Internal conversion, rapid

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... 

This considerable enhancement in redox properties may however remain chemically hidden. Several causes may converge to mask these properties. First of all electron transfer is an intermolecular act of reactivity even when thermodynamically feasible it may have to compete with very rapid intramolecular acts of deactivation (fluorescence, phosphorescence, internal conversion)99. The rate of electron transfer is given by the Rehm-Weller equation96,100... 

Acetaldehyde is methyl-substituted formaldehyde, and has a number of similarities with its smaller cousin. In particular, when photodissociated at 308 nm, internal conversion to is rapid, and acetaldehyde can decompose via analogous radical and molecular channels... 

The much larger energy difference between Si and S0 than between any successive excited states means that, generally speaking, internal conversion between Si and S0 occurs more slowly than that between excited states. Therefore, irrespective of which upper excited state is initially produced by photon absorption, rapid internal conversion and vibrational relaxation processes mean that the excited-state molecule quickly relaxes to the Si(v0) state from which fluorescence and intersystem crossing compete effectively with internal conversion from Si. This is the basis of Kasha s rule, which states that because of the very rapid rate of deactivation to the lowest vibrational level of Si (or Td, luminescence emission and chemical reaction by excited molecules will always originate from the lowest vibrational level of Si or T ... 

We saw in the last section that because of the rapid nature of vibrational relaxation and internal conversion between excited states an electronically-excited molecule will usually relax to the lowest vibrational level of the lowest excited singlet state. It is from the Si(v = 0) state that any subsequent photophysical or photochemical changes will generally occur (Kasha s rule). 

This behaviour may be explained by considering that the azulene molecule has a relatively large S2-Si gap, which is responsible for slowing down the normally rapid S2 to Si internal conversion such that the fluorescence of azulene is due to the S2 —> S0 transition. The fluorescence emission spectrum of azulene is an approximate mirror image of the S0 — S2 absorption spectrum (Figure 4.6). 

When the excited triplet state is populated, rapid vibrational relaxation and possibly internal conversion may occur (if intersystem crossing takes place to an excited triplet of greater energy than Ti). Thus the excited molecule will relax to the lowest vibrational level of the Ti state, from where phosphorescence emission can occur in compliance with Kasha s rule. 

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. 

In summary, spectroscopic studies show that at low temperatures higher excited states of chromium(III) complexes undergo rapid internal conversion to the metastable T2g or 2Eg levels. Intersystem crossing from the 4T2g to the aEg level occurs with near unit efficiency in many chromium(III) complexes. Phosphorescence competes, usually unfavorably, with radiationless decay from the 2Eg state. Studies of the excited states of Mo(III), (4d)3, based on absorption spectra of its complexes in solution, have recently been reported.134-137... 

What is most difficult to accommodate is that the principal mode of deactivation of the 1T2g state should be rapid internal conversion to the 1Tlg state (making aquation or other reactions of the 1T2g state improb-... 

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