Excitation substrate-mediated

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. 

The theoretical description of photochemistry is historically based on the diabatic representation, where the diabatic models have been given the generic label desorption induced by electronic transitions (DIET) [91]. Such theories were originally developed by Menzel, Gomer and Redhead (MGR) [92,93] for repulsive excited states and later generalized to attractive excited states by Antoniewicz [94]. There are many mechanisms by which photons can induce photochemistry/desorption direct optical excitation of the adsorbate, direct optical excitation of the metal-adsorbate complex (i.e., via a charge-transfer band) or indirectly via substrate mediated excitation (e-h pairs). The differences in these mechanisms lie principally in how localized the relevant electron and hole created by the light are on the adsorbate. 

Figure 3.8 illustrates the photon-induced excitation of NO chemisorbed on Pt(lll) based on the scenario of substrate mediated excitation by hot electrons (see Section 4.8.1.1). Hot electrons (and hot holes) are created in the optical skin depth... 

When desorption takes place from a metal surface, many hot charge carriers are generated in the substrate by laser irradiation and are extended over the substrate. Then, the desorption occurs through substrate-mediated excitation. In the case of semiconductor surfaces, the excitation occurs in the substrate because of the narrow band gap. However, the desorption is caused by a local excitation, since the chemisorption bond is made of a localized electron of a substrate surface atom. When the substrate is an oxide, on the other hand, little or no substrate electronic-excitation occurs due to the wide band gap and the excitation relevant to the desorption is local. Thus, the desorption mechanism for adsorbed molecules is quite different at metal and oxide surfaces. Furthermore, the multi-dimensional potential energy surface (PES) of the electronic excited state in the adsorbed system has been obtained theoretically on oxide surfaces [19, 20] due to a localized system, but has scarcely been calculated on metal surfaces [21, 22] because of the delocalized and extended nature of the system. We describe desorption processes undergoing a single excitation for NO and CO desorption from both metal and oxide surfaces. 

Little is known about the photochemistry of 4-iodoaniline. It is thought to undergo homolytic C—I bond cleavage to yield a phenylamine radical and an I atom220. This has been observed when 4-iodoaniline is adsorbed on the GaN(0001)-(l x 1) surface. The photodissociation of the C—I bond is more likely to proceed by direct excitation of the molecule rather than through a substrate mediated process involving transfer of excited carriers between adsorbate and substrate220. 

One of the key aspects concerning the excitation mechanism is which electronic transitions couple to the coherent nudear motions. As for the surface adsorbate excitations, there are two extreme cases for the electronic transition which leads to surface dynamics. One is the adsorbate localized excitation and the other is the substrate-mediated excitation. In many cases, investigating the reaction yield by changing the characters of inddent photons (polarization, energy, etc.) helps to confirm which mechanism operates. If a substrate-mediated process dominates, the reaction yield follows the features of bulk absorption, whereas a deviation from the bulk absorption property would be observed for the surface localized excitations. 

A more direct evidence of the surface localized excitation mechanism has been obtained by a polarization dependence study. For K/Pt(lll) at 0.36 ML, it has been demonstrated that the coherent excitation of the K—Pt stretching mode occurs with p-polarized excitation and not with s-polarized exdtation. Since the s-polarization absorptance is about one fourth of that with p-polarization under the experimental conditions (2.19 eV photon energy, 70° angle of inddence), the coherent amplitude should be detectable with s-polarization if the substrate-mediated process operates. Therefore, the negligible oscillatory component with s-polarization is inconsistent with the substrate-mediated excitation model and it is indicated that some electronic transitions involving K-induced surface states are responsible for the coherent excitations. 

Furthermore, the results support that NO photodesorption and decay of a dimer itself should be competing channels because the introduced resonant orbital is the only one for a dimer which has anti-bonding character for both N—O and NO—NO. This is consistent with the experimental results that N20 desorption is also observed by substrate-mediated excitation, and its action spectrum exactly mirrors that for NO photodesorption. Again we emphasize our results and analyses are obtained by ah iniio calculations within the model of NEGF-DFT focusing on hot electron transport and a charge injection process. This is an example to show the importance and... 

The above described experiments provide a rather clear proof of the importance of the substrate mediated hot/free electron attachment mechanism. Now, with three mechanisms thought to be operative (direct, complex and substrate excitation) the problem... 

The photodissociation of an adsorbed molecule may occur directly or indirectly. Direct absorption of a photon of sufficient energy results in a Franck-Condon transition from the ground state to an electronically excited repulsive or predissociative state. Indirect photodissociation of adsorbates, involving absorption of photons by the substrate, can take place via two processes. The first one is analogous to the process of sensitized photolysis in gases. The second one, also substrate mediated, implies the phototransfer of an electron from the substrate to an antibonding orbital of the adsorbate, i.e. charge transfer photodissociation. The basic principles of these two excitation mechanisms will be discussed later in this part. 

Another example of a substrate-mediated excitation mechanism is that of O2 adsorbed on Pd(l 11). Figure 27.8 shows the photodissociation and photodesorption yield for p- and s-polarized light, as a function of the incident angle. The bottom panel displays the p-/s-polarized light ratio for the two processes. Clearly, the experimental data are reproduced... 

Figure 27.8 Example of substrate-mediated excitation mechanism O2 adsorbed on Pd(lll). Both photodissociation and photodesorption yields are shown for p- and s-polarized light ( and respectively). The error bars are obtained from the standard deviation of two or three measurements. Within the error the data can only be described by a model, which is based on the absorption in the metal substrate as the primary excitation step. The lower part shows the ratios of the curves in the upper part. Reproduced from Wolf et al, J. Chem. Phys., 1990, 93 5327, with permission of the American Institute of Physics...

A brief overview on why most of the autoxidation reactions develop complicated kinetic patterns is given in Section II. A preliminary survey of the literature revealed that the majority of autoxidation studies were published on a small number of substrates such as L-ascor-bic acid, catechols, cysteine and sulfite ions. The results for each of these substrates will be discussed in a separate section. Results on other metal ion mediated autoxidation reactions are collected in Section VII. In recent years, non-linear kinetic features were discovered in some systems containing dioxygen. These reactions form the basis of a new exciting domain of autoxidation chemistry and will be covered in Section VIII. 

Molecule-molecule interactions can be direct (b) or mediated via the substrate (c). There may also be a substantial interaction wiA thermally excited, low frequency modes (d). 

When the chemisorbed molecule is vibrationally excited this influences not only the metal electrons but also the ion cores in the neighbourhood. The vibrating ion cores can then in turn couple to other molecules and give rise to a short range interaction mediated via the substrate lattice. However, as Cl is much larger than the highest substrate phonon frequency the effect of this interaction is very small , but it can be important for low frequency modes . 

In addition, electrode reactions are frequently characterized by an irreversible, i.e., slow, electron transfer. Therefore, overpotentials have to be applied in preparative-scale electrolyses to a smaller or larger extent. This means not only a higher energy consumption but also a loss in selectivity as other functions within the molecule can already be attacked. In the case of indirect electrolyses, no overpotentials are encountered as long as reversible redox systems are used as mediators. It is very exciting that not only overpotentials can be eliminated but frequently redox catalysts can be applied with potentials which are 600 mV or in some cases even up to 1 Volt lower than the electrode potentials of the substrates. These so-called redox reactions opposite to the standard potential gradient can take place in two different ways. In the first place, a thermodynamically unfavorable electron-transfer equilibrium (Eq. (3)) may be followed by a fast and irreversible step (Eq. (4)) which will shift the electron-transfer equilibrium to the product side. In this case the reaction rate (Eq. (5)) is not only controlled by the equilibrium constant K, i.e., by the standard potential difference be-... 

When one considers the role of the matrix in the particle-induced emission of secondary ions it is no wonder that it is so difficult to unravel all the processes that take place. The matrix is the medium in which the primary excitation occurs. It must also disperse some of that energy to sites at the surface where secondary ion emission occurs. It must provide the species to be desorbed and at the same time mediate the ionization process. In an attempt to understand these complex coupled processes we have tried to simplify the system by first selecting a homogeneous substrate for the energy deposition and then studying the ionization-emission process for species that are present as a submonolayer on the surface (26). 

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