The Coulomb explosion of the hydrogen molecule, after absorption of a 76 eV photon, has been studied by momentum imaging the two electrons and the two protons. Absolute fully differential cross sections of high statistical quality are obtained. A subset of the overall data, namely, equal electron-energy sharing, is used to investigate the effects of molecular orientation on the photoelectron angular distribution. Departures from the first-order helium-like model are evident in detection geometries where electron-electron correlation is "frozen."

Angular correlation in the two-electron continuum of the He double photoionization has been studied, both experimentally and theoretically, for equal and unequal energy sharing conditions at the photon energy of 40 eV above the threshold. The triple differential cross sections have been measured in the plane perpendicular to the photon direction using the multi-coincidence detection technique of Reddish et al at the Daresbury Synchrotron Radiation Source. Recent modifications to the bending-magnet beam line allowed an effective cancellation of the circular polarization at the target, leaving a relatively high degree of linear polarization (Stokes parameters: S 3 = S 2 ∼ = 0, S 1 = 0.8). The measured cross sections are compared with the calculations using the 3C method of Maulbetsch and Briggs and the convergent close coupling method of Kheifets and Bray. Good agreement between theory and experiment has been found in most cases, except for the unequal energy sharing when one of the escaping electrons is detected in a direction close to the polarization axis.

Angular distributions of the two ejected electrons following photodouble ionization have been measured for molecular D 2 and He with E 1 = E 2 = 10 eV. The results were obtained at the Daresbury Synchrotron Radiation Source using a multicoincidence spectrometer consisting of two toroidal analysers. The measured distributions show strong similarities, as well as differences which suggest that molecular effects are manifest at these energy conditions.

Angular distributions of the two ejected electrons following photodouble ionization of molecular deuterium have been measured using a toroidal photoelectron-photoelectron coincidence spectrometer in conjunction with synchrotron radiation. Six ͑g, 2e͒ triple differential cross sections (TDCS) were measured in the plane orthogonal to the photon beam direction with E 1 E 2 10 eV. The angular distributions are similar to those of helium, but with differences which we highlight by comparing the D 2 TDCS with helium TDCS measured under nearly identical conditions. [S0031-9007(97)04143-4] PACS numbers: 33.80.Eh, 32.80.Fb In photodouble ionization (PDI) a single photon is absorbed by an atom or a molecule, followed by the ejection of two electrons. PDI is simplest for the fundamental two-electron systems, helium and molecular hydrogen, as there is only one possible ion state and there are no intermediate states above it:It is not possible for direct PDI to be described within the familiar independent particle model as the photon cannot interact with more than one electron. Consequently, PDI is a manifestation of electron-electron interactions and so is a sensitive test for electron correlation theories.The angular distribution-or triple differential cross section (TDCS)-of the ejected electrons is a particularly sensitive measure of the electron correlation. These measurements [e.g., 1-6] have only been possible during the last few years due to improvements in synchrotron radiation sources and in the necessary photoelectronphotoelectron coincidence (PEPECO) techniques. Recently, a complementary technique has been applied to helium which determines the momentum of the recoiling ion in coincidence with one of the photoelectrons, providing similar information but in momentum space [7]. To date, TDCS has been investigated only for rare gases, and the progress in this area has benefited greatly from a close relationship between experiment and theory (e.g., [8][9][10][11][12][13][14][15][16]). However, such studies have yet to be applied to molecules. In this Letter we present the first measurements of molecular TDCS, having performed them in the most fundamental system: hydrogen.The photodouble ionization of helium results in the classic three-particle Coulomb continuum problem, whose solution determines the correlation factor in the expression for the TDCS [8]. The situation in hydrogen is not unrelated, but is further complicated by the inevitable dissociation of the molecule during double ionization. Therefore, PDI in hydrogen results in four unbound particles, albeit with two types of particles having different charges, masses, and velocities. However, it is perhaps not unreasonable to expect similarities in the angular distributions with helium (final state electronic symmetry 1 P 0 ) if the experimental conditions are chosen such that the electrons' speeds are much greater than those of the protons. The nuclear motion could then be considered as a perturbation which increases as the electron energies decrease.Previou...

Abstract.A helium-like description of photo double ionization of molecular deuterium is extended to derive a dependence of molecular excitation amplitudes on electron energy sharing and dynamical quantum numbers labelling internal modes of excitation of the escaping electron pair. Both linear and circular polarizations are considered, and predictions regarding circular dichroisms in the angular distributions of the electron and ion pairs are given. A detailed comparison of the model with recent (γ, 2e) measurements is also presented.

A photoelectron-photoelectron coincidence spectrometer, incorporating two independent toroidal analyzers, has been developed to study photodouble ionization of gas targets using synchrotron radiation. The energy-resolved and angle-dispersed electron images are recorded using two-dimensional position-sensitive detectors. The design and performance of the spectrometer is described, with particular emphasis on the electron optical properties of toroidal deflectors and their associated electrostatic lenses. The operation and calibration of the spectrometer are discussed and sample (␥,2e) results of helium are presented.

An atom trapping technique for determining absolute, total ionization cross sections ͑TICS͒ out of an excited atom is presented. The unique feature of our method is in utilizing Doppler cooling of neutral atoms to determine ionization cross sections. This fluorescence-monitoring experiment, which is a variant of the "trap loss" technique, has enabled us to obtain the experimental electron impact ionization cross sections out of the Cs 6 2 P 3/2 state between 7 eV and 400 eV. CCC, RMPS, and Born theoretical results are also presented for both the ground and excited states of cesium and rubidium. In the low energy region ͑Ͻ11 eV͒ where best agreement between these excited state measurements and theory might be expected, a discrepancy of approximately a factor of five is observed. Above this energy there are significant contributions to the TICS from both autoionization and multiple ionization.

High resolution threshold photoelectron spectroscopy, in conjunction with synchrotron radiation has been employed to investigate single and double photoionization of argon in the 32-51 eV photon energy range. We have been able to distinguish between satellite and resonance states observed in our spectrum and thereby correct previous misassignments. Furthermore, we have developed a method to extract partial cross sections for satellite states from threshold to 150meV. The excitation functions of these electroncorrelated processes generally show a strong threshold peak, which we attribute to dynamic effects within the excitation complex, as well as other peaks due to the nearby neutral resonances. In addition. the apparent asymmetry of the A?' 'D threshold cusp is discussed in terms of a possible pressure effect, * Hall et a/ (1989).$The estimated uncerfainties of the measured energies below 38.6 eV is i 3 meV and 110 meV above.8 Energies and assignments from Minnhagen (1971) unless otherwise stated./I Tentative assignment. 7 Moore (1949)."Svensson er a/ (1987).t Madden et a/ (1969).