COLLISION ENERGY-RESOLVED PENNING IONIZATION ELECTRON-SPECTRA OF UNSATURATED-HYDROCARBONS WITH HE-ASTERISK(2(3)S) METASTABLE ATOMS

Collision energy dependencies of the partial ionization cross sections of ethylene, propene, butadiene, and benzene upon collision with He* (2 3S) metastable atoms were measured. Band intensities of the pi-bands decrease with the increase of the collision energy. This tendency indicates that interaction potentials are attractive when He* (2 3S) approaches the pi-orbitals of unsaturated hydrocarbons. The intensity of a strong extra band at an electron energy of 3.7 eV for benzene shows a similar collision-energy dependence with those of pi-bands. This extra band is assigned to the ionization from the pi(1e1g) orbitals associated with the excitation from the pi(1e1g) orbitals to the pi*(1e2u) orbitals. The reason why the extra band of the Penning ionization electron spectrum of benzene is more enhanced than that of the ultraviolet photoelectron spectrum can be ascribed to the existence of the He* (2 3S) atom at the time the ionization occurs. The shapes of the sigma and pi-bands change with the collision energy, which reflect the nature of the interaction potential.