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Electronic excitation of polyatomic molecules by fast electron impact Sze, Kong Hung


High resolution electron energy loss spectroscopy has been used to examine the inner-shell and valence-shell electronic excitation of a number of polyatomic molecules, including SF₆, SeF₆, TeF₆, ClF₃, C₂H₃F, C₂H₃Cl, C₂H₃Br, C₂H₃I, Ni(CO)₄, (CH₃)₂SO and SO₂. The inner-shell and valence-shell electron energy loss spectra (ISEELS and VSEELS) were measured under low momentum transfer conditions with impact energy in the range of 1-3.7 keV and 0° scattering angle. Under these conditions the spectra are dominated by electric dipole-allowed transitions. The ISEELS spectra include all accessible core excitations of these molecules below 1000 eV equivalent photon energy. A number of specific investigations have been performed in order to extend present understanding of the physical nature of electronic excitation phenomena, in particular those involving inner-shell electrons. In addition the present work illustrates new applications of ISEELS to the study of chemical phenomena. In the investigation of (Coulombic) potential barrier effects in the "cage" molecules SF₆, SeF₆, TeF₆ and ClF₃, f-type continuum shape resonances are observed for the first time in the spectra of TeF₆ and they show very different spectral behavior from the d-type continuum shape resonances observed in the spectra of SF₆ and SeF₆. Consideration of both the ISEELS and VSEELS spectra indicates that there is a weakening of the potential barrier in going through the series from SF₆ to SeF₆ to TeF₆. The Coulombic potential barrier model provides an extremely satisfactory understanding of (a) the co-existence of intense continuum shape resonances and intense Rydberg transitions plus direct ionization continuum; and (b) the number and symmetry of continuum shape resonances observed in the ISEELS spectra of ClF₃. The physical significance of Coulombic potential barrier effects is further convincingly demonstrated by a comparison of the "central atom" inner-shell spectra of SF₆, ClF₃ and HCl. In contrast to earlier work, the present comparative study of the He(I) and He(II) photoelectron spectra and the VSEELS excitation spectra of the monohaloethylenes (i.e. C₂H₃X; X = F, Cl, Br and I) suggests that the HOMO orbital in C₂H₃I is predominantly of iodine 5p[sub ⊥](out-of-plane) character rather than of π character whereas the reverse situation applies to the HOMO orbitals of C₂H₃F, C₂H₃Cl and C₂H₃Br. Based on a term value correlation analysis, substitutional effects are found to be most prominent in the σ* -type orbitals, while the π* and Rydberg orbitals are less influenced. Linear correlations between the C-X bond strength and the term values for both inner-shell and valence-shell transitions to the σ* (CX) orbital are also observed. The ISEELS and VSEELS spectra of Ni(CO)₄ are compared with the corresponding spectra in free CO. The C and O Is spectra of these two molecules show some notable similarities despite the very different manifold of final states available. In particular the inner-shell spectra of both molecules exhibit intense ls → π* and ls → σ* transitions. High resolution ISEELS has been used to obtain vibrational resolved C Is spectra of Ni(CO)₄ and free CO. The implications and possibilities of studying dπ →pπ back-bonding in transition metal carbonyl complexes by high resolution ISEELS spectroscopy are discussed. The inner-shell (S 2s, 2p, C Is) excitations of (CH₃)₂SO (DMSO) measured by ISEELS are compared with synchrotron radiation studies of the S Is photoabsorption spectrum. The pre-edge regions of these spectra are interpreted as excitations to common manifolds of virtual valence and Rydberg orbitals. A linear correlation between the S-C bond lengths and the term values of S 2p[sub 3/2] → σ* (S-C) transitions is demonstrated for DMSO and a number of other sulfur compounds. The S 2p, 2s and O Is ISEELS spectra of SO₂ as well as the S ls photoabsorption spectrum are compared with and assigned according to the results of multichannel quantum defect theory calculations. The calculated energies and oscillator strengths of spectral features in these spectral regions are generally in good quantitative agreement with the measurements.

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