Internal conversion, of electronically excited

Pecourt J-ML, Peon J, Kohler B (2000) Ultrafast internal conversion of electronically excited RNA and DNA nucleosides in water. J Am Chem Soc 122 9348... 

Zgierski MZ, Patchkovskii S, Fujiwara T, Lim EC (2005) On the origin of the ultrafast internal conversion of electronically excited pyrimidine bases. J Phys Chem A 109 9384-9387... 

It is important to note that the ArnCHa.n-X bond is polar, and no matter how it is stretched the electrons will accommodate. When a polar bond stretches the dipole may either increase, which leads toward heterolysis, or decrease and lead toward homolysis. The internal conversion of electronic excitation energy to vibrational energy could take either course and be solvent sensitive. If the spin of the state is fixed, triplets tend to undergo homolysis and singlets tend to undergo heterolysis, which is based on the Pauli principle (G. S. Hammond, Personal communication, 1999). 

Internal conversion of electronically excited states requires very short time, about The life time of upper excited singlet state is generally less than... 

J and Vrepresent the rotational angular momentum quantum number and tire velocity of tire CO2, respectively. The hot, excited CgFg donor can be produced via absorjDtion of a 248 nm excimer-laser pulse followed by rapid internal conversion of electronic energy to vibrational energy as described above. Note tliat tire result of this collision is to... 

Merchan M, Serrano-Andres L (2003) Ultrafast internal conversion of excited cytosine via the lowest it it electronic singlet state. J Am Chem Soc 125 8108... 

Quenching rate constants for dienes and quadricyclenes have similar sensitivities to the electronic and structural features of the excited aromatic hydrocarbon. However, during this process quadricyclene isomerizes to nor-boraadiene with a quantum yield of 0.52, whereas dienes usually remain unchanged/10 Hammond has suggested that vibrational energy which is partitioned to the acceptor upon internal conversion of the exciplex can lead to isomerization(10a,103) ... 

The rate of internal conversion between electronic states is determined by the magnitude of the energy gap between these states. The energy gaps between upper excited states (S4, S3, S2) are relatively small compared to the gap between the lowest excited state and the ground state, and so the internal conversion between them will be rapid. Thus fluorescence is unable to compete with internal conversion from upper excited states. The electronic energy gap between Si and S0 is much larger and so fluorescence (Si —> S0) is able to compete with Si(v = 0) So(v = n) internal conversion. 

Electronic transfer quenching of t-Sf proceeds at ks = kdifr = 7.1 x 10 M sec in DMF. Similar to radical ion pair, (D /A )soiv formed during ET between donor (D) and acceptor (A) molecules in the excited singlet or triplet state, it is suggested that ET quenching initially gives (St/Bp )soiv with competition of the internal conversion of St to Sf . (St/Bp )soiv then undergoes solvent separation into St and Bp at k or returns to Sf and Bp via back ET at k. Therefore, the fraction of free Bp formed is represented by R = k k + k p). 

Nuclear electromagnetic decay occurs in two ways, y decay and internal conversion (IC). In y-ray decay a nucleus in an excited state decays by the emission of a photon. In internal conversion the same excited nucleus transfers its energy radia-tionlessly to an orbital electron that is ejected from the atom. In both types of decay, only the excitation energy of the nucleus is reduced with no change in the number of any of the nucleons. 

Figure 9.5 Kinetic energy spectrum of internal conversion electrons for a 412-keV nuclear transition in 198Hg. Superimposed on this spectrum is the accompanying spectrum of (i particles from the [1 decay that feeds the excited state. The peaks labeled K, L, and M represent conversion of electrons with principal quantum numbers of 1, 2, or 3, respectively. (From Marmier and Sheldon, 1969, p. 332.) Copyright Academic Press. Reprinted by permission of Elsevier.

The quantum yields for a number of triarylmethane leuconitriles have been reported to be close to unity (44,45,47-49). Therefore, internal conversion of the electronic excited state energy into the vibrational mode of the C—CN bond must take place with almost 100% efficiency. 

Abstract The study of the fate of electronically excited radical and radical cation of aromatic hydrocarbons is an emerging topic in modern chemical dynamics. Observations like low quantum yield of fluorescence and photostability are of immediate concern to unravel the mechanism of ultrafast nonradiative internal conversion dynamics in such systems. The radical cations of polycyclic aromatic hydrocarbons (PAHs) have received considerable attention in this context and invited critical measurements of their optical spectroscopy in a laboratory, in striving to understand the enigmatic diffuse interstellar bands (DIBs). 

Internal conversion is a type of relaxation that involves transfer of the excess energy of a species in the lowest vibrational level of an excited electronic state to solvent molecules and conversion of the excited species to a lower electronic state. 

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