Two-photon photoemission spectroscopy study of 1,3-butadiene on Cu(111): electronic structures and excitation mechanism

The characteristics of 1,3-butadiene (C(4)H(6)) adsorbed on Cu(111) were investigated with temperature-programmed desorption (TPD) and two-photon photoemission spectroscopy (2PPE). Dosed at 90 K, the work function drops by 0.4 eV and TPD provides no evidence for dissociation, but there are four coverage-dependent local maxima located at 195, 135, 121, and 115 K. From the 2PPE spectra, three unoccupied electronic states of the C(4)H(6)-Cu(111) system were identified: the LUMO (pi(1)*, 2a(u)), LUMO + 1 (pi(2)*, 2b(g)), and LUMO + 2 (sigma*, 7b(u)), lying 1.3, 3.4, and 4.8 eV above the Fermi level, respectively. Although the excitation mechanisms for the LUMO and LUMO + 1 are substrate mediated, the excitation of the LUMO + 2 is attributed to intramolecular excitation.