion of the product of two plane waves in terms of D-functions are equivalent to the wavefunctions of the three-body rigid rotator in momentum representation. Possible applications of the developed formalism to the theory of angular distributions in molecular photo and predissociation processes are discussed. Several novel relations between integrals with two Bessel functions are discovered during the calculations.

Nondipole effects in double photoionization of He at 450 eV excess energy

Convergent close-coupling results for the triply differential cross section for double photoionization of He that include dipole-quadrupole terms are shown to have improved agreement (as compared to dipole approximation results) with recent experiments using linearly polarized light (Knapp A et al 2005 J. Phys. B: At. Mol. Opt. Phys. 38 615) for a number of kinematical configurations.

Nondipole Effects in Double Photoionization of He

Lowest‐order nondipole effects are studied in double photoionization (DPI) of the He atom. Ab initio parametrizations of the quadrupole transition amplitude for DPI from the 1S0‐state are presented in terms of the exact two‐electron reduced matrix elements. Parametrizations for the dipole‐quadrupole triply differential cross section (TDCS) and doubly differential cross section (DDCS) are presented in terms of polarization‐independent amplitudes for the case of an elliptically polarized photon. Expressions for the DDCS in terms of the reduced two‐electron matrix elements are also given. A general analysis of retardation‐induced asymmetries of the TDCS including the circular dichroism effect at equal energy sharing is presented. Our numerical results exhibit a nondipole forward‐backward asymmetry in the TDCS for DPI of He at an excess energy of 450 eV that is in qualitative agreement with existing experimental data.