Emission, sensitized, identification excited states

Molecular photophysics, especially the use of steady-state and time-resolved luminescence spectroscopy, have many important applications and there has been a progressive emergence of a new field of analytical chemistry based on these principles. It has been known for many decades that the excited state properties of certain molecules are highly sensitive to the local environment but it is only recently that a concerted effort has been made to use this sensitivity in a practical way. The main approaches to employing variations in photophysical properties as an analytical tool can be divided into two areas namely, (i) development of luminescent probes that respond to changes in the environment and (ii) identification of molecular systems for which the emission... 

As indicated in Fig. 16.10 for Cf ", / r = 0.47 for emission from an excited (J = 5/2) state to a lower-lying (J = 11/2) state, while / r = 0.14 for emission to the ground state. In the case of the J = 5/2 state, it would be appropriate to monitor for fluorescence near 13000cm as well as near 20 000 cm The identification of the mechanisms of non-radiative relaxation of actinide ions in solution as well as in solids [57] remains an important area for research. Extensive experimental results for lanthanide systems are available for comparison with those obtained for actinide ions. It should be possible to explore sensitively bonding differences between selected actinides and lanthanides by examining their excited-state relaxation behavior. 

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

The discussion in Section II-B indicates that optical emission from 02(1E 7+) or 02(1A9) to the ground state may provide a useful method for the identification and estimation of the excited species. In laboratory studies, the (0, 0) bands, lying at about 7620 A and 1.27 [x, respectively, are likely to be the strongest. The emission at 7620 A is relatively easily detected by suitable photomultipliers, and spectra may even be recorded with photographic emulsions sensitive to the near infrared (such as the Kodak N coating). Trialkali (S20) photocathodes combine a high sensitivity with low dark current, and photomultipliers with an S20 cathode... 

Photoluminescence Spectroscopy Photoluminescence (PL) is a type of luminescence in which the spontaneous emission of light takes place from a material under optical excitation. The technique requires very little sample manipulation or environmental control. Because the sample is excited optically, electrical contacts and junctions are not required and high-resistivity materials pose no practical difficulty. In addition, time-resolved PL can be very fast, making it useful for characterizing the most rapid processes in a material. The fundamental limitation of PL analysis is its reliance on radiative events. Materials with poor radiative efficiency such as low-quality indirect band gap semi-conductors are difficult to study via ordinary PL. Similarly, identification of impurity and defect states depends on their optical activity. Although PL is a very sensitive probe of radiative levels, one must rely on secondary evidence to study states that couple weakly with light. 

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