Excitation-relaxation reaction

In the bimolecular reactions studied by Wilson, Hynes, and co-workers, 3,35.9i,92,io5 vibrational activation of a diatomic molecule is necessary for the reactants to climb the barrier. The issue of how this energy is transferred from the solvent into the diatomic (and how it decays back into the solvent from the excited diatomic product after the reagents have crossed the barrier) thereby becomes important. The dynamics of this excitation-relaxation reaction process are closely related to that of vibrational relaxation of diatomic molecules in solution. Vibrational relaxation is a subject that is well beyond the scope of this review, and we refer the reader to several reviews that cover the... 

Mullin A S and Schatz G C (eds) 1997 Highly Excited Molecules Relaxation, Reaction and Structure (ACS Symp. Ser. 678) (Washington, DC American Chemical Society)... 

Flynn G W, Michaels C A, Tapalian C, Lin Z, Sevy E and Muyskens M A 1997 Infrared laser snapshots vibrational, rotational and translational energy probes of high energy collision dynamics Highly Excited Molecules Relaxation, Reaction, and Structure ed A Mullin and G Schatz (Washington, DC ACS)... 

In the excited state, the redistribution of electrons can lead to localized states with distinct fluorescence spectra that are known as intramolecular charge transfer (ICT) states. This process is dynamic and coupled with dielectric relaxations in the environment [16]. This and other solvent-controlled adiabatic excited-state reactions are discussed in [17], As shown in Fig. 1, the locally excited (LE) state is populated initially upon excitation, and the ICT state appears with time in a process coupled with the reorientation of surrounding dipoles. 

Different photoreactions can be initiated in structurally related complexes of a metal ion as a result of the intrinsic properties of the LMCT excited state and radical-ion pairs. The excited-state reactions of azido complexes of Co(III) are one example of this chemical diversity.106-109 Irradiation of Co///(NH3)5N2+ aqueous acidic solutions in the spectral region 214 nm < 2exc < 330 nm produces Coin(NH3)4(H20)N, 6 0.6, and Co(aq)2 +, molar ratio.93 The ammonia photoaquation has two sources that also account for the large quantum yield of the photoprocess. One source competes with the formation of Co(aq)2 + from radical-ion pairs. These pairs must be produced with a quantum yield 0.5. The second source is a process unrelated to the Co(aq)2 + production and it has a quantum yield excited state where a Co-NH3 bond has been considerably elongated and where the electronic relaxation of the excited state has been coupled with aquation. A second rationale for the large aquation quantum yield is that a reactive LF excited state is populated by the LMCT excited state. 

Spectra of both CS and SO could be observed. Surprisingly, for it is the old bond in this reaction, the CS vibration was appreciably excited. Relaxation ofCSf was quite slow [214, 215] and the initial distribution could be determined it was roughly Boltzmann with rvib-1775°K, corresponding to 9% of the reaction energy entering this mode. It was more difficult to measure the relative vibrational concentrations of SO, since the reaction... 

Since it has been shown that in many exothermic reactions a large part of the energy liberated enters the vibration of the newly formed molecule, there has been considerable interest in developing vibrational lasers pumped by chemical reaction. Although a total inversion between vibrational states is not required [223], it is difficult to reach the threshold condition for lasing action if relaxation is faster than excitation by reaction. These effects can be reduced where a light flash (or pulsed discharge) initiates reaction, and stimulated emission has now been observed from a number of systems [224]. 

Since the development of ultrashort lasers, nudear wavepacket dynamics of various matters have attracted continuing attention [1,2]. The research targets extend from gas phase molecules [3, 4] to molecules in solution [5, 6], and solids [7]. In general, an excitation of matter by an ultrashort pulse with sufficient bandwidth leads to the creation of coherence between vibrational (or vibronic) eigenstates [1]. The induced nuclear wavepacket then starts to evolve on a certain potential energy surface and the dynamics is probed by a suitable pump-probe spectroscopy. The direct time-domain observation of the nudear motion provides us with valuable information on photochemical reaction dynamics, vibrational excitation/relaxation mechanisms, electron-vibration (phonon) coupling, and so on. 

The observation that the reaction requires an induction time of tens of picoseconds can be used to differentiate between proposed mechanisms of how shock wave energy localizes to cause chemical reaction. This induction time is expected for mechanisms that involve vibrational energy transfer, such as multiphonon up-pumping [107], where the shock wave excites low frequency phonons that multiply annihilate to excite the higher frequency modes involved in dissociation. It is also consistent with electronic excitation relaxing into highly excited vibrational states before dissociation, and experiments are underway to search for electronic excitations. On the other hand, prompt mechanisms, such as direct high frequency vibrational excitation by the shock wave, or direct electronic excitation and prompt excited state dissociation, should occur on sub-picosecond time scales, in contrast to the data presented here. 

This approach defines the vibrational relaxation of a large polyatomic molecule as the rate of decay of the average molecular energy rather than the rate of decay of a particular vibrational state. For the purpose of describing the excited-state reaction it is assumed that the concentration of SFg remains constant but that the rate constant increases with the average energy. 

Hahn, H.S., A. Nitzan, P. Ortoleva J. Ross. 1974. Threshold excitations, relaxation oscillations, and effect of noise in an enzyme reaction. Proc. 

Chemiluminescence (CL) is a phenomenon whereby the electronically excited state product of a chemical reaction generates optical radiation during relaxation to its ground state. There are two general mechanisms of CL that are employed in the context of detection for CE. In direct CL, the photon(s) are emitted by the excited state reaction product as it relaxes to the ground state. In the second, the relaxation of the excited state takes place via energy transfer to a fluorophore, which subsequently fluoresces this is referred to as sensitized CL, because a fluorophore with a high quantum yield can be used. 

Charge-Transfer Excited-State Relaxation and Excited State Reactions... 

Excited-state reactions are not restricted to ionization. Mai dynamic processes tiiat affect fluorescence can be interpreted in tenns of excited-state reactions. These processes include special relaxation, resonance trans-... 

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