Overlapping phenomena, separating

In (a), two photon waves combine to give a new waveform, which has the same appearance and frequency as the initial separate waves. The photons are said to be coherent, and the amplitude of the waves (light intensity) is simply doubled. In (b), the two photon waves are shown out of step in time (incoherent). Addition of the two waveforms does not lead to a doubling of amplitude, and the new waveform is more complex, composed of a doubled overlapping frequency. In (c), the two waveforms are completely out of step (out of phase) and completely cancel each other, producing darkness rather than light (an interference phenomenon). 

The principal mechanisms discussed above can explain the presence of broad rXH absorption regions consisting of separate or overlapping sub-bands. Although such absorption bands are found in a number of cases [4, 6], it is nevertheless true that in many instances the absorption regions are remarkably smooth in contour (Fig. 1). A number of explanations can be, and have been, advanced to explain this phenomenon. As it is not clear which is the most generally important mechanism—for example different ones may be applicable to different states of matter—we shall just list briefly the principal suggestions. 

According to Equation (5.35), energy transfer by the exchange mechanism is a short-range phenomenon since the exchange term decreases exponentially with the donor-acceptor separation / ao- Since it requires the intervention of an encounter complex (D A) it is also called the overlap or collision mechanism. The Wigner-Witmer spin-selection rules (Section 5.4.1) apply, and the spin-allowed processes... 

Since the transformation of C, into co defined by Eq. 9.15 is independent of both X = z/L and t = ut/L, the solution scheme is applicable to any point in the (x, t) plane if an initial discontinuity is imposed at that point. If discontinuities are imposed at two or more points along the x or the t axis, the solution may be derived by constructing separately the wave solutions for each of these points. However, after a finite period of time, any two wave solutions centered in two different, but adjacent, points will meet each other. An overlap region appears, where the solution is influenced by both sets of data. Such a phenomenon is called wave interaction. From what was explained in the previous section, this interaction takes place during the formation of the isotachic train of any mixture. 

The second class of indicators benefits from a quantum-mechanical phenomenon called fluorescence resonance energy transfer (FRET) that occurs when chromophores of two separate fluorescent proteins are less than 8 nm apart [76]. These proteins can either be linked to each in a fusionprotein (intramolecular FRET) or can be separate entities (intermolecular FRET). The emission spectrum of the donor fluorophore should overlap the excitation spectrum of the acceptor fluorophore, but both excitation spectra should be well enough separated to allow independent excitation. When the ratio of donor and the acceptor molecules is... 

---