Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
J Chem Phys. 2012 Sep 7;137(9):091704. doi: 10.1063/1.4746801.
In this essay we discuss the light-matter interactions at molecule-covered metal surfaces that initiate surface photochemistry. The hot-electron mechanism for surface photochemistry, whereby the absorption of light by a metal surface creates an electron-hole pair, and the hot electron scatters through an unoccupied resonance of adsorbate to initiate nuclear dynamics leading to photochemistry, has become widely accepted. Yet, ultrafast spectroscopic measurements of molecule-surface electronic structure and photoexcitation dynamics provide scant support for the hot electron mechanism. Instead, in most cases the adsorbate resonances are excited through photoinduced substrate-to-adsorbate charge transfer. Based on recent studies of the role of coherence in adsorbate photoexcitation, as measured by the optical phase and momentum resolved two-photon photoemission measurements, we examine critically the hot electron mechanism, and propose an alternative description based on direct charge transfer of electrons from the substrate to adsorbate. The advantage of this more quantum mechanically rigorous description is that it informs how material properties of the substrate and adsorbate, as well as their interaction, influence the frequency dependent probability of photoexcitation and ultimately how light can be used to probe and control surface femtochemistry.
在本文中，我们讨论了引发表面光化学反应的分子覆盖金属表面的光物质相互作用。表面光化学的热电子机制已被广泛接受，即在金属表面吸收光会产生电子-空穴对，然后热电子通过吸附物的未占据共振散射，引发导致光化学的核动力学。然而，对分子-表面电子结构和光激发动力学的超快光谱测量几乎没有为热电子机制提供支持。相反，在大多数情况下，通过光诱导的衬底到吸附物的电荷转移来激发吸附物共振。基于最近对相干在吸附物光激发中的作用的研究，正如光学相位和动量分辨双光子光电子发射测量所测量的那样，我们批判性地检查了热电子机制，并提出了一种基于电子从衬底到吸附物的直接电荷转移的替代描述。这种更严格的量子力学描述的优点是，它告知了衬底和吸附物的材料特性以及它们的相互作用如何影响光激发的频率相关概率，以及光如何用于探测和控制表面飞秒化学。
