Vondrak Tomas, Plane John M C, Meech Stephen R
School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK.
J Chem Phys. 2006 Dec 14;125(22):224702. doi: 10.1063/1.2397675.
Photoemission from an ice film deposited on Cu(111) as a function of thickness has been observed in the presence and absence of sodium atoms at the surface-vacuum interface. For either adsorbate alone and photon energies below 4 eV, two-photon photoemission from the Cu(111) substrate dominates. The Cu(111) photoelectron spectrum is perturbed by low coverages of Na, and its intensity is strongly attenuated by a few monolayers of ice. For a low density amorphous ice film, strong charging effects are observed. For ice films annealed to yield either the dense amorphous or crystalline phase, this effect is absent. Deposition of only 0.02 monolayer of Na leads to a dramatic decrease in the threshold for photoemission to 2.3+/-0.2 eV. Thus, photoelectrons are generated by visible radiation in a one-photon process with a cross section that exceeds 10(-18) cm(2). The initial state for the photoemission is identified as a metastable surface trapped electron, which decays thermally with an activation energy of 10+/-2 kJ mol(-1). Quantum calculations are described which support this model and show that the Na atom is accommodated in the first layer of the ice surface.
在表面 - 真空界面存在和不存在钠原子的情况下，观测了沉积在Cu(111)上的冰膜的光发射随厚度的变化。对于单独的任何一种吸附质以及低于4 eV的光子能量，来自Cu(111)衬底的双光子光发射占主导。Cu(111)光电子能谱会受到低覆盖度钠的扰动，并且其强度会被几层冰强烈衰减。对于低密度非晶冰膜，观察到强烈的充电效应。对于退火以产生致密非晶相或结晶相的冰膜，这种效应不存在。仅沉积0.02单层的钠会导致光发射阈值急剧降低至2.3±0.2 eV。因此，光电子是通过单光子过程由可见光辐射产生的，其截面超过10^(-18) cm²。光发射的初始状态被确定为一个亚稳态表面俘获电子，它以10±2 kJ mol⁻¹的活化能进行热衰减。描述了支持该模型的量子计算，并表明钠原子位于冰表面的第一层。
