Electronic Excited States of Adsorbed Molecules

当分子吸附在金属表面时，在费米能级(E_F)附近形成新的电子态。识别吸附诱导的电子态，作为表面化学反应的前线轨道，仍然是一个实验挑战。双光子光电子发射(2PPE)光谱用于探测通常未被占据的电子态，但同时具有探测被占据态和未被占据态的能力。本文用2PPE谱观察到苯在铜(111)面上吸附引起的成键电子态。在0.3 eV的电子轰击E_F处发现了吸附诱导的成键态，与像势态的共振大大增强了成键态的光电子能谱，这在传统的单光子光电子能谱中是很弱的。对光子能量为5.45 eV的苯吸附的铜(111)表面的角分辨光电子能谱研究表明，苯与铜的杂化引起的吸附诱导态沿表面呈现电子动量弥散。有效电子质量比m^*/mC=0.9的抛物线很好地再现了色散。这表明成键状态从被吸附的分子延伸了许多铜晶格间距。成键轨道被认为是金属表面分子长程相互作用的根源。

When a molecule is adsorbed on a metal surface, new electronic states are formed in the vicinity of the Fermi level (E_F). Identification of the adsorption-induced electronic states, which act as frontier orbitals in chemical reactions at surfaces, is still an experimental challenge. Two-photon photoemission (2PPE) spectroscopy is used to probe normally unoccupied electronic states, but has a capability to probe the occupied and unoccupied states at the same time. Here a bonding electronic state induced by benzene adsorption on Cu (111) has been observed by means of 2PPE spectroscopy. The adsorption-induced bonding state is found at 0.3 eV blow E_F. Photoemission from the bonding state, which is weak in conventional one photon photoemission, is greatly enhanced by resonance with the image potential state. An angle resolved photoemission work for a benzene-adsorbed Cu (111) surface with photon energy of 5.45 eV has revealed that an adsorption-induced state due to hybridization of benzene pi^* and copper shows dispersion with electron momentum along the surface. The dispersion is well reproduced by a parabola with an effective electron mass ratio m^*/m_C=0.9. This suggests that the bonding state extends many copper lattice spacing from the adsorbed molecule. The bonding orbital is thought to be the origin of long range interactions of molecules on a metal surface.