Paul R. Keller

Angle‐resolved photoelectron spectroscopy of CCl4: The Cooper minimum in molecules

Angle‐resolved photoelectron spectra have been taken for the first five orbitals in CCl4 (2t1, 7t2, 2e, 6t2, and 6a1) using a variable photon source extracted from synchrotron radiation. From these data, the partial cross sections and angular distribution parameters β have been determined for photon energies from 16 to 80 eV. In the case of the lone pair orbitals (2t1, 7t2, and 2e) minima in both the cross sections and β values were noted experimentally in the photon energy range of 40 to 50 eV. The behavior is similar in appearance to that observed with the 3p subshell of argon and is believed to be the molecular counterpart of the Cooper minimum in atoms. Calculations of the cross sections and β values have been made for each of the different orbitals of CCl4 measured experimentally using the multiple scattering Xα theory. Agreement between theory and experiment for the lone‐pair orbitals is qualitatively good and in the case of photoionization in the 2t1 orbital it has been possible to identify which c...

Angle‐resolved photoelectron spectroscopy of CO2 with synchrotron radiation

The angular asymmetry parameter β has been determined for the first four bands in the photoelectron spectrum of CO2 as a function of photon energy: X 2Πg(15–50 eV), A 2Πu(19–28 eV), B 2Σu+(19–28 eV), and C 2Σg+(21–67 eV). Use is made of a monochromatized beam of photons from the Wisconsin Synchrotron Radiation Center. The experimental results are compared with theoretical calculations employing the multiple scattering method and the overall agreement is good. In addition, the theoretical predictions for the effect on β of shape resonances have been verified except that the experimental widths are broader. This broadening has been partially accounted for in recent calculations of Swanson et al. by including the effects of vibrational motion. Finally, sharp changes in β as a function of vibrational level have been noted for the second and fourth electronic bands. Such behavior has been found for a variety of photon energies, and the results are examined against prevailing theory.

Angle resolved photoelectron spectroscopy of the valence shells in HI and CH3I as a function of photon energy from 13 to 90 eV

Angle‐resolved photoelectron spectroscopy was carried out on the valence shells of HI and CH3I over a photon energy range of 13–90 eV with the use of synchrotron radiation. From these data, partial cross sections and angular distribution parameters were obtained for bands corresponding to the 1 2Π and 1 2Σ+ states of singly ionized HI and the 1 2E, 1 2A and 2 2E states of singly ionized CH3I. Calculations were carried out on HI for the same parameters with the use of the multiple scattering Xα method. The results, both experimental and theoretical, are examined in terms of the Cooper minimum. Clear evidence for minima in both the cross sections and β values are found for the lone‐pair orbitals. Bonding orbitals involving the iodine 5p subshell also show minima in the energy dependence plot of the β values, but the effects are less pronounced. These results are compared with similar studies on chlorine‐containing molecules. The lone‐pair orbitals showed spin‐orbit resolved bands in the photoelectron spectr...

Angle-resolved photoelectron spectroscopy of HCl from a photon energy of 16 to 80 eV

Angle‐resolved photoelectron spectroscopy was performed on HCl using synchrotron radiation over a photon energy from 16 to 80 eV. The partial cross sections and angular distribution parameters β were obtained for photoionization of both the 2π and 5σ orbitals. Multiple scattering Xα calculations were also carried out for the cross sections and β values. The calculations, together with previously published results on the cross section using a Hartree–Fock model, were compared with experiment and gave reasonable qualitative agreement. Both experimental and theoretical results were examined with particular regard to the nature of the Cooper minimum, and the differences between the behavior of the minima for the two orbitals are discussed in detail.

Angle‐resolved photoelectron spectroscopy of N2O measured as a function of photon energy from 14 to 70 eV

Angle‐resolved photoelectron spectra have been taken of the first four bands of N2O, using photons from synchrotron radiation. From these data, the partial cross section and the angular distribution parameter β have been obtained over a range of photon energies from 14 to 70 eV for the first four valence orbitals: 2π, 7σ, 1π, and 6σ. Calculations, based on the multiple scattering Xα method, also have been performed to obtain the partial cross sections and β values for comparison with experiment. An important element in the understanding of these data is the presence of shape resonances particularly in the photoionization of the 7σ and 6σ orbitals. Although the calculations are not in good quantitative agreement with experiment, they are able qualitatively to give a correct account of the overall behavior. In the case of the photoelectron spectra of the 7σ and 6σ orbitals, the vibrationally resolved bands were studied independently, particularly for their β values. Evidence of autoionization was found from...

Angle-resolved photoelectron spectroscopy of the chloro-substituted methanes☆

Abstract The angular distribution parameter, β, was determined for the valence orbitals (IP ′ 21.2 eV) of CCl4, CHCl3, CH2Cl2, and CH3Cl in the 10–30 eV photon energy range using dispersed polarized synchrotron radiation. The energy dependence of β in the photoelectron energy range of 2 to 10 eV for the non-bonding chlorine n(Cl) orbitals of these molecules was found to be similar for all n(Cl) orbitals investigated. The energy dependence of β for the σ orbitals in these molecules was similar to that observed previously for other σ orbitals. The experimental CCl4 results were compared with theoretical CCl4 results obtained using the Xα multiple scattering formalism. Theory predicts the existence of two strong shape resonances in each of the valence orbitals of CCl4. The overall agreement between experiment and theory is evaluated along with the experimental evidence concerning the verification of the predicted shape resonances.

Angle-resolved photoelectron measurements on the first two orbitals of acetylene using synchrotron radiation

Abstract The angle-resolved photoelectron spectrum of acetylene has been measured using synchrotron radiation over a range of photon energies from 12 to 28 eV.

Angle‐resolved photoelectron spectroscopy of CS2 and COS measured with synchrotron radiation

Angle‐resolved photoelectron spectra of CS2 and COS have been taken as a function of photon energy. From these data the angular asymmetry parameter β has been obtained for the five valence orbitals of CS2 and the three least tightly bound valence orbitals of COS. Monochromatized polarized photons produced from synchrotron radiation have permitted studies from near ionization threshold up to 29 eV. The experimental data were used to evaluate previous theoretical calculations which employ a multiple scattering method. The agreement for the least tightly bound orbitals of the nonpolar CS2 molecule were good. There were some problem areas, particularly with the more tightly bound orbitals of CS2 and the polar molecule COS. However, when previous results for CO2 are taken together with the results on CS2 and COS, the calculations in general give a reasonable account of the experimental β values and suggest that the multiple‐scattering method will be useful for the study of differential cross sections of polyat...

Trend of shape resonance-induced features in the angular distribution parameter as a function of photon energy for carbon, silicon and germanium tetrac

Abstract The angular distribution parameter as a function of photon energy, β( h ν), has been measured in the 13–27 e V range using synchrotron radiation for the five valence orbitais in silicon and germanium tetrachlorides. The results have been compared with those previously reported for carbon tetrachloride. In particular, the trend of shape resonanceinduced features has been followed along the valence isoelectronic series CCl 4 -SiCl 4 -GeCl 4 for the 1 e and 2 t 2 orbitals. As the atomic number of the central atom increases, the shape resonances move toward the threshold ionization energy for a particular molecular orbital.

Angle-resolved photoelectron spectroscopy of formaldehyde and methanol☆

Abstract Angle-resolved photoelectron spectroscopy was employed to obtain the angular distribution parameter, β, for the valence orbitals (IP