The Inner Shell Electron Energy Loss Spectra of Some Cyclic Organic Molecules

Abstract

<p>Inner shell electron energy loss spectroscopy has been used to obtain the core excitation spectra of some cyclic molecules. Recorded under the conditions of small momentum transfer (typically 2.5 keV impact energy and small angle scattering, <5°), the spectra are equivalent to those produced by soft x-ray photoabsorption. The molecules investigated are aromatic (benzene, pyridine, furan and pyrrole), unsaturated (cyclopentene, cyclohexene and cyclo-octatetraene) and saturated (cyclopentane, cyclohexane, tetrahydrofuran, pyrrolfdine and piperdine) species. The heterocyclic species contain a single heteroatom which is either nitrogen (pyridine, pyrrole, pyrrolidine and piperdine) or oxygen (furan and tetrahydrofuran). In all cases the spectra are dominated by shape resonance features. The carbon K-shell and heteroatom K-shell spectra are quite similar indicating the transitions are to a similar set of unoccupied orbitals.</p> <p>In certain cases, condensed phase x-ray 'photoabsorption' spectra (recorded by partial electron yield) were available for the purposes of comparision. Such comparisions are very useful in unambiguously identifing Rydberg, π* and δ* states. As a result the carbon K-shell excitation spectrum of benzene has been reassigned. A previously unidentified class of (1s -> π*(CH2)) transitions has been identified and characterized in the spectra of the saturated molecules.</p> <p>All of the spectra studied show continuum resonances which could be assigned to δ*shape resonances. The saturated cyclic systems show a second continuum resonance which is attributed to the overlap of in-ring atomic orbitals. This feature is strongest in the five membered ring systems with a heteroatom. The relationship between the shape resonance position and bond length is extended to these molecules. In the cases where there is extensive delocalization the simple relationship begins to breakdown.</p>