ELECTRONIC EXCITATION OF GASEOUS PYRROLE AND PYRAZOLE BY INNER-SHELL ELECTRON-ENERGY-LOSS SPECTROSCOPY

The spectroscopy of the unoccupied molecular orbitals of gaseous pyrro le and pyrazole has been studied by electron impact under electric dip ole scattering conditions (2 KeV, small angle) and the inner-shell ele ctron energy loss method. The core-excitation spectra have been record ed at the C1s and N1s edges with 0.2 eV resolution, resolution allowin g to observe separately the transitions from the carbon atoms with dif ferent chemical environment. Ab initio calculations, using the equival ent core model, were performed to help in the interpretation of the sp ectral features. The spectra are dominated by transitions to the pi a nd sigma molecular orbitals. The calculations confirm that the 3b(1)( pi) orbital is the lowest-energy unoccupied one in pyrrole. In severa l cases, the intensity of the Rydberg excitations is enhanced by an im portant valence sigma character of the Rydberg orbitals. (C) 1998 Ame rican Institute of Physics. [S0021-9606(98)01237-9].