Double photoionization of helium at an excess energy of 60 eV using left- and right-elliptically-polarized light

Abstract

Helium double photoionization $(\ensuremath{\gamma},2e)$ triple differential cross sections (TDCSs) 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, Japan). Energy-sharing ratios ${(R=E}_{2}{/E}_{1})$ 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 $({\ensuremath{\varphi}}_{12}\ensuremath{\approx}110\ifmmode^\circ\else\textdegree\fi{}--200\ifmmode^\circ\else\textdegree\fi{})$ is determined from the experimental data for both $R=5$ 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 ${\ensuremath{\varphi}}_{12}$ range) or theoretical circular dichroism for these R values at this excess energy.