The angular and energy dependence of circular dichroism in photo-double-ionization of helium at a photon energy of 99 eV is investigated. Using cold target recoil ion momentum spectroscopy the absolute fivefold differential cross section has been obtained by a coincident measurement of the vector momenta of one electron and the recoiling He 21 ion covering all relative azimuthal and polar angles. The experimental results are contrasted with numerical calculations using different helium ground state wave functions and forms of the dipole operator. [S0031-9007(98)06337-6] PACS numbers: 32.80.Fb, 33.55.AdHelicity adds a new twist to photo-double-ionization of helium: How is the handedness of a circularly polarized photon, absorbed by the spherical symmetric He 1S ground state, transferred to the three-body Coulomb continuum? The handedness leads to a symmetry break in the phase space of the diverging two electrons and the target nucleus. This additional aspect provides a novel tool for the investigation of the photon-induced fragmentation of helium, which is one of the simplest and hence most fundamental manifestations of electron-electron correlation. Helicity dependence can also be used to prove symmetry principles of the three-body Coulomb problem as parity conservation and time reversal invariance.Helicity dependence in photoionization is termed circular dichroism (CD). CD is well known in magnetic solids and chiral or aligned molecules (for a recent review see [1]). However, Berakdar and Klar [2] have theoretically shown that CD does not require a chiral or aligned initial state. They predicted that CD may also be observable in a coincidence measurement (e.g., of the two electrons) in photo-double-ionization of helium from its ground state. In general an appropriate experimental approach to investigate a many body system is to measure the fully differential cross section for the fragmentation of the system by a coincident determination of the vector momenta of all outgoing particles. In their pioneering coincidence experiment Schwarzkopf et al.[3] reported the first fivefold differential cross sections (5DCS) [4] for photo-double-ionization of helium using linearly polarized light. Also the technique of cold target recoil ion momentum spectroscopy (COLTRIMS) has already been used to perform fully differential experiments on photo-double-ionization of helium with linearly polarized light [5]. In 1996 Viefhaus et al. [6] reported the first experimental evidence for CD in the helical photo-double-ionization of helium. They obtained the relative 5DCS for five electron energies each at three fixed angles.In the present experiment we have used COLTRIMS [7-9] to investigate the helical photo-double-ionization of helium at an energy of 20 eV above the double ionization threshold. We have measured the 5DCS for all relative polar and azimuthal angles in the electron energy range of 11.5 through 20 eV. A coincidence solid angle of 10% has been achieved, which is an increase of nearly 3 orders of magnitude compared to t...

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The X-ray electron-nuclear double transitions (XENDT) are processes in which a transition effected by an inner atomic electron takes place simultaneously with a nuclear electromagnetic transition. We give expressions for the cross sections of electric and magnetic XENDTs of various multipole orders. We calculate the rate of deexcitation of isomeric nuclei induced by XENDTs for the case when the holes in the atomic shells are produced by incident ionizing electrons and find that the induced nuclear deexcitation rate becomes comparable to the natural decay rate for ionizing fluxes of the order of 10 14 W cm −2 . We show that for E1 and M1 nuclear processes for which there is a matching between the electron and the nuclear transition energies, the XENDTs can be used to produce pulses of Mössbauer radiation, with yields of the order of 10 4 Bq mA −1 .

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