Decay channels, electronic relaxation

Owing to the electron-vibrational interaction in molecules, there is one more possible decay channel for SES. This is the nonradiative relaxation (internal conversion), in which the electron energy is transferred into vibrational energy of molecules (in the condensed phase, into thermal energy of the medium). If the molecule fluoresces, there may also occur fluorescence from the lowest excited state. (According to the empirical rule of Kasha,64 the molecular fluorescence occurs from the lowest excitation level irrespective of the wavelength of the exciting radiation.)... 

The rates k/ and k correspond to any nonradiative decay channel, which couple to the levels /) and /n). It should be kept in mind, however, that reverse collisionally induced electronic transitions may follow vibrational relaxation in the /) manifold, thus leading to further emission. This effect must be taken into account when the jj) level is not the lowest one in the j>) manifold (see Section III.B). 

In molecular reactions, electronic excitations as a consequence of the chemical transformation are often associated with light emission called chemiluminescence. As outlined in Section 3.1, with metal surfaces the relaxation times of electronic excitations (from delocalized band states) are much shorter ( r j 10 s) than those for photons s), so this decay channel will be operat-... 

The foregoing discussion has described facts about excited state relaxation that derive from conventional experiments with total pressures above about 10 torr. They reveal the presence of at least three electronic relaxation channels representing radiative decay, intersystem to a triplet state, and chemical relaxation. They also reveal an intriguing dependence of the relative rates of these processes on vibrational excitation. However, the fast vibrational relaxation in the gas phase at those pressures precludes a detailed study of the vibrational effect. It is difficult to classify the levels from which relaxation occurs more precisely than the headings thermal levels and higher levels indicate. 

The results of these studies were clear. At very low pressures (ca 0.01 torr) where the collision interval exceeds the lifetime ( 10 sec) by a factor of about 100, collisional influences on electronic relaxation disappear. Yet nonradiative decay still persists, and in fact about 70% of the decay from the isolated molecules uses that channel. Furthermore, observations of butene-2 isomerization - indicated that the triplet state is present under these conditions so that at least part of this isolated molecule relaxation is intersystem crossing. The thermal data described in preceding sections imply that the nonradiative channel may be entirely intersystem crossing, but the butene-2 method cannot confirm this. That method does not give quantitative results at very low pressures. ... 

An adsorbed atom or a molecule being in its excited state is characterized by a finite lifetime which is determined by the reciprocal of the decay rate of this state. The finiteness of the lifetime leads to a broadening of the lines in the optical spectra of the adsorbate. Besides spontaneous emission which occurs also for free atoms and molecules, adsorbed species have other specific channels of relaxation, conditioned by their proximity to the surface. Any relaxation process must obey the conservation law of energy and therefore it takes place only if there is a substrate excitation which can accept the energy that the excited adsorbate releases. Therefore, possible decay mechanisms are determined by the energy spectrum of the substrate and thus generally are different for metals, semiconductors and dielectrics. They can be broadly classified as being mediated by photons, phonons, electron-hole pairs and conduction electrons. 

In the case of adsorption on a metal surface, there may be an additional channel of relaxation through the transfer of adsorbate energy to electron-hole pairs. The number of accessible electron states in the metal increases linearly with the transition frequency, coq, and so does the decay rate through... 

Knowledge of the underlying nuclear dynamics is essential for the classification and description of photochemical processes. For the study of complicated systems, molecular dynamics (MD) simulations are an essential tool, providing information on the channels open for decay or relaxation, the relative populations of these channels, and the timescales of system evolution. Simulations are particularly important in cases where the Bom-Oppenheimer (BO) approximation breaks down, and a system is able to evolve non-adiabatically, that is, in more than one electronic state. 

Kim et al. observed a very fast ion pair formation (below their detection limit of about 1 ps) from transient absorption spectra of fullerenes in the presence of aromatic amines such as /V,/V-dimcthyl- or /V,/V-dicthyl-anilinc, corresponding to a rate > 1 X 1012 M-1 s-1. An explanation for such extremly fast electron transfer is most likely a ground-state complex of fullerene and amine. Excitation leads to the neutral aminc/ C 0 contact pair followed by electron transfer. The decay of the both transient absorption from Cfo and Qo/amine occurs with the same rate suggesting that charge recombination is the major nonradiative relaxation channel [138],... 

Figure 2a illustrates the concepts of radiative and nonradiative decay, fluorescence quantum yield, and fluorescence decay. A molecule in an excited electronic state can relax by several channels. Molecules excited to a vibrational level in the excited state undergo vibrational relaxation (cooling, yellow arrows in Fig. 2a) to the lowest vibrational levels of the excited state in a... 

The state s) is a particular vibronic level 2, V2) in the upper electronic state. If thermal relaxation within this electronic manifold is fast relative to the timescale F , then the overall population in this electronic manifold decays into channel 7 (7 = L,Ri ) at a rate given by the thermal average of (18.3), that is, (compare Eqs (12.34) and (12.35))... 

This diagram represents ionization of a core electron via a shape resonance orbital. In the first step the core electron is excited to an orbital above the ionization threshold that has a large amplitude n the vicinity of the molecule. The second step represents separation of the electron from the molecule. The subsequent relaxation then is identical to the situation where the core electron is ionized directly. These channels, depicted in Table II, are Auger decay... 

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