Energy transfer in electronically excited

COLLISION-INDUCED INTRAMOLECULAR ENERGY TRANSFER IN ELECTRONICALLY EXCITED POLYATOMIC MOLECULES... 

The title of this chapter seems to promise a general discussion of the nature of collision-induced intramolecular energy transfer in electronically excited polyatomic molecules. If interpreted as just stated, the title promises more than can be delivered at this time. It is only recently that advances in experimental technique have permitted the study of the pathways of intramolecular energy redistribution following collision, and the few results now available were neither anticipated nor can they yet be fully accounted for by the available theories of collision-induced energy transfer. This chapter describes a preliminary synthesis of the limited experimental and theoretical information in hand and discusses some of its implications. It will be seen that more questions are raised than are answered. 

The experimental data surveyed in Section III have several striking features. Although the data base is too small to permit definitive conclusions to be drawn, it seems likely that the extremely large cross-sections and the existence of propensity rules will be characteristic of all collision-induced intramolecular energy transfer in electronically excited molecules. 

The survey of theory and data presented in this chapter shows that we are barely on the threshold of an imderstanding of collision-induced intramolecular energy transfer in electronically excited molecules. So few experimental data are available that there are likely many qualitatively new phenomena as yet undiscovered. And, at least at present, the theory of collisions has not provided predictions or qualitative concepts which can be used to guide experimental studies, nor has it been developed to the point that it can fully account for the observations in hand. Clearly, much remains to be done in this subject area. 

Collision Induced Intramolecular Energy Transfer in Electronically Excited Polyatomic Molecules... 

S.A. Rice, Collision-induced intramolecular energy transfer in electronically excited polyatomic molecules. Adv. Chem. Phys. 47, 237 (1981)... 

M.H. Alexander, A. Benning, Theoretical studies of collision-induced energy transfer in electronically excited states. Ber. Bunsenges. Phys. Chem. 14, 1253 (1990)... 

Excited n molecules are not discussed here. However, energy transfer in-electronically excited states has been recently studied. In this case a complete preparation of different initial quantum states is possible by laser excitation. The subsequent collisional redistribution can be studied by dispersing the fluorescence or probing the neighboring levels with a second laser. In this way state-to-state data can be obtained. Systems investigated include ZnH, CdH and CaF(A n 2 3/2 gases. 

It can be seen from the above that it is necessary to account for coUisional effects on the emission quantum yields when interpreting chemiluminescence profiles to deduce mechanistic information. In this section, we summarize the results of a series of experiments on quenching and energy transfer in electronically excited OH and CH, which are pertinent to such flame studies. 

In addition, the quenching of the fluorescence of fluorophore groups in protein molecules by neighboring groups(35) and its temperature dependence, t36) energy transfer of electronic excitation and its dependence on excitation wavelength,(1) the type of emission decay kinetics,(1,2) and changes... 

V.M. Agranovich and M.Yu. Galanin, Energy Transfer of Electronic Excitation in Condensed Media, Nauka, Moscow, 1978, p. 383 (in Russian). 

Understanding energy and charge transfers in electronically excited molecules is of fundamental importance to photochemistry and photobiology. Traditionally, excited-state dynamics of organic molecules are described in terms of the low-lying tttt and... 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

In a quite general manner, the scheme of an energy transfer through electronic excited states may be written in one of the following forms ... 

Energy transfer permits electronic excitation of molecules A that do not absorb the incident light. This is exploited, for example, for light harvesting in photosynthetic... 

Energy transfer from electronically excited molecules to ground-state molecules of different chemical composition represents a highly important intermolecular deactivation path. In general terms, energy transfer occurs according to Eq. (1-7) from a donor to an acceptor, the latter frequently being referred to as a quencher. 

Intramolecular proton transfer in electronically excited molecules has been reviewed in salicyclic add esters it is said to lead to deexdtation of the exdted electron the free energy change involved in that proton transfer is reported as about 0.13-0.22 eV. ... 

Proton transfers in electronically excited states have not been amenable to any reasonable interpretation in terms of the theory of Marcus, in part due to the implicit assumption of the symmetry of the potential energy curves of reactant and product [24,38]. In contrast, ISM provides a simple interpretation of this kind of reactions [39]. The excited-state reactions appear to follow the same basic principles of their ground-state analogues the transition state bond order does not change appreciably from the ground to the excited state. However, the mixing entropy parameter X decreases an enhancement of the dipole moment upon eletronic excitation can increase the suddenness of the repulsive wall of the reaction and decreases X. 

When the energy of the electron drops below 5 eV (Case d) there is no more possibility for energy loss in electron excitation processes. The electron scatters in the medium losing energy in very slow energy transfer processes to the vibration/rotation levels of the molecules. 

We shall concentrate in this contribution on energy transfer in electronically adiabatic phenomena involving collisions of atoms with diatomics and with polyatomics. We shall not deal with collisions involving electronic excitations. The formalism can be written down for these cases but not much has yet been done to develop the computational methods required in applications. This is in great part due to the lack of information on interaction-potential energies of electronically excited states and on their couplings due to nuclear motions, for polyatomic systems. Similarly, the formalism can be extended to include rearrangement collisions. Little is known however about interaction potentials for reactions... 

Energy disposal can also be achieved by energy transfer from Ar [120 to 122], Ar + NH3- NH2(5<) + H +Ar. In this energy transfer process, electronically excited NH2 radicals are obtained [122]. NH2(X) radicals were observed upon energy transfer from N2(A) to NH3 [123]. 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Agranovioh V M and Galanin M D 1982 Electronic Excitation Energy Transfer in Condensed Maffer (Amsterdam Elsevier/North-Flolland)... 

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