Emission frequencies

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

It is interesting to note that for a resonant transition (i.e. coinciding absorption and emission frequencies), the reciprocal of the radiative lifetime is equal to the Einstein coefficient Ai for spontaneous emission (see Box 3.2). 

VM vd monomer and excimer emission frequency. A excimer, AHa enthalpy of excimer formation. 

Values of the radiative rate constant fcr can be estimated from the transition probability. A suggested relationship14 57 is given in equation (25), where nt is the index of refraction of the medium, upper states. It is assumed that the absorption and emission spectra are mirror-image-like and that excited state distortion is small. The basic theory is based on a field wave mechanical model whereby emission is stimulated by the dipole field of the molecule itself. Theory, however, has not so far been of much predictive or diagnostic value. 

Table 2.1 Unidentified astronomical IR emission frequencies and their typical assignments...

In this approach, ethylene gas was detected using the 944.2 cm-1 laser emission frequency down to concentration levels of 100 ppm. 

The authors in 1920 compared the frequencies of certain L emission lines with the differences in frequency between corresponding L and M absorption limits.1 Kossel first pointed out that these quantities should be equal on Bohr s theory. At that time we found each L emission frequency to be equal to a corresponding difference in absorption frequencies within the limits of experimental error. Coster2,8 has recently extended... 

In each case the emission frequency exceeds the difference in absorption frequencies, but by an amount that is less than 0.3 of a per cent. 

Burdick, D.S., Tu, X.M., McGown, L.B., and Millican, D.W., Resolution of multi-component fluorescent mixtures by analysis of the excitation-emission-frequency array, J. Chemom., 4, 15-28, 1990. 

The time dependent emission frequency is finally obtained by subtracting from Sasit) the corresponding values of SK(t), defined in Eq. (7-45), calculated at the same t. 

In certain cases the absorption and emission frequencies may be distinguished for CIP and SSIP so that the concentration ratios may be measured [22, 31]. However, the use of time resolved measurements are necessary when superimposed spectra of the two ion pairs are found. This type of measurement was made in the case of -naphtholate anion [24, 35] and the main results are... 

Condition 2. Both vs and vs should be very different from emission frequencies of the molecule M to avoid confusion of sensitized with unsensitized emission. 

The determination of the photoluminescence parameters (excitation frequency, emission frequency, Stokes shift, fine structure parameter, and lifetime) can lead to information which, at the simplest level, indicates the presence of an electronically excited state of a species, but which can be sufficiently detailed so as to lead to a clear identification and characterization of the photoluminescent sites(J6-44). Moreover, measurements of the variations in the intensity and positions of the bands as a function of time (time-resolved photoluminescence) provide valuable kinetic data representing the reactions occurring at the surface. Although most of the photoluminescence measurements have been carried out at low temperatures for specific reasons (see Section III.C.2), there is much evidence that some of the excited states involved are present even at higher temperatures and that they play an important role in catalytic and photocatalytic reactions. Therefore, it is clear that the information obtained by photoluminescence techniques is useful and important lor the design of new catalysts and photocatalysts. 

When absorption measurements were made, also at liquid helium temperatures, we showed that for one suspected X-trap the absorption and emission frequencies were coincident within 2 cm l and that its vibrational intervals corresponded to those of anthracene itself, so confirming the X-trap character. The second X-trap was not found. Moreover we showed [132] that under irradiation the 2-hydroxyanthracene located at one of the trapping sites was destroyed, presumably by joining an anthracene molecule to form a chemically bonded pair, analogous to dianthracene, of which the optical properties were different. Chemical analysis supported the mixed pair interpretation. 

A pp chromophore dissolved in a polar solvent readjusts its polarity in response to the polarity of the surrounding, and its properties are affected accordingly. But other more subtle phenomena can be predicted, including amplification of NLO responses [74, 87], and inhomogeneous broadening effects as observed e.g in vibrational and resonant-Raman spectra [74]. Particularly interesting results were obtained modeling time-resolved absorption and fluorescence spectra of pp chromophores in polar solvents [88], were we were able to reproduce quantitatively the time evolution of the emission frequency and bandshapes as experimentally observed for several dyes. 

In passing, we note that parametric four-wave mixing processes could in principle contribute to the effects described above. Thus, in the single-beam case, the CARS process co + co cOi + CO2 single center could also result in the synergistic excitation of two chemically different centers. Equally in the double-beam case, the four-wave interaction coi +o 2-> (o + (o followed by absorption of the frequency co, could contribute to the excitation of a pair of neighboring molecules of the same species. However, both of these four-wave interactions will be relatively ineffectual unless (one of) the emission frequencies is stimulated by an additional source moreover, the processes described here are not associated with the wave-vector matching characteristics of CARS and related phenomena. 

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