The angle-resolved N 1s photoelectron spectrum of has been measured with high resolution in the threshold region. An analysis of the vibrational fine structure yields a vibrational energy of and a lifetime broadening of for the core-ionized molecule. As in the case of C 1s ionization of CO, the vibrational fine structure changes considerably with photon energy in the region of both the shape resonance and the double excitations. Both the vibrationally resolved partial photoionization cross sections and asymmetry parameters have been determined in this photon energy range. The calculated cross sections in the literature are in reasonable agreement with experiment. In addition, we have measured the cross sections and asymmetry parameters of the most intense direct and conjugate shake-up satellites. The behaviour of the satellites is found to differ significantly from that of the corresponding C 1s satellites of CO.

The absolute photoabsorption cross section of benzene (C6H6), encompassing the C 1s−1 π*e2u resonance, the C 1s threshold, the satellite thresholds, and extending up to 800 eV, has been measured using synchrotron radiation. Measurements of the discrete absorption structure from below the C 1s ionization threshold have been performed at high resolution. In order to unambiguously assign all structure present in the photoabsorption cross section, C 1s photoelectron spectra were measured from the C 1s threshold region up to 350 eV along with satellite spectra. The C 1s−1 single-hole and the satellite cross sections have been derived in absolute units, and their angular distributions have been determined. Resonant and normal Auger spectra were taken on the main features of the photoabsorption and single-hole cross sections. From the best resolved photoelectron spectra the underlying structure in the asymmetric benzene photoelectron peak can be partly disentangled. The experimental data show that at least two vibrational modes play a role in the C 1s photoelectron spectrum. The behavior of the investigated shake-up structure closely resembles that of ethene and ethyne, where the satellite bands due to π→π* excitations gain intensity towards threshold, an observation which may be attributed to conjugate shake-up processes. These processes lead to a significant contribution of the satellite intensity to the production of the absorption features traditionally assigned to the carbon shape resonances in benzene. An EXAFS analysis of the wide range oscillations present on the photoabsorption cross section has been performed, and reveals the C–C nearest-neighbor distance.

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The resonant Auger spectrum of CO has been measured at both the C and O K-edges. At the C resonance the decay spectrum was recorded selectively at the energies of the = 0, 1 and 2 vibrational states. Vibrational fine structure was not only resolved on the participator but also on the spectator lines. For the O resonance the photon energy bandwidth was sufficiently low that different, vibrationally distinct regions of the absorption profile could be selected. Ab initio calculations and their detailed analysis are presented for both the C and O decay spectra. At the C resonance the calculation explicitly accounts for the vibrational fine structure including vibrational-lifetime interference effects. The appearance of the spectator part of the spectrum is shown to be strongly influenced by the existence of unbound states and diabatic interactions. The nodal structure of the vibrational wavefunction of the core-excited state is reflected in the decay spectrum.

The partial photoionization cross sections and angular distributions of the C 1s ' and 0 1s ' singlehole states of CO2 have been measured with high precision near threshold. The 4o. "shape resonance is well reproduced in the 1s single-hole cross sections, but strong site-specific effects are observed with respect to both its energy and intensity. A striking feature of the present data is the presence of fine structure in the 1s single-hole cross sections o. » due to multielectron excitations: virtually all spectral details in the total 1s cross sections o.&, are also present in o. sH implying that multielectron excitations couple strongly to the underlying 1s continuum. The decay dynamics of the double-excitation feature at 303.5 eV in the C 1s cross section of CO2 have been characterized by Auger spectroscopy. Partial cross sections and angular distributions have also been measured for the first group of strong shake-up satellites in the C 1s photoelectron spectrum. Similar to the C 1s ionization of CO, a strong enhancement of intensity near threshold is observed for some satellite channels as well as the occurrence of features that are not present in the sudden limit. The latter are attributed to conjugate shake-up.PACS number(s): 33.80.Eh, 33.60.Fy I. INTRQDUCTIGNA considerable body of experimental and theoretical work on photoabsorption and photoionization cross sections has shown that resonances and multielectron effects are commonly encountered in core-level excitation of small molecules. In some studies these features have been carefully characterized and provide models for the interpretation of similar efFects in larger molecules [1,2]. The current upsurge of interest in such K-shell properties is due to recently improved instrumentation, in particular high-resolution monochromator s in combination with undulator sources, for absorption and photoelectron spectroscopy experiments, e.g. , [3 -6]. These allow detailed examination of the energetics and dynamics of core-level photoionization.The excitation of a core electron into an unoccupied bound molecular orbital or the continuum is generally accomplished by a strong relaxation of the remaining system of charges. Provided that sufhcient energy is available from the absorbed photon, there is a relatively high probability that a valence electron and vibrations are excited simultaneously with the core electron, giving rise to shake-up satellites in the photoelectron spectra [7 -10] and to vibrational structure in both photoabsorption [3 -5] and photoemission spectra [6,11]. Transitions to unoccupied states accompanied by shake-up often produce characteristic features in the continuum just below the onset of the shake-up satellites seen in the photoelectron spectra and have been referred to as double excitations. In the same energy region, i.e. , within the erst tens 'Present address: Max-Planck-Arbeitsgruppe "Rontgenbeugung, " Hausvogteiplatz 5-7, 10117Berlin, Germany. Present address: Hamburger Synchrotronstrahlungslabor at DESY, Notkestralje 85, 22603 Hamb...