Collision-energy-resolved metastable impact electron spectroscopy of thiophene and 2,2′-bithiophene: Stereodynamics in gas phase and adsorbed layer on Ag(110)

2,2′-bithiophene in the gas phase was studied by ultraviolet photoelectron spectroscopy (UPS) and two-dimensional Penning ionization electron spectroscopy (2D-PIES) to obtain the information of stereodynamics in Penning ionization and anisotropic interaction around the ionization region. Thiophene and the 2,2′-bithiophene layer on a Ag(110) surface were investigated by two-dimensional metastable impact electron spectroscopy (2D-MIES). From the relative intensity in the UPS and PIES spectra and the collision energy dependence of partial ionization cross sections (CEDPICS) for 2,2′-bithiophene, ionization bands were assigned to π orbitals. A comparison with the CEDPICS for a gas molecule and the molecular layer on Ag(110) revealed that the attractive interaction between He*(2 ³S) and both thiophene and 2,2-bithiophene was weak due to the formed molecular layer.