Intermolecular energy transfer population

Although excimer (exciplex) fluorescence is also exhibited by most dinucleotides,133 the observed phosphorescence from these systems, and from DNA, is characteristic of the lowest molecular triplet state. In the case of DNA at low temperatures this is identified132 as the triplet state of thymine which, in the absence of molecular intersystem crossing, must be populated by intermolecular energy transfer in the triplet manifold or by intersystem crossing from the XAT exciplex.134... 

C. Intermolecular Electronic Energy Transfer Quenching of one excited-state species may result in transfer of electronic energy to the quenching species. Intermolecular energy transfer has become an important technique in photochemistry because it often permits the selective population or depopulation of one specific excited-state species. 

In related experiments, COF, (formed in situ) has been shown to reduce the output power of DF-CO, transfer chemical lasers [553,611]. These lasers, which achieve population inversion by intermolecular energy transfer to the asymmetric stretching mode of CO, viz. 

As already discussed in Section 13.1, the multiphonon pathway for vibrational relaxation is a relatively slow relaxation process, and, particularly at low temperatures the system will use other relaxation routes where accessible. Polyatomic molecules take advantage of the existence of relatively small frequency differences, and relax by subsequent medium assisted vibrational energy transfer between molecular modes. Small molecules often find other pathways as demonstrated in Section 13.1 for the relaxation ofthe CN radical. When the concentration of impurity molecules is not too low, intermolecular energy transfer often competes successfully with local multiphonon relaxation. For example, when a population of CO molecules in low temperature rare gas matrices is excited to the v = 1 level, the system takes advantage ofthe molecular anhannomcity by undergoing an intermediate relaxation of the type... 

In order to maintain state-specific populations, low pressures are required. If pressures are raised to the point where collisions are frequent, then specific effects of laser excitation are lost and intermolecular energy transfer heats the entire sample. Reactions induced in this fashion then yield products characteristic of thermal processes. A particularly interesting aspect of this pyrolysis technique is the homogeneous nature of the reaction. Since heating is confined to the region near the laser beam, heterogeneous effects due to hot walls in a conventional pyrolysis reactor are avoided. 

Chemiluminescence has been observed from chelate complexes of Eu ". For the case of the complex Eu(dbm)3(pyridine), the Do excited state can be populated by energy transfer from the electrogenerated singlet state exciplex BF /TPTA (where BP is benzophenone and TPTA is tri-p-tolylamine). A similar chemiluminescent system has been observed in the thermal decomposition of trimethyl-1,2-dioxetane to a triplet excited state in the presence of Eu(Ha)3(phen). The major fraction of the emission is observed from the Do state of Eu ", which is populated by intermolecular energy transfer. " ... 

Figure 18 Direct and indirect intermolecular vibrational energy transfer in a polyatomic liquid mixture consisting of molecules A and B. In polyatomics, direct transfer does not often compete efficiently with VER. Indirect transfer from A to B occurs when A undergoes VER, which produces phonons, which pump vibrations on B. Indirect transfer is efficient only when the density of excited vibrations is large enough to significantly increase the phonon population.

Clarke (326) has studied the optical electron spin polarization in triplet anthracene and has observed ESR emission at 1.5°K which was attributed to a non-Boltzman distribution over the triplet spin levels at low temperature. The dynamics of optical spin polarization in triplet naphthalene at 1.6°K was also reported by Sixl and Schwoerer (327a) and van der Waals et al. (327b). have used a general method to study dynamics of populating and depopulating triplet spin levels by microwave-induced delayed phosphorescence. These experiments enable measurements of the lifetimes of each triplet spin state and thus can provide important information about intramolecular decay processes and intermolecular triplet energy transfer. 

The preparation of nonequilibrium level or species populations is the first step in any kinetic experiment. The introduction of lasers to chemical research has opened up new possibilities for preparing, often state-selectively, the initial nonequilibrium states. However, the subsequent time evolution of the molecular populations occurs almost invariably along several relaxation pathways. Some of which, like intra- and intermolecular vibrational energy transfer in infrared multiphoton absorption experiments, may interfere with the exciting laser pulse and/or with the specific process investigated. In such cases, as in chemical laser research, one has to interpret the behavior of complex nonequilibrium molecular systems in which the laser radiation plays of course a major role. This establishes the link between the present article and the general subject of this volume. 

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