Multiple electrons relaxation processes

A special version of UV-vis spectroscopy is the detection of the sample light emission after irradiative molecular excitation. This phenomenon is called luminescence and specified as fluorescence in case of relatively fast electron relaxation processes without changing spin multiplicity (time scale of microseconds and below). 

Radiationless transitions among electronic states of molecules represent a class of relaxation processes that are electronic in nature. The general term electronic relaxation appears to be appropriate for these processes,23 but it is convenient to divide those transitions involving a change in the bound electronic states of a molecule into two categories Transitions between states of the same multiplicity, referred to as internal conversion, and transitions between states of different multiplicity, referred to as intersystem crossing. Although there are several early experimental... 

There is good reason to believe that the pseudo Jahn-Teller effect commonly assumes an important role in a number of molecular processes including electronic relaxation. Large classes of heterocyclic molecules whose zero-order BO electronic states may be very close to one another are ideal candidates for these types of vibronic interaction.127 Should the lowest state 0X of a given multiplicity be strongly coupled to a nearby electronic state 02 the following spectral effects can be expected ... 

An isolated atom has a characteristic set of discrete energy levels. Then if one electron is removed from an inner shell of the atom, electronic relaxation occurs due to one of the outer shell electrons filling the electron hole left in the inner shell. This leads to the emission of a characteristic X-ray. In such cases, only one spectral line, originating from the X-ray transition between an inner shell and one of the discrete outer shells, can be observed. However, when multiple ionization occurs during a single excitation process, as in the case of enei etic ion impact, a fine structure or finger pattern is necessarily observed in the spectrum. In PIXE,... 

Higher-order processes contribute in both the excitation of the core electron and relaxation of the core hole. These multi-electron processes are significant because of the strong perturbation caused by the creation or the annihilation of a core hole. In these processes, additional electrons are excited (shake-up) or ionized (shake-off). Two electron processes, double autoionization and double Auger decay, were mentioned above. The final states reached in core hole decay may be excited states and also may autoionize. It is clear that excitation of a core electron and the relaxation of the core hole provide many paths leading to multiple-electron excited states. These states have a unique chemistry relative to the single-electron excited states produced by arc lamp, laser, or vacuum ultraviolet (VUV) excitation. 

Features corresponding to fixed kinetic energies, located at fixed positions from the end of the spectrum, are assigned to the accumulation of electrons relaxed in high density of state levels in the conduction band, above the vacuum level following the multiple relaxation processes suffered by incident electrons. 

Jablonski diagram illustrates the electronic states, properties and relaxation processes of an excited organic molecule (Fig. 6.4) [3]. The electronic states are arranged vertically by energy and grouped horizontally by spin multiplicity. 

Internal conversion If the molecule is excited to a higher-energy excited singlet state than Sj (such as S2 in Figure 1), a rapid non-radiative relaxation usually occurs to the lowest-energy singlet excited state (Sj). Relaxation processes between electronic states of like spin multiplicity such as Sj and S2 are called internal conversion . These processes normally occur on a time scale of 10 s. 

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