No abstract available

No abstract available

No abstract available

No abstract available

[en] Non-dipolar angular-distribution parameters for valence photoemission have been measured for the first time. Results were obtained for the electric-quadrupole parameter γ for Ne 2s photoemission and for the sum of γ and the magnetic-dipole parameter δ for Ne 2p photoemission in the 900-1200 eV photon-energy range. Both valence lines exhibit stronger non-dipole effects than predicted by theory. In addition, the electric-dipole parameter β was measured for Ne 2p photoemission between 650 and 1200 eV and does not agree with earlier data or theory. The presence of significant non-dipolar effects in valence photoemission at photon energies below 1 keV illustrates that these effects need to be accounted for; earlier experiments on atoms, molecules, and solids using soft-x-ray photons may need to be re-examined in light of these findings

[en] Full text: An x-ray emission spectrometer was designed to observe the polarization of x-ray fluorescence resulting from the excitation of a sample by synchrotron radiation using beamline 9.3.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. The incident photons are intense, monoenergetic, and linearly polarized along the plane of the ALS storage ring orbit. The emission spectrometer records the entire K- α spectrum by dispersing the emitted radiation with a curved Si (111) crystal and detecting the resulting radiation with a resistive anode position sensitive detector; both of which are situated on a Rowland circle. A sample is in a fixed position and located in the middle of the Rowland circle. The spectrometer can be positioned to detect fluorescence emitted 0 deg (parallel) and 90 deg (perpendicular) with respect to the polarization/propagation direction of the incident x-rays. When measuring freon-13, the K- x-ray emission intensity was isotropic at energies above the chlorine 1s ionization threshold with an intensity ratio of 2/1 in both directions by comparing the intensity of α₁ to α₂ (2p_3/2 to 2p_1/2 ). Below the ionization threshold, there was a small polarization dependence of the emission intensity; the largest effect seen when the excitation was to the np Rydberg state; yielding 1.7/1 in the parallel and 2.4/1 in the perpendicular

[en] Photoelectron spectroscopy is a powerful technique because it directly probes, via the measurement of photoelectron kinetic energies, orbital and band structure in valence and core levels in a wide variety of samples. The technique becomes even more powerful when it is performed in an angle-resolved mode, where photoelectrons are distinguished not only by their kinetic energy, but by their direction of emission as well. Determining the probability of electron ejection as a function of angle probes the different quantum-mechanical channels available to a photoemission process, because it is sensitive to phase differences among the channels. As a result, angle-resolved photoemission has been used successfully for many years to provide stringent tests of the understanding of basic physical processes underlying gas-phase and solid-state interactions with radiation. One mainstay in the application of angle-resolved photoelectron spectroscopy is the well-known electric-dipole approximation for photon interactions. In this simplification, all higher-order terms, such as those due to electric-quadrupole and magnetic-dipole interactions, are neglected. As the photon energy increases, however, effects beyond the dipole approximation become important. To best determine the range of validity of the dipole approximation, photoemission measurements on a simple atomic system, neon, where extra-atomic effects cannot play a role, were performed at BL 8.0. The measurements show that deviations from open-quotes dipoleclose quotes expectations in angle-resolved valence photoemission are observable for photon energies down to at least 0.25 keV, and are quite significant at energies around 1 keV. From these results, it is clear that non-dipole angular-distribution effects may need to be considered in any application of angle-resolved photoelectron spectroscopy that uses x-ray photons of energies as low as a few hundred eV

[en] The dynamics of near-threshold photoionization is a complex phenomenon in which the many-electron character of the wavefunctions plays an important role. According to generalized time-independent resonant scattering theory, the transition matrix element from an initial state to a final state is the summation of the amplitudes of direct photoionization and an indirect term in which intermediate states are involved and the resonant behavior is embedded. Studies of the interference effects of intermediate states have been explored in the cases where the direct term is negligible. In the present work, electron time-of-flight spectra of the Ar 2p satellites were measured at two angles (magic and 0 degrees) in the dipole plane with the exciting photon energy tuned in the vicinity of the Ar 2s threshold. For excitation far below or above the 2s threshold, the 2p satellites spectrum is dominated by 3p to np shakeup contributions upon the ionization of a 2p electron

[en] Measurements of the photoelectron angular-distribution asymmetry parameter β for Xe 5s photoionization have been performed in the 80--200 eV photon-energy region. The results show a substantial deviation from the nonrelativistic value of β=2 and provide a clear signature of significant relativistic effects in interchannel coupling

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