Electronic Excited States of Adsorbed Molecules

当分子吸附在金属表面时，在费米能级（E_F）附近会形成新的电子态。吸附诱导的电子态，在表面的化学反应中作为前线轨道的识别，仍然是一个实验上的挑战。双光子光电发射（2 PPE）光谱用于探测通常未占据的电子态，但具有同时探测占据和未占据态的能力。本文用2 PPE谱观察到苯在Cu（111）表面吸附诱导的成键电子态。在E_F以下0.3eV处发现吸附诱导成键态。从键合态的光发射，这是在传统的单光子光发射弱，大大增强了与图像的潜在状态的共振。利用光子能量为5.45 eV的苯吸附Cu（111）表面的角分辨光电子能谱工作揭示了由于苯π ^* 和铜的杂化导致的吸附诱导态显示出电子动量沿着表面的色散。有效电子质量比m^*/m_C=0.9的抛物线很好地再现了色散。这表明键合状态从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上分子间长程相互作用的起源。

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.