Excited states physical processes

Any excited-state relaxation process necessarily involves the dissipation of energy from the electronically excited molecular system to the environment. This energy may appear as light, in emission as discussed above, as heat, as electrical energy (in some heterogeneous systems) and/or in some chemical form. The emphasis in this chapter is on the physical processes. 

Intermolecular Physical Processes of Excited States Photosensitization Processes... 

This volume contains seven chapters written by international experts on the photochemishy and photophysics of organic, inorganic, and biological molecules. The first chapter by Steer and coworkers summarizes the current status of upper excited state physics of organic and inorganic molecules. Almost five decades ago it was believed that no measurable processes are likely from S, S3, T, and Tj states however, Chapter 1 makes it evident that this belief is no longer valid. 

Excited states can also be quenched. Quenching is the same physical process as sensitization, but the word quenched is used when a photoexcited state of the reactant is deactivated by transferring its energy to another molecule in solution. This substance is called a quencher. 

Fig. 13.11. A schematic drawing of the potential energy surfaces for the photochemical reactions of stilbene. Approximate branching ratios and quantum yields for the important processes are indicated. In this figure, the ground- and excited-state barrier heights are drawn to scale representing the best available values, as are the relative energies of the ground states of Z- and E -stilbene 4a,4b-dihydrophenanthrene (DHP). [Reproduced from R. J. Sension, S. T. Repinec, A. Z. Szarka, and R. M. Hochstrasser, J. Chem. Phys. 98 6291 (1993) by permission of the American Institute of Physics.]...

In turn, 1O2 is a very electrophilic excited state species of molecular oxygen that interacts efficiently with electron-rich molecules, such as aminoadd residues of proteins like histidine, metionine, tryptophan, tyrosine, etc., by both physical and chemical quenching processes, eqns. 9 and 10 (Davies, 2003 Bisby et al., 1999). 

Figure 2 (a) The optimized electric field as a function of time for the H2(v = 0,) = 0) — H2 (v = 0,7 = 2) rotational excitation process, (b) Absolute value of the Fourier transform of the optimized electric field, (c) The change in populations of the ground-and target excited-state shown as a function of time. Taken from Ref [24] with permission from Qinghua Ren, Gabriel G. Balint-Kurti, Frederick R. Manby, Maxim Artamonov, Tak-San Ho, and Herschel Rabitz, 7. Chem. Phys. 124, 014111 (2006). Copyright 2006, American Institute of Physics. 

Gas-phase ion chemistry is a broad field which has many applications and which encompasses various branches of chemistry and physics. An application that draws together many of these branches is the synthesis of molecules in interstellar clouds (Herbst). This was part of the motivation for studies on the neutralization of ions by electrons (Johnsen and Mitchell) and on isomerization in ion-neutral associations (Adams and Fisher). The results of investigations of particular aspects of ion dynamics are presented in these association studies, in studies of the intermediates of binary ion-molecule Sn2 reactions (Hase, Wang, and Peslherbe), and in those of excited states of ions and their associated neutrals (Richard, Lu, Walker, and Weisshaar). Solvation in ion-molecule reactions is discussed (Castleman) and extended to include multiply charged ions by the application of electrospray techniques (Klassen, Ho, Blades, and Kebarle). These studies also provide a wealth of information on reaction thermodynamics which is critical in determining reaction spontaneity and availability of reaction channels. More focused studies relating to the ionization process and its nature are presented in the final chapter (Harland and Vallance). 

Figure 3 Different processes for losing energy from the excited state (1) direct CL (2) molecular dissociation (3) chemical reaction with other species (4) intramolecular energy transfer (5) intermolecular energy transfer (in case of a fluorophore, indirect CL) (6) isomerization (7) physical quenching. (Adapted from Ref. 1.)...

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