Optical-absorption

Optical absorption and emission in conjugated oligomers and polymers 

In this section, the basic features of light absorption and emission (luminescence) processes in conjugated systems are reviewed. The discussion will focus on poly(/ -phenylenevinylene), PPV, compounds, which provide typical examples of the physical phenomena to be highlighted in the context of polymer-based light emitting devices. 

Another most notable aspect relates to the vibronic coupling, that is the coupling between electronic excitations and vibrational modes. As has been stressed many times in the literature, much of the rich and fascinating physics of conjugated polymers is based on the strong 

Cornil et al. have studied the optical absorption spectra of PPV oligomers containing from two to five phenyl/phenylene rings and analysed the extent to which the vibronic couplings affect the lineshape of the spectra11. It is useful to set first the theoretical 

1 eV for the five-ring compound, comes from a large mixing of configurations involving electron transitions between delocalized levels15. 

To derive the shift in the optical band gap, Tiedje et al (1984) used the conduction and valence densities of states corresponding to the model of free electrons and holes in a one-dimensional periodic potential shown in Fig. 5a. The parameters of the model were chosen as follows bulk amorphous silicon band gap 1.8 eV conduction- and valence-band-edge discontinuities at the a-Si H/a-SiN t H interfaces [4= 1.0 eV and [4 = 0.6 eV, respectively 

Fermi level and and Uy are the discontinuities in the conduction and valence bands, respec- 

To check that there are no systematic errors in the extrapolated gap due to differences in the total thickness of the a-Si H material (sum of all the a-Si H layers), we also calculated Eq from the relation (oEy = const X (E— Eq) (Cody et al., 1982), which is less sensitive to the a-Si H total thickness. Both approaches gave the same increase in band gap with layer thickness. 

Change in optical band gap AEq as a function ofa-Si H layer thickness . Solid line is the extrapolated gap from the model data in Fig. 6 open circles are extrapolated values of Eq derived from the experimental data in Fig. 4 and Eq. (2). [From Tiedje et al. (1984).] 

Commonly, excited singlet states have very short lifetimes and can only be detected by means of femtosecond absorption spectroscopy. A typical case is illustrated in Fig. 1.24, which shows the differential transmission spectrum of MEH-DSB (see Chart 1.18). 

Chart 1.17 Chemical structure of poly (phenyl vinyl ketone). 

The phenomenon of spectral narrowing is attributed to a cooperative effect in light emission, the so-called amplified spontaneous emission effect, which involves the coherent coupling of a large number of emitting sites in a polymer matrix. 

Aexc = 532 nm. Film thickness 210 nm [74]. Adapted from Lemmer et al. [75] with permission from Wiley-VCH. 

Chart 1.19 Chemical structure of poly[2-butyl-5-(2 -ethylhexyl)-l,4-phenylene vinylene], BuEH-PPV. 

In general, semicontinuum models explain the band maximum position rather well. GJ calculate the energy at maximum absorption as 0.9 and 2.0 eV respectively for eam and eh, which compare reasonably well with the experimental values, 0.8 and 1.7 eV respectively. 

Much worse than the oscillator strength is the line shape. The calculated absorption spectra has no similarity with what is experimentally seen. The calculated half-width is always smaller, typically by a factor of 2 the exact reasons for this are only speculated. It is common knowledge that a photodetachment process is capable of giving a very broad absorption spectrum, but a satisfactory method has not been developed to adopt this with the bound-bound transition of the semicontinuum models. Higher excited states (3p, 4p, etc.) have been proposed for the solvated electron, but they have never been identified in the absorption spectrum. 

The position of absorption maximum shifts to lower energy with temperature, attributed by Jortner (1959, 1964) to the temperature dependence of 

Another method of detection of charge carriers in nonpolar liquids is by means of their optical absorption. 

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Our intention is to give a brief survey of advanced theoretical methods used to detennine the electronic and geometric stmcture of solids and surfaces. The electronic stmcture encompasses the energies and wavefunctions (and other properties derived from them) of the electronic states in solids, while the geometric stmcture refers to the equilibrium atomic positions. Quantities that can be derived from the electronic stmcture calculations include the electronic (electron energies, charge densities), vibrational (phonon spectra), stmctiiral (lattice constants, equilibrium stmctiires), mechanical (bulk moduli, elastic constants) and optical (absorption, transmission) properties of crystals. We will also report on teclmiques used to study solid surfaces, with particular examples drawn from chemisorption on transition metal surfaces. 

Knickelbein M B and Menezes W J C 1992 Optical response of small niobium clusters Rhys. Rev. Lett. 69 1046 Ceilings B A, Athanassenas K, Lacombe D, Rayner D M and Hackett P A 1994 Optical absorption spectra of AUy,... 

Alvarez M M efa/1997 Optical absorption spectra of nanocrystal gold molecules J. Phys. Chem. B 101 3706... 

Krelbig U and Genzel L 1985 Optical absorption of small metallic particles Surf. Sol. 156 678... 

ColvinVL, Cunningham K L and Alivisatos A P 1994 Electric field modulation studies of optical absorption in CdSe nanocrystals dipolar character of the excited state J. Chem. Phys. 101 7122... 

Computed optical properties tend not to be extremely accurate for polymers. The optical absorption spectra (UV/VIS) must be computed from semiempiri-cal or ah initio calculations. Vibrational spectra (IR) can be computed with some molecular mechanics or orbital-based methods. The refractive index is most often calculated from a group additivity technique, with a correction for density. 

Allen, H. C. Brauers, T. Finlayson-Pitts, B. J. Illustrating Deviations in the Beer-Lambert Law in an Instrumental Analysis Laboratory Measuring Atmospheric Pollutants by Differential Optical Absorption Spectrometry, /. Chem. 

Cm ORINE OXYGEN ACIDS AND SALTS - Cm OROUS ACID, Cm ORITES, AND Cm ORINE DIOXIDE] (Vol 5) Optical absorption spectra... 

Eig. 27. Optical absorption spectra of thin, 1 p.m-films of novolac, polyhydroxystyrene and polyacrylate polymers. The novolac resin is transparent only above 300 nm. While polyhydroxystyrene also absorbs strongly below 300 nm, it exhibits a region of adequate transparency centered near 248 nm. The... 

In each of these approaches, imaging is confined to the top of a single polymeric film by adjusting optical absorption. The penetration depth of the silylation agent and the attendant swelling of the polymer film must also be controlled to avoid distortion of the silylated image. Resists of this type are capable of very high resolution (Fig. 37). 

Transmission. The spectral transmission of glass is determiaed by reflectioa at the glass surfaces and the optical absorption within the glass. Overall transmission of a flat sample at a particular wavelength is equal to (1 — R), where P is the absorption coefficient, t the thickness of glass, and... 

The optical absorption spectra of Pu ions in aqueous solution show sharp bands in the wavelength region 400—1100 nm (Fig. 4). The maxima of some of these bands can be used to determine the concentration of Pu ions in each oxidation state (III—VI), thus quantitative deterrninations of oxidation—reduction equiUbria and kinetics are possible. A comprehensive summary of kinetic data of oxidation—reduction reactions is available (101) as are the reduction kinetics of Pu + (aq) (84). 

Optical absorption measurements give band-gap data for cubic sihcon carbide as 2.2 eV and for the a-form as 2.86 eV at 300 K (55). In the region of low absorption coefficients, optical transitions are indirect whereas direct transitions predominate for quantum energies above 6 eV. The electron affinity is about 4 eV. The electronic bonding in sihcon carbide is considered to be predominantiy covalent in nature, but with some ionic character (55). In a Raman scattering study of vahey-orbit transitions in 6H-sihcon carbide, three electron transitions were observed, one for each of the inequivalent nitrogen donor sites in the sihcon carbide lattice (56). The donor ionization energy for the three sites had values of 0.105, 0.140, and 0.143 eV (57). 

The changes in the optical absorption spectra of conducting polymers can be monitored using optoelectrochemical techniques. The optical spectmm of a thin polymer film, mounted on a transparent electrode, such as indium tin oxide (ITO) coated glass, is recorded. The cell is fitted with a counter and reference electrode so that the potential at the polymer-coated electrode can be controlled electrochemically. The absorption spectmm is recorded as a function of electrode potential, and the evolution of the polymer s band stmcture can be observed as it changes from insulating to conducting (11). 

For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

It should be noted that during CL observations intensity variations may arise due to sample morphology (e.g., surface roughness), which may lead to nonuniform excitation and to local variations in optical absorption and reflecdon losses. 

Depth resolution depends on the (spectrally dependent) optical absorption coefficient of the material. Near-surface analysis (first 50 nm) frequendy can be per-... 

As NRA is sensitive only to the nuclei present in the sample, it does not provide information on chemical bonding or microscopic structure. Hence, it is often used in conjunction with other techniques that do provide such information, such as ESCA, optical absorption. Auger, or electron microscopy. As NRA is used to detect mainly light nuclei, it complements another accelerator-based ion-beam technique, Rutherford backscattering (RBS), which is more sensitive to heavy nuclei than to light nuclei. 

Ceo samples [66]. At higher photon energies, dipole-allowed transitions ean occur, and the optical absorption increases dramatieally. 

As shown in Fig. 7, a large increase in optical absorption occurs at higher photon energies above the HOMO-LUMO gap where electric dipole transitions become allowed. Transmission spectra taken in this range (see Fig. 7) confirm the similarity of the optical spectra for solid Ceo and Ceo in solution (decalin) [78], as well as a similarity to electron energy loss spectra shown as the inset to this figure. The optical properties of solid Ceo and C70 have been studied over a wide frequency range [78, 79, 80] and yield the complex refractive index n(cj) = n(cj) + and the optical dielectric function... 

Applications The differential optical absorption spectrometer has been used to monitor concentrations of gases or intermediate compounds such as SO, NO, O5, HCHO, HNO, CS, NO, and OH in the atmosphere.In atmospheric measurements with open paths of 100 to 1000 m, a detection limit of about 1 ppb can be achieved. In the emission measurements, the path length across the duct or the plume can range in meters. 

Edner, Hans, Anders Sunesson, Sune Svanberg, Leif Llneus, and. Svante Wallin. Differential Optical Absorption Spectroscopy System Used for Atmospheric Mercury Monitoring. Applied Optics 25 (1986), pp. 403-409. 

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