High resolution, vibrationally resolved, near-edge x-ray absorption fine structure ͑NEXAFS͒ spectra at the C 1s and N 1s ionization thresholds of pyridine and deuterated d 5-pyridine in the gas phase have been recorded. The high resolution of 65 meV ͑150 meV͒ at the C s ͑N 1s͒ ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and density functional theory ͑DFT͒ calculations were performed to interpret the experimental spectra and to assign the observed peaks. In particular we focused on the previously unexplained intensity ratio for the two components of the C 1s→1* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and leads to the experimentally observed ϳ2:3 intensity ratio between the two * components in the C 1s spectrum rather than the ϳ3:2 ratio obtained without vibrational effects. After inclusion of relaxation effects in the excited states, in addition to the vibrational effects, both theoretical methods yield almost perfect agreement with experiment.

Neutral C60 is well known to exhibit a giant resonance in its photon absorption spectrum near 20 eV. This is associated with a surface plasmon, where delocalized electrons oscillate as a whole relative to the ionic cage. Absolute photoionization cross-section measurements for C+60, C2+60, and C3+60 ions in the 17-75 eV energy range show an additional resonance near 40 eV. Time-dependent density functional calculations confirm the collective nature of this feature, which is characterized as a dipole-excited volume plasmon made possible by the special fullerene geometry.

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High-resolution photoabsorption spectra of CO B.lld NO below the 0 Is ionization threshold are presented. For both molecules, the vibrational finestructure of the 0 ls-t 'II"• a.nd 0 ls-1 Ryd (Rydberg) excitations could be resolved, allowing a derivation of the vibrational energies and intra.moleculal' distances of the core-excitation states in CO and NO from Ftanck-Condon analyses. The spectral features of the 0 ~s-1 Ryd region in CO are reassigned on the basis of the new experimental results. The results obtained for the 0 ls-1 3s Rydberg state in NO support the idea of a WP.akt>.nihg of the molecular bond upon an 0 15-J. ionization process.

Data and analysis for the ratio of double to single ionization in helium is reviewed for impact by photons and charged particles. In the case of photoionization there are two processes, namely, (i) photoionization where the photon is annihilated, and (ii) Compton scattering where the photon is inelastically scattered. In the case of charged particle scattering the ratio of total cross sections tends toward an asymptotic high energy value of 0.26% which is well below the value observed for photons of 1.7% at photon energies between 2 and 12 keV. Theoretical relations between various ratios have been predicted and to some extent confirmed by observations.

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We report high-resolution C 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the C6-ring-containing molecules benzene (C6H6), 1,3- and 1,4-cyclohexadiene (C6H8), cyclohexene (C6H10), cyclohexane (C6H12), styrene (C8H8), and ethylbenzene (C8H10) which allow us to examine the gradual development of delocalization of the corresponding pi electron systems. Due to the high experimental resolution, vibrational progressions can be partly resolved in the spectra. The experimental spectra are compared with theoretical NEXAFS spectra obtained from density-functional theory calculations where electronic final-state relaxation is accounted for. The comparison yields very good agreement between theoretical spectra and experimental results. In all cases, the spectra can be described by excitations to pi*- and sigma*-type final-state orbitals with valence character, while final-state orbitals of Rydberg character make only minor contributions. The lowest C 1s-->1pi* excitation energy is found to agree in the (experimental and theoretical) spectra of all molecules except for 1,3-cyclohexadiene (C6H8) where an energy smaller by about 0.6 eV is obtained. The theoretical analysis can explain this result by different binding properties of this molecule compared to the others.

Anion-yield spectroscopy using x rays is shown to be a selective probe of molecular core-level processes, providing unique experimental verification of shape resonances. For CO, partial anion and cation yields are presented for photon energies near the C K edge. The O- yield exhibits features above threshold related only to doubly excited states, in contrast to cation yields which also exhibit pronounced structure due to the well-known sigma* shape resonance. Because the shape resonance is completely suppressed for O-, anion spectroscopy thus constitutes a highly selective probe, yielding information unobtainable with absorption or electron spectroscopy.