Electronic excitation of gaseous pyrrole and pyrazole by inner-shell electron energy loss spectroscopy

The spectroscopy of the unoccupied molecular orbitals of gaseous pyrrole and pyrazole has been studied by electron impact under electric dipole scattering conditions (2 KeV, small angle) and the inner-shell electron energy loss method. The core-excitation spectra have been recorded at the $C1s$ and $N1s$ edges with 0.2 eV resolution, resolution allowing to observe separately the transitions from the carbon atoms with different chemical environment. Ab initio calculations, using the equivalent core model, were performed to help in the interpretation of the spectral features. The spectra are dominated by transitions to the $π*$ and $σ*$ molecular orbitals. The calculations confirm that the $3b1(π*)$ orbital is the lowest-energy unoccupied one in pyrrole. In several cases, the intensity of the Rydberg excitations is enhanced by an important valence $σ*$ character of the Rydberg orbitals.