Light emissions

The panel Emitted in Fig. 5.1 represents the EM Helds that are involved in any process where the molecule is acting as a source of light. Eemi is in fact the electric field emitted by the molecule as in the absence of the metal nanoparticle, while Ere/ is the field originated by the interaction of Eem/ with the metal nanoparticle. We have also highlighted the fact that part of the emitted field is actually penetrating the metal particle and is dissipated there. The field that results from the superposition of Eem/ and Ere/ is the total field Etot that can be measured by a far- or a near- field probe. 

As mentioned before, our discussion is following a classical electrodynamics treatment of the EM fields. Spontaneous emission requires QED for a proper description. However, with some ad hoc assumptions, spontaneous emission can be described in a classical framework. Within this framework, spontaneous emission is understood as the emission of light due to an electrical dipole oscillating at a frequency ty corresponding to the de-excitation from the molecular state n to the ground state 0. To calculate the radiative decay rate yr of an excited molecule within this theory, one has to preliminary calculate the total power emitted by the dipole fid. 

to heuristically introduce quantization of EM fields, one can assume that this power is due to the emission by the molecule of photons each carrying an energy tuo. By applying this procedure to a free-standing molecule (i.e. no metal nanoparticle), one would find that (see Sec. 1.5)  

We now compare this expression with the proper one coming from QED  

Many treatments of molecular plasmonics phenomena based on classical electrodynamics bypasses this possible source of mistakes by focusing on the ratio between the rates for the molecule close to the nanoparticle and the free-standing molecules, in such a way that the molecular dipole cancels out (for linear media). 

The exact relative locations of the lowest excited Bu and Ag states are difficult to predict on a theoretical basis. Indeed, they sensitively depend on the interplay between electron correlation effects and bond-length alternation effects, as shown for instance by Soos and his co-workers23. Strong effective bond alternations favour the 1BU state as the S, state this is the case in polyparaphenylene and PPV due to the presence of phenylene rings. The effective bond alternation is much weaker in polyacetylene while it is intermediate in polythiophene where the 2Ag state is found to lie slightly above the 1BU state. 

Another aspect worth mentioning is that it is usually the case that the 2Ag state has a covalent (neutral) character while the 1BU state has a more ionic character. As a result, in polyenes end-capped by strong donor and acceptor groups, the 1 Bu state becomes very much stabilized and passes below the 2Ag state24 substitution of the 

Finally, it should be stressed that photoluminescence has usually a larger quantum yield in solution than in the solid state27. In the latter case, inter-chain interactions appear to quench the luminescence properties through a mechanism that is not fully understood yet. In this context, it is, however, useful to note that the possibility that the polaron-excitons evolve into excimers28 is ruled out by the absence of Stokes shifts in good-quality PPV oligomer and polymer samples14. 

Beljonne, Z. Shuai, T. Hagler, I. Campbell, C. W. Spangler, K. Mullen, D. D. C. Bradley and J. L. Bredas, Chem. Phys. Lett., in press. 

Beljonne, F. Meyers andj. L. Bredas, submitted for publication. 

FIGURE 7.17 Additional examples of absorption (top) and transmission (bottom) spectra. Upper left, iron-o-phenanthroline upper right, toluene in heptane lower left, benzene in heptane lower right, potassium permanganate. 

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Krug J T II, Wang G D, Emory S R and Nie S 1999 Efficient Raman enhancement and intermittent light emission observed in single gold nanocrystals J. Am. Chem. Soc. 121 9208-14... 

The amount of light emitted by a source is measured by its luminance or by its luminous intensity, which are defined in Figure 18.2. Intrinsic light emission relates to the amount of light emitted per unit area (luminance). Table 18.1 lists approximate luminances for some common light sources. 

Eig. 3. Depiction of the light extraction, ie, escape cones of light emission, for various LED chip stmctures consisting of absorbing substrate devices having (a) thin window layers (top cone) (b) thick window layers (top cone and four one-half side cones) (c) thin window plus the implementation of a distributed Bragg reflector between the active layer and the substrate (top and bottom cone). Also shown is (d), the optimal stmcture for light extraction, a... 

Fig. 7. Light-emission spectra of various LEDs emission from (—) AIGalnP LEDs results from direct gap recombination, whereas that of (-) GaP...

Reaction takes place ia aqueous solution with hydrogen peroxide and catalysts such as Cu(II), Cr(III), Co(II), ferricyanide, hernia, or peroxidase. Chemiluminescent reaction also takes place with oxygen and a strong base ia a dipolar aprotic solvent such as dimethyl sulfoxide. Under both conditions Qcis about 1% (light emission, 375—500 am) (105,107). 

Organism Common name Wavelength of light emission, nm... 

The intensity of bioluminescence emission is > 2 x 10 photon /s-cm in the dinoflageUate Gonyaulax and the spectmm of light emission ranges from 450—490 nm (blue) in deep sea species, 490—520 nm (green) in coastal water species, and 510—580 nm (yeUow-green) in terrestrial and freshwater species. 

Hg ", Zn ", Cd " ) light emission is shifted to the red (610—615 nm). In vitro a flash of light is produced (< 1 s) that decays rapidly. Glow-type emission is obtained ia the presence of detergents (Triton X-100), polymers (PEG 6000), coen2yme A, inorganic pyrophosphate, and cytidine nucleotides (206,207). 

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

The membrane enzyme luciferase, responsible for light emission in fireflies, is sensitive to anesthetics (20,21), and the concentrations of inhalational agents which inhibit luciferase are the same as those which cause general anesthesia. Studies of various classes of inhalational agents and luciferase demonstrated that above a certain chain length in a homologous series, a point is reached where higher members are not anesthetic. The same cut-off effect in efficacy is observed in anesthesia (22). This effect is not explainable by Hpid theory. 

A light-emitting diode (LED) is a forward-biasedp—n junction in which the appHed bias enables the recombination of electrons and holes at the junction, resulting in the emission of photons. This type of light emission resulting from the injection of charged carriers is referred to as electroluminescence. A direct band gap semiconductor is optimal for efficient light emission and thus the majority of the compound semiconductors are potential candidates for efficient LEDs. 

Chemiluminescent labels, in which the luminescence is generated by a chemical oxidation step, and bioluminescent labels, where the energy for light emission is produced by an enzyme-substrate reaction, are additional labeling types (39,42). Luminol [521 -31 -3] CgHyN202, and acridine [260-94-6] C H N, derivatives are often used as chemiluminescent labels. 

HEED = high energy electron diffraction IILE = ion-induced light emission INS = ion-neutralization spectroscopy IRS = infrared spectroscopy ISS = ion-scattering spectroscopy LEED = low energy electron diffraction LEIS = low energy ion scattering ... 

This is a transient discrete electric discharge which takes place between two conductors which are at different potentials, bridging the gap in the form of a single ionization channel (Plate 4). Based on light emission measurements of sparks with symmetrical electrode geometry, the energy is dissipated approximately uniformly along the channel. This is in contrast with asym-... 

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