Absorption coefficients visible, infrared

In the preceding discussion of Beer s Law, it was argued that x-ray absorption is a simpler process than the absorption of ultraviolet, visible, and infrared wavelengths. This greater simplicity becomes particularly obvious when x-ray absorption coefficients are examined. 

The spectrum k(p,T) thus calculated should be corrected, since, as pointed out by KRO, the asymptotic value p( ) from their calculations should be increased by 1050 cm (i.e. by a factor of 1.066) in order to obtain the correct experimental Na D transition frequency. Previously (L2) our correction procedure has been to multiply the frequency scale of the calculated spectrum by this factor of 1.066. It has been pointed out to us (D.D. Konowalow, private communication) that a more appropriate correction procedure is to increase the separation of the KRO potential curves involved by 1050 cm", i.e. to add 1050 cm to the frequency scale of the spectrum as calculated in the previous paragraph. We have adopted this latter procedure and a resulting spectrum of the reduced absorption coefficient k(i, T)/[Na] is given by the fully drawn curves in Figure 3 for T= 2000 K. The spectrum contains four partly overlapping contributions due to the four optically allowed Na2 transitions in the visible and near-infrared part of the spectrum, namely X Sg,... 

For the purpose of measuring the infrared spectra of unstable species such as free radicals in solution, it is advisable to place a small Fourier transform infrared (FT-IR) spectrometer into an inert-gas glovebox system, in which the concentrations of oxygen and water vapor can be kept at <0.1 ppm. In such an experimental setup, it is possible to generate unstable species in appropriate solvents and measure their infrared spectra in solution [6]. Preferably, a small ultraviolet/visible spectrometer should also be placed within the glove-box system. Then, it will be possible to determine the concentration of the unstable species under study by additionally measuring its ultraviolet/visible absorption spectrum, provided the molar absorption coefficient of the ultraviolet/visible absorption band of the unstable species is known. 

Spectroelectrochemistry encompasses a group of techniques that allow simultaneous acquisition of electrochemical and spectroscopic information in situ in an electrochemical cell. A wide range of spectroscopic techniques may be combined with electrochemistry, including electronic (UV-visible) absorption and reflectance spectroscopy, luminescence spectroscopy, infrared and Raman spectroscopies, electron spin resonance spectroscopy and ellipsometry. Molecular properties such as molar absorption coefficients, vibrational absorption frequencies and electronic or magnetic resonance frequencies, in addition to electrical parameters such as current, voltage or charge, are now being used routinely for the study of electron transfer reaction pathways and the fundamental molecular states at interfaces. In this article the principles and practice of electronic spectroelectrochemistry are introduced. 

Khalil OS, Yeh S J, Lowery MG, Wu X, Hanna CF, Kantor S, Jeng TW, Kanger JS, Bolt RA, de Mul FF. Temperature modulation of the visible and near infrared absorption and scattering coefficients of human skin. Journal of Biomedical Optics 2003, 8, 191-205. 

Ultraviolet and visible absorption spectroscopy has been a powerful aid to studies of free radicals in the gas phase but is of less value in the solid phase because of line broadening and consequent lack of resolution. Under suitable conditions, however, solid state infrared spectra can give information on the structure of free radicals and on the forces holding the constituent atoms in their equilibrium positions. The technique is less sensitive than e.s.r. because the extinction coefficients of vibrational transitions are generally low but the ability to observe non-para-magnetic molecules in addition to free radicals is sometimes advantageous. 

The metalloporphyrin V-cation radical spectrum. An extremly broad band covering the whole visible range of the spectrum and often extending into the near infrared is typical of a one-electron oxidation product of the porphyrin ligand. Sometimes strong visible absorption bands around 700 nm are also found with these products, but the broad visible absorptions are always present. The Soret band usually has a low extinction coefficient and is broadened considerably [e.g. Fuhrhop (76)]. 

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