Marcel Pont

Absolute cross sections for double photoionization of helium at energies from 0 to 80 eV above threshold

We have performed, using an ab initio basis-set method, calculations of the absolute cross section for double photoionization of helium for energies from 2 to 80 eV above threshold, and we have extended the results down to 0 eV through spline interpolation. Our results are consistent with the Wannier threshold law, and agree well with the near-threshold measurements. At higher energies (20 eV or so above threshold) significant discrepancies with experiment are evident. We find a simple relation between the integrated cross section for double photoionization and the singly differential cross section at the midpoint of the energy distribution.

Angular distributions for double photoionization of helium: a comparative study

We have performed calculations of the triply differential cross section (TDCS) for double photoionization of helium. Our calculations are based on two different ab initio methods, one where the final continuum state is represented by a product of three Coulomb wavefunctions, with the dipole matrix element evaluated directly, the other where the final state is represented by a product of two screened Coulomb wavefunctions, which is substituted into a flux formula that in principle yields exact results. We compare the results of the two calculations with each other and also with two different sets of experimental data, for various geometries, for various ratios of the energies of the two photoelectrons, and for various photon energies such that the total energy of the photoelectrons ranges from 0.6-20 eV above threshold. The theoretical results for the shapes of the TDCS are in good agreement with the experimental data and also with each other, although the calculated absolute values of the TDCS differ. We comment on various features of the angular distributions.

Double Photoionization of Helium

The differential cross section for double photoionization of helium provides a sensitive tool for exploring electron-electron correlation since the absorption of one photon cannot lead to double escape without the two electrons interacting. However, differential cross sections for double photoionization are neither easy to calculate nor easy to measure. Moreover, while a great deal has been learned about the dynamics of double photoionization since the pioneering work by Wannier1 in 1953, a complete understanding has yet to be achieved. Over the last several years we have developed a method,2,3 based on the earlier work of Rudge and Seaton,4 for calculating differential cross sections for double photoionization of helium. In this paper we present some of our results and we compare them with selected data from recent measurements in the energy range 0 to 80 eV above threshold.