Carbon <i>K</i>-shell excitation of gaseous and condensed cyclic hydrocarbons: C3H6, C4H8, C5H8, C5H1, C6H1, C6H12, and C8H8

Abstract

The carbon K-shell excitation spectra of gaseous cyclic hydrocarbons, both saturated (cyclopropane, cyclobutane, cyclopentane, cyclohexane) and unsaturated (cyclopentene, cyclohexene, and cyclooctatetraene), have been recorded by electron energy loss spectroscopy under dipole-dominated conditions. These are compared to the NEXAFS spectra of multilayers and monolayers of C4H8, C5H8, C6H12, and C8H8 on Pt(111). Multiple scattering Xα calculations of the spectra of cyclopropane, cyclobutane, and cyclohexane are also reported. In most cases the gas and solid spectra are essentially the same indicating that intramolecular transitions dominate in the condensed phase. The NEXAFS polarization dependence of the condensed phases has assisted spectral assignments and the determination of the molecular orientation in the monolayer phase. In the saturated species a sharp feature about 3 eV below the carbon 1s ionization threshold is identified as a transition to a state of mixed Rydberg/valence character with the π*(CH2) valence component dominating. Except for cyclopropane the positions of the main σ * resonances correlate with the C–C bond lengths in a manner similar to that reported previously for noncyclic aliphatic molecules. In the spectra of monolayer C6H12, C5H8, and C8H8 spectral broadening and weak additional features are observed which are attributed to molecule–surface interactions.