Vibrational spectroscopy absorbed species

Infrared spectroscopy records the molecular vibrations of absorbing species, occurring as a result of their interaction with electromagnetic energy. The... 

The use of vibrational spectroscopy for the qualitative analysis of absorbed surface species is first considered, and a Table is then included which summarises a number of the key features of the various quantitative techniques. We then proceed to summarize these in groups depending not upon the probe used (as in the preceding chapters), but in terms of the signal emitted by the specimen which is used in each identification process. 

Raman and IR spectroscopies are complementary to each other because of their different selection rules. Raman scattering occurs when the electric field of light induces a dipole moment by changing the polarizability of the molecules. In Raman spectroscopy the intensity of a band is linearly related to the concentration of the species. IR spectroscopy, on the other hand, requires an intrinsic dipole moment to exist for charge with molecular vibration. The concentration of the absorbing species is proportional to the logarithm of the ratio of the incident and transmitted intensities in the latter technique. 

Good summaries of accepted experimental techniques can be found in the references that are cited for individual radionuclides in the sections below. Nitsche (1991) provides a useful general summary of the principles and techniques of solubility studies. A large number of techniques have been used to characterize the aqueous speciation of radionuclides. These include poten-tiometric, optical absorbance, and vibrational spectroscopy. Silva and Nitsche (1995) summarize the use of conventional optical absorption and laser-based photothermal spectroscopy for detection and characterization of solution species and provide an extensive citation list. A recent review of the uses of Raman and infrared spectroscopy to distinguish various uranyl hydroxy complexes is given by Runde et al. (2002b). 

Heptavalent neptunium and plutonium can be prepared in highly alkaline aqueous media via electrochemical or chemical oxidation of An, Arr, or An species, with Np being more easily obtained and isolated than Pu. The complexes formed have been characterized in solution primarily by optical absorbance and vibrational spectroscopy, and more recently by NMR and EXAFS, and in the solid state by EXAFS and X-ray diffraction. Most research in this area was conducted at the Russian Academy of Sciences. 

A solvent for ultraviolet/visible spectroscopy must be transparent in the region of the spectrum where the solute absorbs and should dissolve a sufficient quantity of the sample to give a well-defined analyte spectrum. In addition, we must consider possible interactions of the solvent with the absorbing species. For example, polar solvents, such as water, alcohols, esters, and ketones, tend to obliterate vibration spectra and should thus be avoided to preserve spectral detail. Nonpolar solvents, such as cyclohexane, often provide spectra that more closely approach that of a gas (compare, for example, the three spectra in Figure 24-14). In addition, the polarity of the solvent often influences the position of absorption maxima. For qualitative analysis, it is therefore important to compare analyte spectra with spectra of known compounds measured in the same solvent. 

The symmetric dire< t product finds its most extensive use in vibrational spectroscopy. When more than one quantum of u degenerate vibration is absorbed, the symmetry of the resulting states (overtone) are obtained from the symmetric direct product. In general, the species deriving from the pth overtone of a vibrational state of symmetry r(o() is... 

Photoelectron spectroscopy can also be carried out by measuring the distribution of flight times of photoemitted electrons. This method is useful in systems in which the absorbing species are produced by a pulsed source and the photolysis radiation is also pulsed. These electron time of flight methods have been used to elucidate structures of transient species such as free radicals and clusters produced in pulsed photolysis sources and in assessing the vibrational-state purity of ions produced in multiphoton ionization processes, particularly those in which the final photon absorption process is from a Rydberg state whose geometry is similar to that of the ion. 

Changes in the absorbance behavior of the vibrational bands of both bidentate carbonate species, as the amount of introduced CO was gradually inCTeased, were monitored using FT-IR spectroscopy. Two species, A and B in Chart 12.1, were detected when a small amount of COj was introduced onto MgO. The observed... 

Resonance Raman spectroscopy (RRS) leads to increased selectivity in Raman spectral measurements. The Raman spectrum of individual components in a complex mixture can be selectively enhanced by a judicious choice of laser wavelength. Only the Raman bands of the chromophore which is in resonance at the wavelength of excitation are significantly enhanced. Raman bands of non-absorbing species are not enhanced and do not interfere with those of the chromophore. Clearly, resonance Raman is a very sensitive analytical tool capable of providing detailed molecular vibrational information. 

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

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