Surface emission relaxation mechanism

In Section III.A.l we did not discuss the way the surface emission is excited. The radiative behavior of the surface shows that emission (normal to the surface) is observed as soon as the K = 0 state is prepared. This state may be prepared either by a short ( 0.2ps) resonant pulse, or by relaxation from higher, optically prepared excited states. It is obvious that the quantum yield of the surface emission will critically depend on the excitation, owing to intrasurface relaxation accelerated by various types of fission processes (see Fig. 2.8) and in competition with fast irreversible transfer to the bulk (3.30), which is also a surface relaxation, at least at very low temperatures. Thus, the surface excitation spectra provide key information both on the upper, optically accessible surface states and on the relaxation mechanisms to the emitting surface state K = 0. 

The effect of temperature on deep trap emission is similar to that observed for bandgap emission, with the intensity of the emission decreasing as the temperature increases. This can be explained by the involvement of nonradiative recombination processes dominating at higher temperature. Nonradiative relaxation in CdSe nanoclusters has been assigned to the involvement of a multiphonon relaxation mechanism mediated by a vibrational mode of the surface phenylse-lenolate ligands. ... 

An adsorbed atom or a molecule being in its excited state is characterized by a finite lifetime which is determined by the reciprocal of the decay rate of this state. The finiteness of the lifetime leads to a broadening of the lines in the optical spectra of the adsorbate. Besides spontaneous emission which occurs also for free atoms and molecules, adsorbed species have other specific channels of relaxation, conditioned by their proximity to the surface. Any relaxation process must obey the conservation law of energy and therefore it takes place only if there is a substrate excitation which can accept the energy that the excited adsorbate releases. Therefore, possible decay mechanisms are determined by the energy spectrum of the substrate and thus generally are different for metals, semiconductors and dielectrics. They can be broadly classified as being mediated by photons, phonons, electron-hole pairs and conduction electrons. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

The PAES mechanism, first demonstrated in 1987 [1], can be outlined as follows (1). A positron implanted at low energy diffuses to and gets trapped at the surface. (2). A few percent of the trapped positrons annihilate with core electrons leaving atom in excited state. (3). The atom relaxes via emission of an Auger electron. The PAES mechanism is contrasted with that of electron induced Auger Spectroscopy (EAES) in Figurel2.1. 

Under incident radiation or bombardment by an electron beam surfaces emit photons, electrons, or both. The emission properties of solid surfaces differ widely, just as their mechanisms or relaxation after excitation by high-energy radiation differ. Many surface-sensitive experimental techniques providing information related to the electronic properties of surfaces are based on these processes, for example. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). These are discussed below. 

Current explanations of tribochemical reactions state that the more obvious consequence of mechanical treatments, the increase of the surface area of a solid, is a minor factor, which contributes only to 10% of the reactivity increase. The more important effect is due to the accumulation of energy in lattice defects which can relax either physically by the emission of heat, or chemically by the ejection of atoms or electrons, formation of excited states on the surface, bond breakages, and other chemical transformations. Mechanical stress can be applied as single or periodic shocks, rapid loads, etc. An example is that of the mechanochemical decomposition of aluminum hydride, which increases with the frequency of the applied stress (Fig. 2).i ... 

While the diabatic mechanism we just discussed is typical of photochemical processes, there are some other less common yet interesting paths, summarized in Figure 16.16. In an adiabatic reaction (Figure 16.16 A), the conversion from reactant geometry to product geometry occurs on just one surface, the excited state surface. This is then followed by relaxation back down to the ground state. This relaxation could in principle be emissive, such that we would excite the reactant and see fluorescence from the product. 

The most complex situation is sketched in Figure 7(b) for intermediate separation distances the chromophores excited either by plane waves from the dielectric side or by a surface plasmon mode excited from the prism side relaxes vibronically to the bottom of the excited state level of the chromophore but then can back-couple to the metal, thereby exciting a red-shifted siuface plasmon mode. This mode in turn can re-radiate via the prism (or the grating) and lea to an enhanced fluorescence emission. The optimum dye-metal separation for this decay mechanism has been reported to be in the range of d = 20 nm . 

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