J. P. J. Carney

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.

Existence of near-zero minima in elastic photon–atom scattering

Abstract We consider near-zero minima in the photon–atom elastic scattering cross section, possible signatures of the presence of a particular element in a target, using the S-matrix approach. However, the usual formulation of this approach performs an averaging over magnetic substates at the level of the scattering amplitude that is approximate (except for the case of fully filled subshells). We consider whether the predicted near-zero minima persist in a more proper treatment. We find that near-zero minima seen below thresholds in scattering from ground-state atoms generally persist, but near-zero minima seen above thresholds in scattering from excited states may be spurious.

Elastic photon scattering from excited states of atoms and ions

Abstract The elastic scattering of photons from excited state configurations of carbon atoms and ions is considered, using the S–matrix approach, for configurations involving varying numbers of K and L electrons, with any remaining electrons being placed in an outer ( N ) shell. Situations where the scattering cross section is much larger or smaller than that for the ground state configuration are identified.

Understanding the importance of dynamic, correlation, and exchange effects in new low-energy photon scattering experiments

We consider the current status of theory in photon scattering, and new approaches to both elastic and inelastic scattering, in the light of recent x-ray scattering measurements on neon and helium in the range 4–22 keV, obtained using the APS third-generation synchrotron. The precision of these experiments requires a detailed consideration of the effects of nonlocal exchange, electron correlations, and dynamic effects in making predictions. These effects can often be regarded as perturbations, though this assumption fails as threshold regions are approached. As well as considering procedures for making composite predictions including all significant effects, we also consider the approximations that are often employed in describing elastic (form-factor approximation) and inelastic (incoherent-scattering-factor and impulse approximations) scattering. Dynamic effects and nonlocal-exchange effects can be significant in elastic scattering. The incoherent-scattering-factor approximation works best for inelastic ...