Surface optical excitation

Figure Al.6.26. Stereoscopic view of ground- and excited-state potential energy surfaces for a model collinear ABC system with the masses of HHD. The ground-state surface has a minimum, corresponding to the stable ABC molecule. This minimum is separated by saddle points from two distmct exit chaimels, one leading to AB + C the other to A + BC. The object is to use optical excitation and stimulated emission between the two surfaces to steer the wavepacket selectively out of one of the exit chaimels (reprinted from [54]).

Shank C V, Yen R and Hirlimann C 1983 Femtosecond-time-resolved surface structural dynamics of optically excited silicon Phys. Rev. Lett. 51 900-2... 

Homola, J., "Thin Films Study by Means of Optically Excited Surface Plasmons, Nano 94—International Coirference on Nanometrology Scanning Probe Microscopy and Related Techniques, Proceedings, Nano 94, Bmo, Czech Rep., 1994, pp. 84-87. 

The electron is excited from a filled initial state f below the Fermi level F to an empty final state f above F. Momentum conservation will be provided by a lattice vector or in some cases by a surface vector. The transition probability is mainly determined by the optical excitation matrix element containing the joint density of states. 

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

Thus, optical excitation to the Ui, U2 or U3 levels should be followed by vibrational relaxation to the minimum point of the respective potential energy surfaces and subsequent radiative decay to the ground state surface as illustrated in Figure 3. 

Figure 1. Potential energy plot of the reactants (precursor complex) and products (successor complex) as a function of nuclear configuration Eth is the barrier for the thermal electron transfer, Eop is the energy for the light-induced electron transfer, and 2HAB is equal to the splitting at the intersection of the surfaces, where HAB is the electronic coupling matrix element. Note that HAB << Eth in the classical model. The circles indicate the relative nuclear configurations of the two reactants of charges +2 and +5 in the precursor complex, optically excited precursor complex, activated complex, and successor complex.

Figure 3. Optical excitation of surface plasmon-polaritons (surface plasmon) by the attenuated...

At infrared wavelengths extinction by the MgO particles of Fig. 11.2, including those with radius 1 jam, which can be made by grinding, is dominated by absorption. This is why the KBr pellet technique is commonly used for infrared absorption spectroscopy of powders. A small amount of the sample dispersed in KBr powder is pressed into a pellet, the transmission spectrum of which is readily obtained. Because extinction is dominated by absorption, this transmission spectrum should follow the undulations of the intrinsic absorption spectrum—but not always. Comparison of Figs. 10.1 and 11.2 reveals an interesting discrepancy calculated peak extinction occurs at 0.075 eV, whereas absorption in bulk MgO peaks at the transverse optic mode frequency, which is about 0.05 eV. This is a large discrepancy in light of the precision of modern infrared spectroscopy and could cause serious error if the extinction peak were assumed to lie at the position of a bulk absorption band. This is the first instance we have encountered where the properties of small particles deviate appreciably from those of the bulk solid. It is the result of surface mode excitation, which is such a dominant effect in small particles of some solids that we have devoted Chapter 12 to its fuller discussion. 

The dynamics of the interfacial electron-transfer between Dye 2 and TiOz were examined precisely by laser-induced ultrafast transient absorption spectroscopy. Durrant et al.38) employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of Dye 2 adsorbed onto the surface of nanocrystalline Ti02 films. Detailed analysis indicates that the injection is at least biphasic, with ca. 50% occurring in <150 fsec (instrument response limited) and 50% in 1.2 0.2 psec. 

Surface electromagnetic waves or surface polaritons have recently received considerable attention. One of the results has been a number of review articles1, and thus no attempt is made here to present a comprehensive review. These review articles have been concerned with the surface waves, per se, and our interest is in the use of surface electromagnetic waves to determine the vibrational or electronic spectrum of molecules at a surface or interface. Only methods using optical excitation of surface electromagnetic waves will be considered. Such methods have been the only ones used for the studies of interest here. 

Recently, schemes for computational architectures, which make use of the quantum properties on the molecular scale rather than avoiding them, have been proposed, such as cascading CO molecules on metal surfaces [35], computing with optical excitations [36] or controlling the interference pattern of electron transport through aromatic molecules by modifying their chemical side-groups [37] (Fig. 2). 

Free charge carriers generated upon optical excitation either get trapped at the surface vacancies or undergo charge recombination (1) to (3). 

Possible applications of ZnO nanostructures are UV lasers (up to now only with optical excitation) [84], chemical sensors [85], or transparent substrates for thin film solar cells, e.g. as an alternative to Ti02 in injection type solar cells or in organic solar cells [79,86,87]. In the latter two applications the increased effective surface of arrays of ZnO nanowires leads to... 

Niemeyer et al. have reported the design of quantum dot/enzyme nanohybrids that are capable of catalyzing an organic transformation upon optical excitation of semiconductor quantum dots (QDs) [31]. The hybrid device was composed of semiconductor CdS nanoparticles and cytochrome p450BSp enzyme. It has been proposed that irradiation of QDs leads to formation of excitons (h+-e pairs) that on dissociation generate superoxide and hydroxyl radicals in interfacial electron transfer process (see Chapter 7). These radicals in turn activate the enzyme adsorbed at the QD surface. The activated enzyme is able to catalyze mono-oxygenation of fatty acids, but has a lower activity than the native enzyme [31]. 

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