Surface photochemistry

Associative desorption of hydrogen or the reaction between CO and O on a Ru(0 001) initiated by intense IR fs laser pulses, as outlined at the end of Chapter 3, are reactions caused by heating up the electron gas of the metal. The repetitive transition between ground-state and excited potentials in the DIMET process is responsible for the occurrence of the reaction that can still be considered as thermal since the electron gas equilibrates rapidly to an electron temperature and the process is still dominated by the ground-state potential. It is characteristic of this mechanism that the yield increases stronger than linear with the photon flux, reflecting its multiple excitation mechanism. 

Irradiation of adsorbate-covered surfaces with higher energy photons (typically up to 6.4 eV) with lower intensities opens the possibility of direct valence excitation. Since the lifetimes of electronic excitations at metal surfaces are much shorter than those for nuclear motion, photochemical reactions appear rather improbable. Surprisingly, however, the cross sections determined for photodesorption were found to be comparable to those found for reactions with free molecules, mainly because the short lifetime of the excited state is compensated by a much larger cross section for absorption of the light [32,62-64]. This process takes place in the near-surface region of the metal (within about 10 nm), where relaxation of the photoexcited electrons leads to rapid establishment of a transient energy distribution. As depicted in Fig. 4.11, these hot electrons may scatter at the surface or are resonantly attached to an empty level of the adsorbate. 

FIGURE 4.11. Energy diagram for an adsorbate covered metal surface under the influence of light absorption. 

This implies a transition from the ground-state potential to that of an excited state. If the latter is repulsive, the nuclear motion becomes accelerated and may take up sufficient kinetic energy so that the particle can leave the surface after return to the ground state. This picture is equivalent to the MGR model of DIET, as discussed in Section 4.4. 

FIGURE 4.12. Photodesorption of ammonia from a Cu(l 11) surface. Model potential energy surfaces for the ground state and the excited state, respectively, as a function of the coordinates x and z. The lower part shows a contour plot of the ground-state PES and the solid line displays a typical desorption event on a calculated trajectory that had spent 9 fs on the excited-state PES [65]. 

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Surface photochemistry can drive a surface chemical reaction in the presence of laser irradiation that would not otherwise occur. The types of excitations that initiate surface photochemistry can be roughly divided into those that occur due to direct excitations of the adsorbates and those that are mediated by the substrate. In a direct excitation, the adsorbed molecules are excited by the laser light, and will directly convert into products, much as they would in the gas phase. In substrate-mediated processes, however, the laser light acts to excite electrons from the substrate, which are often referred to as hot electrons . These hot electrons then interact with the adsorbates to initiate a chemical reaction. 

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Surface photochemistry Monitoring the reflectivity of high-quality crystals (defects minimized) allows one to follow in time slow photoreactions, especially autocatalytic photoreactions.120... 

FrigoSP (1994) The physical aspects of halosilane soft X-ray surface photochemistry. Ph.D. thesis (unpublished), University of Wisconsin, Madison... 

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