Visible/ultraviolet absorption experiments

Table XIV. Visible/Ultraviolet Absorption Spectroscopy Experiments...

The spectroscopy experiments are further subdivided into atomic spectroscopy found in Table XII, infrared and Raman spectra found in table XIII, visible and ultraviolet absorption spectroscopy found in table XIV, and luminescence spectroscopies found in table XV. 

Direct correlations between the results of MO calculations and those of experiments (on electronic structure) have until recently been plagued by a number of problems related to the experimental state of the art The data obtained from visible and ultraviolet absorption spectra arise from transitions between occupied and unoccupied levels, and hence represent a convoluted density of states. In addition, valence electronic transitions in solids in the range of 10-40 eV are accessible experimentally only under limited and difficult conditions deeper spectral regions require varied experimental techniques to probe them. 

Dye lasers, frequency doubled if necessary, provide ideal sources for such experiments. The radiation is very intense, the line width is small ( 1 cm ) and the wavenumber may be tuned to match any absorption band in the visible or near-ultraviolet region. 

Low-temperature, photoaggregation techniques employing ultraviolet-visible absorption spectroscopy have also been used to evaluate extinction coefficients relative to silver atoms for diatomic and triatomic silver in Ar and Kr matrices at 10-12 K 149). Such data are of fundamental importance in quantitative studies of the chemistry and photochemistry of metal-atom clusters and in the analysis of metal-atom recombination-kinetics. In essence, simple, mass-balance considerations in a photoaggregation experiment lead to the following expression, which relates the decrease in an atomic absorption to increases in diatomic and triatomic absorptions in terms of the appropriate extinction coefficients. 

Almost all spectroscopic studies of free radicals have been carried out in the visible and ultraviolet regions, and as a consequence only those radicals that have discrete spectra in these regions have been amenable to detailed investigation. Our inability to find any spectroscopic evidence for certain radicals, such as BH3, C2H, CF3, and others, in spite of their almost certain presence under the conditions of our experiments, is probably due to the fact that these radicals have no stable excited states, and therefore give rise only to (weak) continuous absorptions, which are difficult to detect and identify. (If CH4 were a free radical, it would be exceedingly difficult to detect spectroscopically, since it has only continuous absorptions except, of course, in the infrared.)... 

In 1964, the spin echo experiment was extended to the optical regime by the development of the photon echo experiment (3,4). The photon echo began the application of coherent pulse techniques in the visible and ultraviolet portions of the electromagnetic spectrum. Since its development, the photon echo and related pulse sequences have been applied to a wide variety of problems including dynamics and intermolecular interactions in crystals, glasses, proteins, and liquids (5-8). Like the spin echo, the photon echo and other optical coherent pulse sequences provide information that is not available from absorption or fluorescence spectroscopies. 

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