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K-shell excitation of molecules by fast electron impact Wight, Gordon Robert

Abstract

Energy loss spectra of 2.5 keV electrons, scattered by molecular targets through small angles, have been studied in the regions of the respective carbon, nitrogen, oxygen and fluorine K-edges and the sulfur LII,III edges. Electron energy loss spectra for diatomic, triatomic and polyatomic molecules have been studied. Discrete excitations have been interpreted in terms of the promotion of the respective K-shell electron to unfilled valence molecular orbitals and Rydberg orbitals. Most spectra show considerable structure above the respective K-edge, in addition to the normal K-continuum. This structure represents the simultaneous transitions of a K-shell and valence shell electrons (i.e. shake-up and shake-off events following the creation of an inner hole). In the case of molecular nitrogen and carbon monoxide, a simple core model was shown to provide an accurate description for the K-shell excited molecule. On the basis of this model, excitation and ionization energies for some exotic chemical species have been predicted from the relative energies observed in the K-shell energy loss spectra of a number of molecules. The agreement between the estimated (core analogy) and observed K-shell excitation energies for larger molecules is less satisfactory, possibly because of the large changes in molecular geometry which occur as a result of an election promotion. Finally, the carbon K-shell energy loss spectra of carbon disulfide, carbonyl sulfide and carbon tetraf1uoride show features which are possibly associated with the existence of an effective potential barrier in these molecules.

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