Electronic Excited States of Adsorbed Molecules

当一个分子被吸附在金属表面时，在费米能级（E_F）附近会形成新的电子态。在表面化学反应中充当前沿轨道的吸附诱导电子态的识别仍然是一个实验挑战。双光子光电发射光谱（2PPE）用于探测通常未占据的电子态，但具有同时探测已占据和未占据电子态的能力。本文用2PPE光谱法观察了苯在Cu（111）上吸附引起的成键电子态。在E_F为0.3 eV时，发现了吸附诱导的键合状态。传统单光子发光弱的键合态的光发射，通过与像势态的共振而大大增强。对光子能量为5.45 eV的苯吸附Cu（111）表面的角度分辨光发射功表明，由于苯π ^*和铜的杂化引起的吸附诱导态沿表面表现出电子动量的色散。有效电子质量比m^*/m_C=0.9的抛物线可以很好地再现色散。这表明，键合状态从被吸附的分子中扩展了许多铜晶格间距。成键轨道被认为是金属表面分子间远距离相互作用的起源。

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.