Absorption spectroscopic sensing

Direct absorption spectroscopic sensing, in some cases, fulfills the above requirements. In many cases, however, lack... 

Absorption spectroscopic sensing, general discussion, 271 Acetic acid sensors, 334 Activities of ions, relation to interfacial potential difference, 369... 

Absorption Bands. Success of remote NIR spectroscopic sensing depends on the identification and isolation of unique spectral bands for the targeted analytes. These analyte bands must be distinguishable from those of water and other analytes in the sample. Water has large absorption bands with peak maxima at 6876, 5267, and 3800 cm in the NIR (12). The spectral region of 5000-4000 cm corresponds to an optical window between two large water absorption bands with a minimum around 4500 cm ... 

Wavelengths preferable for gas detection are listed in Table 14.4. NIR absorption bands are typically broad, overlapping, and 10-100 times weaker than their corresponding fundamental mid-IR absorption bands. These characteristics severely restrict sensitivity in the classical spectroscopic sense and require chemometric data processing to relate spectral information to sample properties. The low absorption coefficient, however, permits high penetration depth and, thus, an adjustment of sample thickness. 

This knowledge and understanding may be helpful to characterize local conformations of other optically active polysilanes in solution. For example, poly(methyl-(-)-(3-pinanylsilane) [(+)-7 Mw = 10,200] prepared by Shinohara and co-workers.281 showed a bisignate CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at 15°C. Since the spectroscopic features are quite similar to those of i,28d-28e it is possible that the main chain in 7 may contain diastereomeric helical motifs with opposite screw senses and different screw pitches. 

It is possible that the helicity is a result of the chiral substitution itself and that the polymers with achiral substituents have, in fact, all-anti conformations. While this possibility cannot be directly ruled out, comparison of the spectroscopic data for the polymers with chiral substituents and achiral substituents, for example, 47 and 48, respectively, indicates similar main-chain dihedral angles, since the UV absorption maxima are so similar. Both polymers should therefore be latent helical, that is, contain segments of opposite screw sense separated by strong kinks (helix reversal points), with the difference being that in the case of 47 the overall numbers of P and M turns are equal, whereas for 48, one of the screw senses predominates, resulting in net helicity and optical activity. 

Interaction-induced absorption by the vibrational or rotational motion of an atom, ion, or molecule trapped within a Ceo cage, so-called endohedral buckmin-sterfullerene, has excited considerable interest, especially in astrophysics. The induced bands of such species are unusual in the sense that they are discrete, not continuous they may also be quite intense [127]. Other carbon structures, such as endohedral carbon nanotubes, giant fullerenes, etc., should have similar induced band spectra [128], but current theoretical and computational research is very much in flux while little seems to be presently known from actual spectroscopic measurements of such induced bands. 

When deciding to study the dynamics of electronic excitation energy transfer in molecular systems by conventional spectroscopic techniques (in contrast to those based on non-linear properties such as photon echo spectroscopy) one has the choice between time-resolved fluorescence and transient absorption. This choice is not inconsequential because the two techniques do not necessarily monitor the same populations. Fluorescence is a very sensitive technique, in the sense that single photons can be detected. In contrast to transient absorption, it monitors solely excited state populations this is the reason for our choice. But, when dealing with DNA components whose quantum yield is as low as 10-4, [3,30] such experiments are far from trivial. 

The sun, an important factor to take into account in remote sensing using spectroscopic imaging instruments. The passive optical system and the atmosphere through which the energy passes, both from the sun to the earth s surface and back to the instrument, interferes with the data collected. Atmospheric distortions include the effect of scattered dry air molecules (haze) and absorption by air molecules. 

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