Recent progress in ESCA studies of gases

Correlation Effects in the Ionization of Molecules: Breakdown of the Molecular Orbital Picture

The quadratic and the most important cubic force constants of benzene have been determined from ab initio Hartree–Fock calculations with a double‐zeta basis set. Some constants have also been recalculated using other basis sets, including a polarized one. A few empirical scale factors, applied to the ab initio force field, allow the reproduction of a large number of observed vibrational frequencies, isotope shifts, and Coriolis constants within the uncertainties of experiment and the harmonic model. It is shown that the simultaneous utilization of ab initio and spectroscopical information is sufficient for the conclusive resolution of the uncertainties and alternatives in previous empirical force fields. The resulting scale factors can be used directly to obtain force fields for other aromatic hydrocarbons from ab initio calculations. Reproduction of the observed infrared intensities is only moderately successful, even with the polarized basis set. The calculated vibronic coupling constants show qualitative agreement but important deviations from previous simpler calculations. The predicted vibrational patterns confirm Lindholm’s assignment of the photoelectron spectrum of benzene.

Force field, dipole moment derivatives, and vibronic constants of benzene from a combination of experimental and ab initio quantum chemical information

Synchrotron-based high-resolution photoemission and X-ray absorption near-edge spectroscopy (XANES) have been used to study the interaction of NO2 with polycrystalline surfaces of metallic zinc and zinc oxide. NO2 exhibits a complex chemistry on metallic zinc. After adsorbing nitrogen dioxide, N, O, NO, NO2, and NO3 are present on the surface of the metal. At room temperature the NO2 molecule mainly dissociates into O adatoms and gaseous NO, whereas at low temperatures (<250 K) chemisorbed NO2 and NO3 dominate on the surface. NO2 is a very good oxidizing agent for preparing ZnO from metallic zinc. Zn reacts more vigorously with NO2 than metals, such as Rh, Pd, or Pt which are typical catalysts for the removal of NOx molecules (DeNOx process). At 300 K, the main product of the reaction of NO2 with polycrystalline ZnO is adsorbed NO3 with little NO2 or NO present on the surface of the oxide. No evidence was found for the full decomposition of the NO2 molecule (i.e., no NO2 → N + 2O). The results of density ...

Reaction of NO2 with Zn and ZnO: Photoemission, XANES, and Density Functional Studies on the Formation of NO3

Experimental valence ionisation energies of 143 hydrocarbons, CnHm, have been determined from their He (IIα) (40.80 eV) and He (Iα) (21.22 eV) excited photoelectron spectra. Ionization energies, usually up to 26 eV are given in tabular form for 108 hydrocarbons with n≤6, together with their (tentative) assignment. The ionization energies up to approximately 25 eV, of 35 selected hydrocarbons with 7≤n≤10, are presented by means of their He (IIα) photoelectron spectra.

Valence Ionization Energies of Hydrocarbons

The ionization potentials of NO2 up to 28 eV have been measured using He i and He ii high‐resolution photoelectron spectroscopy. The assignment of the ionized levels below 16 eV is straightforward, including the singlet–triplet splittings, and allows us to assign Rydberg series in the vacuum uv leading to the first five ionization potentials. Above 16 eV the spectrum is complicated by the absence of obvious singlet–triplet splittings. The assignments we propose fit in with a coherent scheme for the molecular‐orbital energy levels of 16‐, 17‐, and 18‐electron triatomic molecules. The SCF calculations have been performed on several states of NO2 and NO2+ (both open shell and closed shell) and the results are in quite good agreement with the experimental ionization potentials.

Electronic Structure of NO2 Studied by Photoelectron and Vacuum‐uv Spectroscopy and Gaussian Orbital Calculations

Ionization energies have been obtained for all the valence orbitals of some simple hydrocarbons and also some of their isoelectronic analogues in which a nitrogen or an oxygen atom replaces CH or CH2 respectively. This required the use of a helium discharge run under conditions such that a high proportion of 30.4 nm radiation was emitted. In order to utilize the full range of the 41 eV photon it was further necessary to use filters, usually polystyrene films about 100 nm thick, selectively to absorb 58.4 nm radiation.Analysis of the results show that in many molecules the summed binding energies of the molecular orbitals built from 2s atomic orbitals may be divided up into atomic contributions which agree in magnitude with those obtained for the same atoms in other molecules, i.e., the atomic contributions are additive as is expected from the application of simple theory. A similar additive behaviour was also found for the orbitals built from 2p and 1s(H) atomic orbitals though theory does not directly indicate this for incompletely filled orbital systems of this type.Certain weak photoelectron bands have been found at high energies which arise from transitions to configurationally mixed ionized states. The experimental results are compared with theoretical predictions.

Ultra-violet photoelectron data on the complete valence shells of molecules recorded using filtered 30.4 nm radiation

Photoelectron spectra of all the fluorobenzenes (except 1,2,3-trifluorobenzene) have been obtained with photon energies up to 40.8 eV. The changes in spectral features arising from the progressive substitution of fluorine, as well as those associated with the various stereoisomers of these molecules, illustrate clearly many important features of the benzene orbitals and assist in their correct assignment. The nature of the Jahn–Teller effect in states arising from the ionization of the doubly degenerate π and σ orbitals is shown to be markedly different, and this in turn differs from the effect of substitutional splitting in these degenerate orbitals.The analysis leads to the following reassignment of adiabatic ionization potentials for C6H6; (1a2u)–1, 11.490 eV; (3e2g)–1, 11.570 eV; (3a1g)–1, 15.446 eV and (2b1u)–1, 16.848 eV. Other changes relative to previous band assignments arise from the recognition of large electronic Jahn–Teller splittings in the σ systems. In the π systems, because of their much weaker binding properties, the correspondingly weak Jahn–Teller forces are found to be just strong enough to cause excitation of e-type vibrations without giving rise to any electronic splitting. A study of the isotopic blue shifts of the band origins observed for benzene-d6 reflects the bonding character and hydrogen dependence of the orbitals and supports the analysis. Selective enhancement of (3e2g)–1 relative to (1a2u)–1 in the C6D6 spectrum obtained with Ne I radiation was used to identify these states.

Photoelectron spectra of benzene and some fluorobenzenes

The 30.4 nm spectra of some saturated hydrocarbons are reported and it is shown that in most cases the energy spacing of the inner valence s type orbitals of these molecules can be accurately described in terms of simple Huckel expressions. The results of the analysis are used to comment on the “anomalous nature” of s type orbitals in the lower members of the cyclic paraffins. Vibrational analysis of the outermost s type bands of the methylmethanes is used as a basis for discussion of these orbitals and the nature of the C 2s orbital unit.

Photoelectron spectra of inner valence shells. Part 1.—Saturated hydrocarbons

The electronic structures of C6H6‐nFn, 1⩽n⩽6, C2H4, and C2F4 are evaluated by extending the new ’’spectroscopic’’ self‐consistent‐field CNDO/S2 molecular orbital model to include a parameterization of fluorine. As in our earlier studies of conjugated hydrocarbons, the CNDO/S2 model provides an adequate description of photoelectron spectra associated with states of ionization potential I≲13 eV. The intensities of vibrational progressions evident in the photoelectron spectra are analyzed to extract the linear coupling constants of electrons in the various molecular orbitals to the totally symmetric (a1g) molecular vibrations and, when allowed by symmetry, to e2g molecular vibrations. These coupling constants differ substantially from those obtained for benzene and deuterobenzene. Their values reflect the consequences of the heavy fluorine mass and the incompressibility of the charge density on the fluorine substituent. They lead, moreover, to explicit predictions of increased insulating behavior for fluorob...

A. W. Potts

Papers

Electronic Structure of NO2 Studied by Photoelectron and Vacuum‐uv Spectroscopy and Gaussian Orbital Calculations

Ultra-violet photoelectron data on the complete valence shells of molecules recorded using filtered 30.4 nm radiation

Photoelectron spectra of benzene and some fluorobenzenes

Photoelectron spectra of inner valence shells. Part 1.—Saturated hydrocarbons

Electronic structure of the fluorobenzenes, ethylene, and tetrafluoroethylene