The stimulation of surface reactions by light is a basic chemical physics phenomenon that is important for a large number of real-world applications and processes. The review here focuses exclusively on its most fundamental aspects by emphasizing reaction dynamics. It also limits the discussion to substrate crystals with a finite band gap, that is, a semiconductor or metal oxide. Such a finite band gap inhibits or modifies the flow of any photogenerated carriers across the adsorbate/ crystal interface. Finally, the adsorbate systems discussed in this review are limited to the case of relatively weakly adsorbed molecules; this assumption allows one to compare the photoreaction event, at least in the first approximation, to related processes for the isolated molecule. This approach to understanding surface photodynamics allows o ne to see clearly that a surface alters the isolated-molecule chemical response in several major and distinct ways. First, the surface may "align" an adsorbate molecule and, thus, direct the trajectory of photofragments into specific spatial and angular coordinates. In practice, this process may inhibit hot-fragment reactions on the surface simply by directing fragments away from other adsorbate molecules. More subtle effects are possible; for example, a fixed orientation in conjunction with a polarized laser beam may allow only one specific molecular transition to occur due to the directional nature of molecule transition dipole moments. Second, the surface may provide a source of low-energy photoelectrons that can be captured by adsorbate molecules after tunneling through or over (by photoemission) the substrate surface barrier. In practice, this process can greatly enhance (or even allow) a reaction rate on the surface over its value in the gas or liquid phase. Third and conversely, a nearby surface can quench a photoreaction if resonant charge transfer or simply charge transfer is energetically allowed for the photoexcited adsorbate molecule or negative ion, respectively. Quenching is most clearly seen in the case of direct UV excitation to a repulsive state in the case of molecules directly adsorbed on the surface of a metallic or small-band-gap substrate. The results given above show that dissociative electron attachment is also quenched, but not completely so. Fourth, a surface can also modify the local optical electric field due to the crystal dielectric response. In some cases, such as metallic nanospheres, this can lead to optical field enhancement, whereas in others, such as a flat surface, destructive interference occurs. Both enhancement and reduction affect the rate of any optically controlled photoreactions. What are emerging areas for ph otodynamics studies, or in other words, what still needs to be learned? One important area is to obtain an atomic-level understanding of the interaction of energetic or hot photofragments with a bare surface. Recent work using STM measurements (see the description in section 4.8) shows a very powerful experimental path to reaching this goal. A second emerging area is the investigation of nonadiabatic effects involving highly internally excited photofragments. For example, recent work by Wodtke, Auerbach, and co-workers on the interaction of highly vibrationally excited diatomics with metal surfaces has indicated that nonadiabatic effects at surfaces can be extremely important. Finally, a third direction or challenge is the utilization of a specific photodynamics effects or phenomenon to accomplish a new surface modification procedure. For example, it has been shown that surface photoreactions involving CH3Br result in selective bromination of certain group III-V semiconductor surface or group IV surface sites. Such a selective reaction can be employed to alter the surface reconstruction or passivate surface sites. An exciting challenge is then to determine if other photoreaction processes achieve other even more subtle or powerful effects. © 2006 American Chemical Society.
