Ralf Wehlitz

Chapter 1 – Simultaneous Emission of Multiple Electrons from Atoms and Molecules Using Synchrotron Radiation

Abstract The removal of two (or more) electrons by a single photon from an atom or molecule is a fundamentally important process as it is a direct probe of electron correlation. Synchrotron radiation provides photons over a wide range of energies from infrared to X-rays to photoexcite or -ionize gaseous samples. With the help of a monochromator the energy of the photons can be tuned to study the photon-energy dependence of the double-photoionization process in which two electrons are emitted by a single photon. In this chapter, I will discuss the photon-energy dependence of the double-photoionization cross sections and the associated double-to-single photoionization ratios for some atoms from threshold to high photon energies. I will also present recent experimental results on the triple-photoionization process, particularly for the text book example of lithium. Furthermore, recent results on multiple ionization by a single photon for selected molecules will be presented. In contrast to atoms, not only ionization but also fragmentation plays an important role in the double-photoionization experiments.

Inner-shell photoionization and core-hole decay of Xe and XeF2

Photoionization cross sections and partial ion yields of Xe and XeF2 from Xe 3d(5/2), Xe 3d(3/2), and F 1s subshells in the 660-740 eV range are compared to explore effects of the F ligands. The Xe 3d-ϵf continuum shape resonances dominate the photoionization cross sections of both the atom and molecule, but prominent resonances appear in the XeF2 cross section due to nominal excitation of Xe 3d and F 1s electrons to the lowest unoccupied molecular orbital (LUMO), a delocalized anti-bonding MO. Comparisons of the ion products from the atom and molecule following Xe 3d photoionization show that the charge-state distribution of Xe ions is shifted to lower charge states in the molecule along with production of energetic F(+) and F(2+) ions. This suggests that, in decay of a Xe 3d core hole, charge is redistributed to the F ligands and the system dissociates due to Coulomb repulsion. The ion products from excitation of the F 1s-LUMO resonance are different and show strong increases in the yields of Xe(+) and F(+) ions. The subshell ionization thresholds, the LUMO resonance energies, and their oscillator strengths are calculated by relativistic coupled-cluster methods and agree well with measurements.

Double photoionization of halogenated benzene

We have experimentally investigated the double-photoionization process in C6BrF5 using monochromatized synchrotron radiation. We compare our results with previously published data for partially deuterated benzene (C6H3D3) over a wide range of photon energies from threshold to 270 eV. A broad resonance in the ratio of doubly to singly charged parent ions at about 65 eV appears shifted in energy compared to benzene data. This shift is due to the difference in the bond lengths in two molecules. A simple model can explain the shape of this resonance. At higher photon energies, we observe another broad resonance that can be explained as a second harmonic of the first resonance.

Double photoionization of tropone and cyclooctatetraene

We have studied the double-photoionization process of tropone (C7H6O) and cyclooctatetraene (C8H8) as a function of photon energy using monochromatized synchrotron radiation between 18 and 270 eV. We compare our results with previously published data for partially deuterated benzene (C6H3D3), which exhibits three distinct features in the ratio of doubly to singly charged parent ions, whereas pyrrole (C4H4N) exhibits only two of these features. The question that we address in this paper is how molecules with different molecular structures (pentagonal, hexagonal, heptagonal, and octagonal carbon rings) affect the photon-energy dependence of this ratio.

Lithium inner-shell resonances in the 70?77 eV photon energy region

We have measured the relative ion-yield of lithium between 70 and 81 eV and investigated the 1s3?n? (? = s, p, d; n ? 3) and 1s4?n? (? = s, p, d, f; n ? 4) inner-shell double excitations. Employing an undulator beamline with a resolving power of 13?000 we have tracked the autoionizing resonances to higher principal quantum numbers than in previous experiments. Using Fano profiles to fit our ion yield spectra we have determined the resonance positions, widths and profile parameters for the 1s3?n? resonances. From the resonance positions we have derived the series limits and quantum defects of six Rydberg series. We find very good agreement with the resonance positions and widths of the R-matrix calculations by Chung and Chang (Chung and Chang 2000 Phys. Rev. A 61 030701). Our energy positions of the 1s4?n? resonances agree well with a previous measurement.

Core polarization effects in the decay of 1s →np (n = 2-6) resonantly excited beryllium

The photoelectron angular distribution parameter of the relevant decay channels following the resonant 1s →np (n = 2-6) excitations in beryllium was measured. The 2s and 3s photoelectrons present at the 2p and 3p resonances, respectively, have β = 2 as predicted by theory. For the mp spectator channels, small but distinct deviations from the expected complete isotropy were found. An explanation of these deviations is proposed in terms of the polarization of the 2s2 subshell due to the excited np electron.