Relaxation from, “higher” vibrational levels

No dependence of on [Q2] was found for emission from higher vibrational levels of the B state whereas Ix — AtJCh] was found for emission from lower levels. It was concluded that Qj was effective in relaxing higher vibrational levels to lower levels of the B state, and that lower levels were more effectively electronically quenched by chlorine atoms, giving an order < 1 in [Oa] for low vibrational levels. It was argued that the reverse of reaction (8) can occur for higher vibrational levels near the dissociation limit before the product is stabilized by thermonuclear collision to low levels of the B state. 

Increase of third-body concentration (at constant [Cl]) shifts the emission to the red (emission from lower vibrational levels)128-130 while increase in [Cl] at constant [M] shifts it to the blue (higher levels)128,129. This indicates vibrational relaxation by the third body. The blue shift with increasing [Cl] may be due to the fact that the shortened lifetime of C12(3I70+M), due to (4), allows less time for vibrational relaxation. 

These authors arrived at a calculated half-width of 150 cm-1, in good agreement with experiment. However, their calculation did not take into account the presence of the as yet unidentified electronic relaxation channel observed by Callomon, Parkin and Lopez-Delgado 9> to be predominant in the radiationless transition from the higher vibrational levels of the lB2u state. This channel may contribute to the broadening in the lB u state. Further extensive photodissociation is observed upon excitation into the state and therefore photodissociation must be considered a possible cause of the line broadening. 

One more point of importance should be emphasized. Si Ti transitions can take place from higher vibrational substates of Si if vibrational overlap with the vibrational substate of Ti is sufficiently large. The fact, as mentioned earlier, that f iza-state vibrational relaxation to the zero-point vibrational level occurs so rapidly, means that essentially all molecules in Si will be in the zero-point vibrational level. Therefore, radiationless transitions tend to occur from the zero-point vibrational level of the upper electronic state to a higher vibrational substate of the lower electronic state even though vibrational overlap between upper vibrational substates of Si and Ti is good. 

Vibrational relaxation (VR) from a vibrational level of a higher electronic state such as S2 to the vibrational ground state is very rapid. Before the molecule can react, the vibrational energy is quickly distributed among the various vibrations and can be partially or completely dissipated by collisions into heat. 

Vibrational Relaxation. The absorption from So to S involves an energy change from the 0th vibrational level of So to some vibrational level of the excited state. The t/ = 0 vibrational level is the level most populated at room temperature for the ground electronic state of a molecule, and i/ = 0 also is the vibrational level of the excited electronic state that is most likely to be populated at equilibrium in condensed phases (i.e., solids or liquids). Unless the molecule dissociates before equilibrium can be obtained, rapid vibrational relaxation (with a rate constant of about 10 s ) converts the higher vibrational level of the excited state to its 0th vibrational level. ... 

The general nature of excited singlet relaxation in benzene vapor can be quickly mapped out by reference to the energy level diagram in Fig. 1. Fluorescence is observed only when excitation is limited to the low vibrational regions of shown by the hatched area. Relaxation from higher levels of and from everywhere in and S3 in the vapor is dominated by nonradiative paths. In fact, nonradiative decay is also observed in the low energy r ions of where it competes with fluorescence. Nowhere does fluorescence become the principal relaxation channel. 

Figure 7.3. Explanation for heat release in NIR sensitizers. Excitation from the lowest vibrational level of the So into a higher vibrational level of the of the Si results in fast vibrational relaxation (VJ into the lowest vibrational level of the same excited state. The lowest vibrational mode of the Si couples in NIR sensitizers very efficient with higher vibrational modes of the ground state So resulting in the release of heat. This occurs fast in NIR dyes and additional heat can be generated by internal vibrational relaxation of higher modes of the ground state to the lowest vibrational mode of the Sg (V"i v". Electron transfer of the Si with A and fluorescence occur as competing processes...

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