Optical absorption/emission

Optical spectroscopy measurement of optical absorption, emission or scattering Offers a direct and rapid, and often highly selective means, of measuring gas concentration with good sensitivity. The gas must have a significant and distinct absorption, emission or scattering in a convenient region of the optical spectrum. 

Optical methods monitor the optical absorption/emission/scattering of a gas species at defined optical wavelengths. The distribution of this optical absorption/emission with wavelength provides an optical fingerprint for any gas species present, and the magnitude typically shows the concentration. 

Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-57]. These applications are reviewed in several excellent sources [1, 54,58], and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-61], and the interested reader is referred to these for details. 

UV/Vis absorption and photoluminescence spectra of semiconductors 368-373 were measured in solution to assess the effect of carbonyl functionalization on the optical absorption/emission spectra and resultant optical HOMO-LUMO energy gaps (Egap) (07CM4864). 

The simple steady-state photolysis apparatus shown in Figure 12.2 consists of a light source such as a high- or medium-pressure mercury vapor lamp, temperature-controlled water bath fitted with optically flat quartz window, shutter, and light filter. Analysis of the reaction products may be carried out using one or more of the standard methods, such as optical absorption, emission, ESR, etc. 

Among the most common unsaturated units, there are mono (poly) cyclic aromatic hydrocarbons, heterocycles, benzofused systems, and olefinic and acetylenic groups, typically paired with various fullerene derivatives. The extent of conjugation/interaction between these units determine the polymer solution/solid-state electronic structure, which in turn control polymer properties, such as, optical absorption/emission, redox... 

Heimel, G., et al. 2005. Breakdown of the mirror image symmetry in the optical absorption/ emission spectra of oligo(para-phenylene)s. / Chem Phys 122 054501. 

Figure 6.66. Density of states in a semiconducting single-walled carixm nanotube. The sharp maxima are van Hove singularities. The dominant optical absorption/emission transitions are indicated by arrows. Reproduced with permission from Weisman, R. B. Subramoney, S. Electrochem. Soc. Interface, Summer 2006,42.

In spectroscopic analysis, species are identified by the frequencies and stmctures of absorption, emission, or scatteting features, and quantified by the iatensities of these features. The many appHcations of optical methods to chemical analysis rely on just a few basic mechanisms of light—matter iateraction. 

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... 

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. 

Figure 10-5. Transient transmission changes AV/Po in PPV for different lime delays between the pump and probe pulse. The pump pulse is a 100 fs laser pulse at 325 nm obtained by frequency doubling ol amplified dye laser pulses, (a) and (b) correspond to different sides of a PPV-film. The spectra in (a) were obtained lor the unoxidized side of the sample while the set of spectra in (b) was measured for the oxidized side of the same sample. The main differences observed are a much lower stimulated emission effect for the oxidized side. The two bottom spectra depict the PL-spectra for comparison. The dashed line indicates the optical absorption (according to Kef. (281).

Electrical measurements on devices with different layer thickness have shown that the diode current depends on the applied field rather than the drive voltage. This is similar to what has been observed with our alternating PPV copolymers [68]. It indicates that field-driven injection determines the electrical characteristics. From Figure 16-39 it is evident that U-OPV5 has the lowest onset for both current and emission. By means of Fowler-Nordhcini analysis of the /-V -charac-teristics and optical absorption measurements, wc estimated the injection barrier for holes and the HOMO-LUMO gap, respectively [119]. The results of... 

Detectors based on established optical absorption and emission techniques,... 

Xie, J.R.H., Cheung, C.F. and Zhao, J.J. (2006) Tuning Optical Absorption and Emission of Suh-Nanometer Gold-Caged Metal Systems M Auj4hy Substitutional Doping. Journal of Computational and Theoretical Nanoscience, 3, 312—314. 

In addition to measuring total recombination coefficients, experimentalists seek to determine absolute or relative yields of specific recombination products by emission spectroscopy, laser induced fluorescence, and optical absorption. In most such measurements, the products suffer many collisions between their creation and detection and nothing can be deduced about their initial translational energies. Limited, but important, information on the kinetic energies of the nascent products can be obtained by examination of the widths of emitted spectral lines and by... 

The above theory is usually called the generalized linear response theory because the linear optical absorption initiates from the nonstationary states prepared by the pumping process [85-87]. This method is valid when pumping pulse and probing pulse do not overlap. When they overlap, third-order or X 3 (co) should be used. In other words, Eq. (6.4) should be solved perturbatively to the third-order approximation. From Eqs. (6.19)-(6.22) we can see that in the time-resolved spectra described by x"( ), the dynamics information of the system is contained in p(Af), which can be obtained by solving the reduced Liouville equations. Application of Eq. (6.19) to stimulated emission monitoring vibrational relaxation is given in Appendix III. 

It is useful to view optical absorption and emission processes in such a system in terms of transitions between distinct vibrational levels of the ground and excited electronic states of a metal atom-rare gas complex or quasi-molecule. Since the vibrational motions of the complex are coupled with the bulk lattice vibrations, a complicated pattern of closely spaced vibrational levels is involved and this results in the appearance of a smooth, structureless absorption profile (25). Thus the homogeneous width of the absorption band arises from a coupling between the electronic states of the metal atom and the host lattice vibrations, which is induced by the differences between the guest-host... 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

Optical Spectra and Fluorescence. The absorption-emission properties of the M[pz(A B )] A = S-R, B = 4,7-diisopropyloxybenzene have been discussed in Section II. 

What is the most important analytical aspect of optical emission methods Why are they in any sense better than optical absorption methods ... 

Fig. 8 Optical absorption of An., and two-photon emission spectrum after excitation at 1,290 nm for Au25 clusters [23]...

The imaginary part of the dielectric function describes the optical absorption in PS and thereby gives information about the bandgap. Details of the optical transitions responsible for absorption and emission of photons in Si are shown in Fig. 7.12 and will be discussed in the next section. The absorbed fraction P(x) of the non-reflected light intensity P depends on the sample thickness % and on the absorption coefficient a according to... 

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