Surface excitation

Translational -> internal energy transfer Surface excitation (phonon, electron)... 

Other techniques to inspect bonding surfaces for contamination have also been proposed, including ultraviolet fluorescence [162], Pulsed ultraviolet light incident on the surface excites fluorescence of organic contamination, which can... 

Experimental results clearly demonstrate that catalytic reaction of dehydration of alcohols on zinc oxide proceeds via formation of radicals. Emission of hydrogen atoms from the catalyzer surface may be associated with structure relaxation of the catalyzer surface excited during the reaction [26]. 

The potential exists for the development of a powerful technique for the study of variations of surface excitation energies with a spatial resolution of 1 nm or better. 

Electrons which are channelled along a flat surface in this way are particularly effective in exciting surface states of the crystal. Since they spend only a small fraction of their time "inside the crystal they are not greatly attenuated by bulk scattering processes and their EELS spectra do not show bulk excitation peaks. They are therefore effective probes for studying surface excitations. 

Beechem f. M., Ameloot M. and Brand L. (1985) Global Analysis of Fluorescence Decay Surfaces Excited-State Reactions, Chem. Phys. Lett. 120, 466-472. 

The collected fluorescence 3F [from Eq. (7.39)] clearly depends on the orientation distribution of the dipoles and the incident polarization through the dependences on 0 and E. We will assume a special but common case here randomly oriented dipoles with a z-dependent concentration near the surface, excited by a p-polarized evanescent wave. 

Figure 4.19 The range of strong diffraction in (a) the Laue case, (b) the Bragg case. The heavy lining shows the region of the dispersion surface excited as the rocking curve is traversed...

In this section we deal with particles which are excited in the gas phase and then approach a surface. Excitation to optically allowed state are probably not important in this context since they will remain in the excited state for a period less than 10 sec. During this period they only travel lmm. However, metastable molecules and ions have long lifetimes and therefore may be de-excited primarily at surfaces. The question, Do these excited molecules react differently from ground state molecules is the topic of this section. Since little information is available, the discussion is highly speculative. 

A plate on which a thin layer of a semieonduetor (e.g. selenium) has been deposited. A very thin transparent layer of silver is sputtered over the selenium to aot as a eolleetor eleetrode. Light falling on the semiconductor surface excites eleetrons which are released to the eolleetor eleetrode. The eurrent thus generated is measured using a galvanometer. 

Some of them, however, excite vibrations of surface atoms or molecules. Their energy is thus reduced by the energy of the vibration. This allows one to take energy spectra of surface excitations. 

A convenient approximation in many applications is to assume that a region of interest with a RDOp p is in contact with a medium at thermal equilibrium. The system and medium are chosen so that the latter can be assumed to remain at equilibrium at all times, with a density operator yeq. In this case it is possible to search for solutions of the equations starting from a factorized density operator for the whole system, r = p 69 A/,q, in a procedure also called a Fano-factorization. [6] This however is not acceptable when the total system is subject to excitations which induce transitions among states of the medium. An example is a molecule adsorbed on a metal surface, excited by visible light which first creates electronic excitations in the substrate. In this case the active medium is described by a DOp evolving in time, and some of the common developments in the literature must be generalized. 

The principal methods of gas activation are thermal and electrical much less common are chemical and photochemical activation. In the most commonly used thermal activation technique - the hot filament technique - a W or Ta wire is arranged in the immediate vicinity of the substrate to be coated by diamond (Fig. 1). The wire is heated until it reaches the temperature when H2 molecules dissociate readily. The gas phase is a mixture of a carbon-containing gas (e.g. methane, acetone or methanol vapor), at a concentration of a few per cent, and hydrogen. Upon the contact of the gas with the activator surface, excited carbon-containing molecules and radicals are produced, in addition to the hydrogen atoms. They are transferred to the substrate surface, where deposition occurs. Table 2 gives an indication of the hot-filament deposition process parameters. 

Section III deals with the surface excitations of the anthracene crystal, confined in the first (SJ, second (S2) and third (S3) (001) lattice planes. The experimental observations are briefly summarized. A simple model shows how the fast radiative decay arises and how the underlying bulk reflection modulates this superradiant emission, as well as why gas condensation on the crystal surface strongly narrows this emission, thus accounting for the observed structures. An intrinsic process is proposed to explain the surface-to-bulk relaxation at low temperatures, observed in spite of the very weak surface-to-bulk coupling for k 0 states. 

In this section we analyze the surface investigation of molecular crystals by the technique of UV spectroscopy, in the linear-response limit of Section I, which allows a selective and sharp definition of the surface excited states as 2D excitons confined in the first monolayer of intrinsic surfaces (surface and subsurfaces) of a molecular crystal of layered structure. The (001) face of the anthracene crystal is the typical sample investigated in this chapter. 

These surface excitation phenomena are investigated in this section, on excitons in the intrinsic-surface-bulk system, and the next section, on disordered 2D excitons. 

Introducing the explicit expression (3.21) for r( >), and putting E = (z2 — co2)/2co0, which is roughly the detuning from the isolated-surface excitation, we obtain (with the notation RK = iB)... 

Here r,(co) shows the effect of the surface reflectivity, which appears as a lorentzian line, centered at the surface resonance, if we neglect the variation of rv(co) with co around the surface resonance ( lOcnU1). The surface excitations are renormalized relative to the bulk-free surface, leading for coupled surface excitons to a frequency shift ds and to a new radiative width rs, both quantities simply related to the complex amplitude of the bulk reflectivity ... 

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. 

Refs. [i] Hard S, Hamnerius Y, Nilsson O (1976) J Appl Phys 47 2433 [ii] Lofgren H, Neuman RD, Scriven LE, Davis HT (1984) J Colloid Interface Sci 98 175 [iii] Laudon R (1984) Ripples on liquid interfaces. In Agranovich VM, Laudon R (eds) Surface excitations. North Holland, Amsterdam, pp 591-638 [iv] Byrne D, Earnshaw JC (1977) J Phys D Appl Phys 10 L207 [v] Lamb H (1945) Hydrodynamics. Dover, New York, p 348 [vi] Zhang Z, Tsuyumoto I, Takahashi S, Kitamori T, Sawada T (1997) J Phys Chem A 101 4163 [vii] Samec Z, Trojanek A, Krtil (2005) Faraday Discuss 129 301... 

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