Fine structure of the two-photon photoemission from benzene adsorbed on Cu(111)

Two-photon photoemission (2PPE) spectroscopy has been applied to benzene adsorbed on a Cu(111) surface. With laser light in the 3.7 to 4.3 eV photon energy region, 2PPE occurs through benzene-induced excited states which are about 1 eV above the Fermi level. The excited state is considered to be the lowest triplet state of adsorbed benzene. The spectrum shows a progression of sharp peaks of less than 0.2 eV width, each separated by about 0.2 eV. The widths of the peaks indicate that the lifetimes of the excited and ionic states are both longer than 3 fs.