Optical absorption spectroscopy

It is a remarkable fact that the contemporary history of absorption and emission spectroscopy began simultaneously, from the simultaneous discoveries that Bunsen and Kirchhoff made in the middle of the 19th century. They observed atomic emission and absorption lines whose wavelengths exactly coincided. Stokes and Kirchhoff applied this discovery to the explanation of the Fraunhofer spectra. Nearly at the same time approximately 150 years ago, Stokes explained the conversion of absorbed ultraviolet light into emitted blue light and introduced the term fluorescence. Apparently, the discovery of the Stokes shift marked the birth of luminescence as a science. 

A rule due to Beer and Lambert is that the light absorbed per imit length in a sample depends at any wavelength only on the incident Ught intensity, i.e.. 

There are a number of different processes, which may generate optical absorption in the visible and ultraviolet wavelength range. The following transitions are of primary importance for minerals (Platonov 1979 Rossman 1988)  

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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. 

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiating in the solid state and at very low temperatures. Then, the intermediates can be measured at the end of the irradiation by optical absorption spectroscopy or ESR. 

Time-resolved optical absorption spectroscopy experiments have shown that arenesul-fonyl radicals decay with clean second-order kinetics14 the values of 2 k,/a h where s2 is the extinction coefficient at the monitoring wavelength, increased linearly with decreasing viscosity of the solvent, further indicating that reaction 16 is clearly a diffusion-controlled process. 

Saito et at.130 studied the salts of TMTSF and the sulfur analogue tetra-methyltetrathiafulvalene, TMTTF, with a polycyano dianion. Although the conductivity of both compounds was low (crrt = 10-5 Scm-1 for TMTSF vs. 10-7 Scm-1 for TMTTF) the conductivity of the Se-donor salt was improved by two orders of magnitude. Optical absorption spectroscopy was also used to assess the materials. The electronic transition between radical cations within the segregated donor columns occurred at considerably lower energy (8800 cm-1) in the TMTSF salt than in the TMTTF (11500 cm-1). A concurrent improvement... 

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. Then, the intermediates can be detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR study of radicals in the solid state is done on single crystals, polycrystalline samples or frozen aqueous solution. In case of polycrystalline samples or frozen aqueous solution the identification of the radicals from the ESR spectra is difficult in many cases and, for better identification, the ESR experiment should be conducted on irradiated single crystals. Later, the method of spin trapping, developed for the liquid phase5, was extended to polycrystalline solids. In this technique the polycrystalline solids are /-irradiated and subsequently dissolved in a solution containing the spin trap. 

UV/vis optical absorption spectroscopy The first and important characterization is optical absorption. The synthesis of clusters can be monitored by the appearance of the optical absorption features. Each cluster has a characteristic absorption spectrum and can be distinguished from each other by their characteristic absorption features. 

Winer, A. M and H. W. Biermann, Long Pathlength Differential Optical Absorption Spectroscopy (DOAS) Measurements of Gaseous HONO, N02, and HCHO in the California South Coast Air Basin, Res. Chem. Intermed., 20, 423-445 (1994). 

Armerding, W., M. Spiekermann, J. Walter, and F. J. Comes, Multipass Optical Absorption Spectroscopy A Fast Scanning Laser Spectrometer for the In-Situ Determination of Atmospheric Trace Gas Components, in Particular OH, Appl. Opt., 35, 4206-4219... 

Brauers, T., M. Hausmann, U. Brandenburger, and H.-P. Dorn, Improvement of Differential Optical Absorption Spectroscopy with a Multichannel Scanning Technique, Appl. Opt., 34, 4472-4479 (1995). 

Plane, J. M. C., and N. Smith, Atmospheric Monitoring by Differential Optical Absorption Spectroscopy, in Spectroscopy in Envi-... 

Platt, U Differential Optical Absorption Spectroscopy (DOAS), in Air Monitoring by Spectroscopic Techniques (M. W. Sigrist, Ed.), Chemical Analysis Series, Vol. 127, pp. 27-84, Wiley, New York, 1994. 

Stutz, J., and U. Platt, Numerical Analysis and Estimation of the Statistical Error of Differential Optical Absorption Spectroscopy Measurements with Least-Squares Methods, Appl. Opt., 35, 6041-6053 (1996),... 

Optical absorption spectroscopy is often carried out on CD films to verify that the films have a bandgap expected from the deposited semiconductor. Additionally, since CD films are often nanocrystaUine and the most apparent effect of very small crystal size is the increasing bandgap due to size quantization (the effect is visible to the eye if the bandgap is in the visible region of the spectrum), absorption (or transmission) optical spectroscopy is clearly a fast and simple pointer to crystal size, since bandgap-size correlations have been made for a number of semiconductor colloids and films. 

In a similar study, the deposition conditions were modified [pH of 8 (by ammonia), no Ag catalyst, and a deposition temperatnre of 80°C] [30], A primary thin film was deposited, followed by a second deposition, resnlting in films several microns thick. Optical absorption spectroscopy gave abandgap of 1.7 eV. The film resistivity was 1.3 X 10 ft-cm (carrier density = 8 X 10 cm mobility = 50 cm V sec ). 

The NiO was confirmed by XRD. Optical absorption spectroscopy was used to estimate a direct bandgap of 1.75 eV (see later). The films were p-type (thermoelectric probe), with a resistivity of 10 O-cm. 

The bandgap, calculated from optical absorption spectroscopy, varied almost linearly with composition between that of CdS (2.4 eV) and ZnS (3.6 eV), providing further evidence for solid solution formation. 

A considerable spectrum of techniques also exists for studying the surfaces of solid oxidation catalysts. Several of these are discussed below. No attempt has been made here to be comprehensive. For example, the powerful tool of optical absorption spectroscopy (particularly infrared) is now so well-known and widely used that there is no need here to emphasize its importance. 

The cross section a is a fundamental property of the molecule and as such is related to the molecular wave functions for the two states between which a transition is induced. Hence it is desirable to separate the contributions to a that arise from purely kinematic quantities such as the impact energy of the electron beam from those that depend solely on the properties of the molecule. To this end, a dimensionless quantity, the oscillator strength, is introduced in optical absorption spectroscopy, defined by the relation22... 

GASCOD-A uses the differential optical absorption spectroscopy (DOAS) technique. It operates in the UV-visible spectral region and enables the detection of the trace gases listed in Table 4. The deliveries of GASCOD-A are total amounts of the trace constituents at zenith and nadir as well as vertical profiles. 

GOME is a nadir sounding spectrometer which observes the up-welling radiance from the top of the atmosphere and the extra-terrestrial solar irradiance between 240 and 790 nm. The resolution of the measurements is chosen to be suitable for the application of the differential optical absorption spectroscopy (DOAS) technique, which was developed for long-path measurements and zenith sky observations (e.g. Platt and Pemer, 1980 Mounter a/., 1987 Eisinger etal., 1997). 

Platt, U. and D. Pemer (1980) Direct measurements of atmospheric HCHO, HONO, O3, NO2, and SO], by differential optical absorption spectroscopy in the near UV. Journal of Geophysical Research 85 7453-7465. 

In the CrCl units the combined distortions along the a-jg and eg coordinates lead to a compressed octahedron, with equatorial Cr-Cl distances approximately 0.1 larger than in the ground state and a much smaller axial compression. Similar distortions were deduced from optical absorption spectroscopy for the 2- state in CrCNHj) (18) as well as the t and states in CoCNHj) (17). There are im-... 

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