Nondipole effects in the triply differential cross section for double photoionization of He

Abstract

Lowest-order nondipole effects are studied systematically in double photoionization ͑DPI͒ of the He atom. Ab initio parametrizations of the quadrupole transition amplitude for DPI from the 1 S 0 state are presented in terms of the exact two-electron radial matrix elements. Analytic expressions for these matrix elements within lowest-order perturbation theory ͑LOPT͒ in the interelectron interaction are also given. The corresponding parametrizations for the dipole-quadrupole triply differential cross section ͑TDCS͒ are presented for the case of an elliptically polarized photon. A general analysis of retardation-induced asymmetries of the TDCS including the circular dichroism effect at equal energy sharing is presented. Numerical LOPT estimates of nondipole asymmetries in photoelectron angular distributions for the cases of linear and circular polarization and of the circular dichroism effect at equal energy sharing are presented. We find that experimental observation of nondipole effects at excess energies of the order of tens to hundreds of eV should be feasible in TDCS measurements. 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.

Ejection of Quasi-Free-Electron Pairs from the Helium-Atom Ground State by Single-Photon Absorption

“…However, very little is known about PDI at higher photon energies. The coupling of higher orders of angular momentum of the incoming light, such as the electric quadrupole term, to the two electrons in the atom has only been addressed theoretically [25][26][27][28][29][30]. So far, no experiments have studied non-dipole effects in PDI due to extremely small cross sections.…”

Nondipole Effects in Double Photoionization of He

“…The An alternative parametrization of the DPI amplitude A in terms of orthogonal vectors p+ = (pl + fi2)/2 and p-= (pl -p2)/2 and the symmetrized (f(g) = fl + f2 and = gl +g2 f 2gs) and antisymmerized (f (u) = f l -f2 and g(U) = gl -g2) amplitudes is rather obvious and can be easily obtained from Eqs. (3), (4), (6), and (7) [2]. The results in Eqs.…”

“…[3], to derive a parametrization of the quadrupole amplitude Aq we use the expansion (5) and the reduction formulae [3] for the rank-2 bipolar harmonics. Thus, Aq may be presented as follows [2,5]:…”

“…D ) + ( {~@~I~. Q~) I~) , (2) for both velocity (V) and length (L) gauges of the electron-photon interaction. {a@ b)2, is the irreducible tensor product of rank 2.…”

“…The dichroic term 1mcs(k-[PI x 821) in Eqs. (8) and (10) results also in an unusual feature of the TDCS, a different symmetry of the TDCS with respect to two transformations: (i) k + -k (i.e., the inversion of the photon beam direction) and (ii) (el, &) + ( a -81, n -&) (i.e., the reflection of the photoelectron pair in the polarization plane) [2].…”