Electronic states of the C6H6/Cu{111} system:: Energetics, femtosecond dynamics, and adsorption morphology

被引:83
作者
Velic, D [1 ]
Hotzel, A [1 ]
Wolf, M [1 ]
Ertl, G [1 ]
机构
[1] Max Planck Gesell, Fritz Haber Inst, D-14195 Berlin, Germany
关键词
D O I
10.1063/1.477468
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Two-photon-photoemission (2PPE) spectroscopy is employed to characterize electronic states of a bilayer C6H6/Cu{111} system at 85 K. The unoccupied benzene pi* e(2u), state is observed with a binding energy of 4.6 eV above the Fermi level. This result agrees with inverse-photoemission (IPE) data and provides a case where the determination of the binding energy is identical for 2PPE and IPE. The pi* e(2u), state is assigned in the 2PPE scheme as a final state which is the first observed final state in 2PPE of adsorbate-surface systems. The dependence of the electron dynamics on the morphology of an incomplete adsorption layer is also investigated. Two (n = 1)-like image potential states A and B are observed which presumably originate from,two different C6H6 adsorption geometries in the bilayer regime. The two image states A and are characterized by electron effective masses of 1.1 and 1.9 m(e), binding energies of 3.30 and 3.45 eV above the Fermi level, and lifetimes of 40 and 20 fs, respectively. The dielectric continuum model and the Kronig-Penney model are employed to simulate the origin of (n=1)-like image states. The work function decreases from 4.9 eV at clean Cu{111} to 4.0 eV at bilayer coverage. The change of the work function and the observation of two image states suggest the redefining of the ratio of the numbers of benzene molecules in the first and the second layers of the bilayer regime to approximately 1:1 instead of 1:2, as previously reported. 2PPE is shown to be sensitive to the changes of morphologies, local work functions, and adsorbate-surface potentials during the layer formation. (C) 1998 American Institute of Physics. [S0021-9606(98)70344-7].
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页码:9155 / 9165
页数:11
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