The mutual angular distributions of the two ejected electrons following direct photodouble ionization have been measured for D 2 at an excess energy (E) of 25 eV using linearly polarized light. These (γ , 2e) 'triple' differential cross sections (TDCSs) were obtained for asymmetric electron energy conditions with energy sharing ratios (R = E 2 /E 1) of R = 24, 11.5, 4 and 2.57. In all cases the 'reference' electron (energy = E 1) was oriented along the direction of the electric field vector (ε) and detected in coincidence with a second electron (energy = E 2) coplanar with ε and the photon beam direction (k γ). For comparison, helium TDCSs were obtained for the same E and R values under nearly identical spectrometer conditions. These show very good agreement with the results of hyperspherical-matrix with semi-classical outgoing waves calculations, thus providing even more confidence in the D 2 TDCSs where there is as yet no accurate ab initio theory. The similarities and differences between the experimental results associated with the two targets are qualitatively discussed in terms of Feagin's model (

Helium double photoionization (␥,2e) triple differential cross sections ͑TDCSs͒ were measured at an excess energy of 60 eV using a dual toroidal spectrometer and synchrotron radiation from a helical undulator ͑BL-28A, Photon Factory, Japan͒. Energy-sharing ratios (RϭE 2 /E 1 ) for the two ejected electrons of 5 and 11 are studied with both right-and left-handed elliptically polarized light. The TDCSs are found to be in good agreement with those obtained using the hyperspherical R matrix with semiclassical outgoing waves theory. The circular dichroism for a limited mutual angular range ( 12 Ϸ110°-200°) is determined from the experimental data for both Rϭ5 and 11, and compared to theoretical calculations performed over the complete range of mutual angles. No dynamic nodes are found in either the experimental ͑within the explored 12 range͒ or theoretical circular dichroism for these R values at this excess energy.

The breakup of D 2 following the absorption of a single photon, leading to four free charged particles interacting via the Coulomb force, has been investigated. Triply differential cross sections ͑TDCSs͒ of the two ejected electrons were measured for selected asymmetric kinematic conditions, namely, a 20 eV electron in coincidence with a 5 eV ''reference'' electron fixed at four different angles. The results are compared with helium TDCSs and, although the respective angular distributions have similar features, there are surprising differences that could have their origin in two-center interference effects.

A systematic study is reported on the convergence of computed triply differential cross sections for photodouble ionization of helium at 20 and 25 eV above threshold, obtained using the hyperspherical R matrix with semiclassical outgoing waves method. Such a study has become feasible due to an essential improvement in the description of the radial correlation coordinate ␣ = arctan͑r 2 / r 1 ͒ at short range, and to the implementation of the code on a more powerful computer. It shows that the size requirements of the inner region R ഛ R 0 , with R = ͱ r 1 2 + r 2 2 , vary considerably depending on the dynamic situation considered. Typically, while R 0 Ӎ 20 a.u. proves sufficient in most circumstances, one has to increase the inner region up to R 0 Ӎ 60 a.u. in order to reach convergence in the triply differential cross sections obtained for very asymmetric energy sharings or in the particular case when one electron is ejected along the electric field for linearly polarized photons. The converged calculations are generally in very good agreement with new experimental data of high statistical quality recorded in the latter case, at 25 eV above threshold, for various energy sharings. The dynamic situations identified here call for further experimental and theoretical studies.