Radiationless process

Radiationless Processes. There were several studies of the fluorescence and phosphorescence (253) of glyoxal at much lower pressures than were used previously, which showed that the photochemistry observed by Parmenter was primarily collision- 

The work of van der Werf et al. (246) has clarified the nonradiative pathways that depopulate Sj. Since the results of this work have been discussed in a previous section, it will not be mentioned here, except to confirm that both the aforementioned low-energy studies involve collision-induced intersystem crossing. 

In summary, glyoxal photodecomposition studies are still in a state of disarray. A complete accounting of the photolysis products is necessary before definite conclusions can be drawn. Very low pressure work where —/vvv- -So internal conversion is 

Electronic Spectrum. Propynal exhibits a sharp, clearly structured absorption band with 382.1 nm as the 0-0 transition. This band has been studied extensively in absorption (38,39) and emission, and has been shown to be that of the iA +- - -A transition. It is only recently, however, that optically excited emission has been observed. At longer wavelengths starting about 420 nm, a weak absorption band was observed. This is probably due to the A - A transition (111). Analysis of the 382.1-nm band has shown that the A (n,ir ) state differs from that of many other carbonyl compounds in that it is planar there is no evidence that a planar excited state is at a potential maximum. 

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C. Resonance energy transfer. The excitation energy can be transferred by resonance energy transfer, a radiationless process, to a neighboring molecule if their energy level difference corresponds to the quantum of excitation energy. In this process, the quantum, or so-called exciton, is transferred. 

FIGURE 7.4 Modified Jablonski diagram showing transitions between excited states and the ground state. Radiative processes are shown by straight lines, radiationless processes by wavy lines. IC = internal conversion ISC = intersystem crossing, vc = vibrational cascade hvf = fluorescence hVp = phosphorescence. 

Figure 5.6 Fluorescence. Absorption of incident radiation from an external source causes excitation of the analyte to state 1 or 2. Excited species can dissipate the excess energy by emission of a photon or by radiationless processes (dashed lines). The frequencies emitted correspond to the energy differences between levels...

As seen in the radiationless process, intercombinational radiative transitions can also be affected by spin-orbit interaction. As stated previously, spin-orbit coupling serves to mix singlet and triplet states. Although this mixing is of a highly complex nature, some insight can be gained by first-order perturbation theory. From first-order perturbation theory one can write a total wave function for the triplet state as... 

The absorption of electromagnetic radiation by molecular species in solution in the UV/visible region is followed by relaxation from excited electronic states to the ground state mostly by a combination of radiationless processes. Vibrational relaxation, where the excess energy is rapidly dis-... 

Fong, F. K.(ed.), Radiationless processes. Topics in applied physics, Springer-Verlag, Berlin, 1976. 

Explain the effects of deuteration and heavy atoms on the relative rates of radiationless processes. 

Fig. 5 A proposed mechanism for enhanced emission (or AIEE) in solid-state organic dye nanoparticles. The dye considered here is trans-biphenylethylene (CN-MBE) compound. The geometry is optimized by the density functional theory (DFT) calculation at the B3LYP/6-31G level. Molecular distortion such as twisting and/or subsequent planarization causes prevention of radiationless processes along with specific aggregation such as the /-aggregate in the nanoparticles...

The same experiment can be carried out quantitatively. By taking into account radiationless processes, namely, internal conversion fcIC, intersystem crossing isc, and bimolecular quenching kQ[Q] with a quencher Q, the time-dependent concentrations of the donor D and the acceptor A in the excited singlet state Si, [Ds,] and [As,] can be expressed as follows ... 

The emitting level must not be at too low an energy. The energy gap law states that radiationless processes become more efficient as the emitting state approaches the ground state.01 12)... 

El-Sayed, M. A. Spin-orbit coupling and the radiationless processes in nitrogen hetero-cyclics. J. Chem. Phys. 38, 2834 (1963). 

Freed, K. F. The theory of radiationless processes in polyatomic molecules. In Topics Curr. Chem. 31, 105 (1972). 

Auger electron spectroscopy spect The energy analysis of Auger electrons produced when an excited atom relaxes by a radiationless process after ionization by a high-energy electron, ion, or x-ray beam. Abbreviated AES. o zha i lek.tran spek tras-ko pe ... 

As well as returning to the ground state by radiative or radiationless processes, excited states can be deactivated by electronic energy transfer. The principal mechanisms for this involve dipole-dipole interactions (Forster mechanism) or exchange interactions (Dexter mechanism). The former can take place over large distances (5 nm in favourable cases) and is expected for cases where there is good overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor and where there is no change in the spin... 

Although, in principle, it is possible for some fraction of the events to follow the Odd surface beyond this second intersection and to thus lead to JOdd product molecules that might fluoresce, quenching is known to be rapid in most polyatomic molecules as a result, reactions which are chemiluminescent are rare. An appropriate introduction to the use of OCD s, CCD s, and SCD s as well as the radiationless processes that can occur in thermal and photochemical reactions is given in the text Energetic Principles of Chemical Reactions, J. Simons, Jones and Bartlett, Boston (1983). 

Di Bartolo, B., ed. (1981). Radiationless Processes" (NATO Advanced Study Institute Series, Vol. B62). Plenum, New York. 

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