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Vibrational branching ratios and photoelectron angular distributions were measured for 4σ-1g photoionization of CO2 in the energy range 20–28 eV. Of particular interest are three vibrational components of the resulting CO+2 C 2Σ+g state—the allowed (000) and (100) bands and the forbidden (101) band. The wavelength dependence of the beta parameter for the forbidden band deviated significantly from that of the two allowed bands, showing instead a strong resemblance to that of the B 2Σ+u state. This behavior suggests that vibronic coupling to the B 2Σ+u state is responsible for the appearance of the forbidden (101) band in the C 2Σ+g state photoelectron spectrum. We also observe evidence for other many-body effects—shape-resonance-induced continuum–continuum coupling and doubly excited autoionizing resonances—in the present data. [ABSTRACT FROM AUTHOR]

Resonant photoemission has been studied above the carbon 1s ionization thresholds in gas-phase benzene and ethylene. The experimental data for both molecules include relative partial cross section and asymmetry-parameter measurements for the C 1s main line and asymmetry-parameter measurements for one C 1s shake-up satellite in each system. Resonances above the C K edge have been analyzed on the basis of their decay to either the C 1s main line or valence-hole states, and have been tentatively assigned as either shape resonances or doubly excited states according to their observed one-electron or many-electron decay, respectively. The importance of determining the resonant behavior of all available photoemission channels in the proximity of a resonance is thus illustrated. [ABSTRACT FROM AUTHOR]

We present vibrationally resolved photoelectron studies of the 000, 100, 200, and 001 modes of the A state (7σ-1) of N2 O+ in the 17.4–26 eV photon-energy range. The vibrational branching ratios σ(100)/σ(000) and σ(001)/σ(000) agree very well with fluorescence measurements by Kelly et al. and qualitatively with recent theoretical predictions of Braunstein and McKoy. The large non-Franck–Condon variations in the σ(100)/σ(000) and σ(200)/σ(000) branching ratios are associated with a predicted 7σ→εσ shape resonance near 20 eV. Overall, the vibrational branching ratios imply lower resonant energies for the stretching modes (100 and 200) and a similar resonant energy for the asymmetric stretch (001), compared with the 000 mode. The vibrational asymmetry parameters (β) display a strong variation with energy which is qualitatively reproduced by theory; however, the experimental values for β(100) and β(001) exhibit additional structure around 20 eV. When combined with theory and recent fluorescence data, these results help to demonstrate a correlation of shape resonance energy with overall molecular length (RN–N+RN–O); this important result implies a resonant state which is localized on the entire triatomic molecular frame rather than on the N–N or N–O components. [ABSTRACT FROM AUTHOR]

Zero kinetic energy (ZKE) spectra of N2, CO, C2H4, and C6H6 were taken across the N1s (N2) and C1s ionization thresholds. Discrete resonances at subthreshold photon energies were observed and were found to become more intense as threshold is approached relative to the same peaks in absorption spectra. For N2 the satellite/main line branching ratios at threshold are: 11(1)% for the 419.7(1) eV binding energy satellite, and 2.3(1.0)% for the 426.5(1) eV binding energy satellite. For CO, the branching ratio for the 304.6(1) eV binding energy satellite is 15(2)% at its threshold. Branching ratios at threshold are also determined for the satellites of C6H6 and C2H4. Decay characteristics and assignments of the continuum features of C6H6 and C2H4 are also discussed. [ABSTRACT FROM AUTHOR]

The core-level photoexcitation and photoionization of SF6 were studied in the vicinity of the resonances below and above the S 2p threshold. The decay channels of the S 2p→6a1g discrete excitation were characterized, with decay leading mostly to valence-shell satellites. The S 2p continuum data show an oscillatory asymmetry parameter β(S 2p) near threshold that is virtually identical to β(Si 2p) in SiF4. It also resembles—but differs from—theoretical curves for β(S 2p) in atomic sulfur and in SF6. Data at the feature assigned as an eg shape resonance indicate strong multielectron properties for this state, because a resonance in the S 2p satellite is observed at the same photon energy as the main-line resonance. We propose a unified model which generally includes configuration interaction both in the continuum-state manifold and between discrete doubly excited states and the continua, to explain this unexpected satellite behavior. Finally, the S(L2,3VV) Auger electron asymmetry parameter shows no significant deviation from zero near the t2g and eg shape resonances. [ABSTRACT FROM AUTHOR]

Autoionizing Rydberg series in the valence-shell spectrum of CO have been studied by determining the high resolution relative photoionization cross section of cooled CO in the energy region 14.0–20.0 eV and by determining the vibrational branching ratios and the photoelectron angular distributions for production of CO+ X 2Σ+, v+=0–2 in the energy region 16.75–18.75 eV. Of particular interest are three prominent spectral features between 17.0 and 17.5 eV that result from interactions involving Rydberg series converging to the excited A 2Π and B 2Σ+ states of the ion. The results are discussed in the context of recent two-step multichannel quantum defect theory calculations by Leyh and Raseev (the following paper). [ABSTRACT FROM AUTHOR]

A discussion is presented on the general nature of shape resonances in small molecules and how they may relate to molecular bond lengths. Criteria for assigning photoabsorption features as shape resonances are described, and the usefulness of photoemission experiments to such assignments is highlighted. Based on these criteria, all unambiguously identified K-shell shape-resonance features in molecules containing B, C, N, O, and F are examined in an attempt to identify empirically a shape-resonance energy/bond length relationship. Although the available data are insufficient to establish a quantitative correlation, they indicate the influence of other variables besides the bond length to the energy position of a shape resonance. [ABSTRACT FROM AUTHOR]

The relationship between shape-resonance energies and bond lengths within agiven molecule is explored further.It is contended that some shape-resonance assignments presented by Stohr and Hitchcock (ref.1) are not convincing.(AIP) [ABSTRACT FROM AUTHOR]