Excitation surface adsorbate

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. 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

A monoenergetic electron beam, 2 - 10 eV, is scattered off a surface and the energy losses between 0.5 eV to bulk and surface phonons and vibrational excitations of adsorbates are measured as a function of angle and energy (also called EELS). 

In the case of a metal substrate, the experimental evidence shows that metal excitation is dominated by surface photon absorption. Optical radiation excites surface charge carriers, usually free or sub-vacuum-level electrons that can efficiently couple to the adsorbate. This often leads to enhanced photolysis cross sections or altered product distributions. Excitation localized on the adsorbed molecule in close proximity to a metallic solid may efficiently couple to the electronic states of the surface, leading to excitation quenching. When light-absorbing molecules are separated from the surface by spacer molecules, the influence of the surface on molecular excitation and relaxation decreases [4,21],... 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

The whole field received a new impetus after the first oil crisis, when Fujishima and Honda reported on the photoelectrolysis of water at Ti02-electrodes [13], Whereas, before the oil crisis, most basic models and results had been published only by 3-4 research groups in the world, many other scientists entered the field after this crisis and studied solar applications, and hundreds of papers were published. Since then, many processes at semiconductor electrodes have been studied more quantitatively by using not only standard electrochemical methods, but also new techniques, such as spectroscopic surface analysis (see e.g. [12]). Naturally, photoeffects played a dominant role in these investigations. These were not only restricted to reactions induced by light excitation within the semiconductor electrode [11], but were also extended to the excitation of adsorbed dye molecules [14,15]. 

Although it has been difficult to separate the effects of excitation and emission enhancement, both of these effects should be extremely sensitive functions of the shape of the metal particle, the orientation of the fluorophore, and the distance between the fluorophore and the metal, because the local-field effects depend strongly on these parameters. Many groups have studied variations in fluorescence intensity as a function of the distance between a layer of fluorophores and a number of nanostructured metal surfaces, adsorbed colloidal particles or suspended colloidal particles. Single-molecule experiments have even provided strong evidence for the existence of a local maximum in the fluorescence intensity versus distance curve. ... 

The flow of energy from laser-heated snbstrate electrons into the adsorbate results in the excitation of adsorbate vibrational modes. For desorption of diatomics from various surfaces, it has been snggested that excitation of the frustrated rotation is responsible for the desorption process [9,14, 39]. Our results are consistent with those observations. We cannot, strictly speaking, dismiss (a contribution from) the Pt-CO stretch vibration, but the frustrated translational mode can be exclnded based on the independently determined electron-coupling times found to be 2.5 and 4 ps for terrace-and step-adsorbed molecnles, respectively (see the Sect. 10.3.2). These coupling times are much longer than those describing the very rapid desorption process. 

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. 

The first picosecond time-resolved observations of a photosensitised charge-injection process in semiconductor particles were carried out by Moser et at. (1985). The rate of electron injection from the excited singlet state of the surface-adsorbed dye eosin, EO(Si), to the conduction band of colloidal Ti02 particles... 

Figure 2. An MGR surface applicable for desorption (coordinate is surface adsorbate, S-A, distance) or dissociation (A-B distance), a, optical excitation, b ultrafast quenching leading to unaltered ground state, c quenching leading to the adsorbate being carried over into a metastable adsorbed state by kinetic energy acquired on the upper surface (Ec), d quenching leading to dissociation (or desorption) due to acquired Ed. If no quenching occurs reaction takes place on the upper surface.

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