What is Exciton Binding Energy? What is Electronic Transitions with extinction coefficients? (Explain concisely)

What is Exciton Binding Energy?
What is Electronic Transitions with extinction coefficients?
(Explain concisely)

Solutions

Expert Solution

a) Exciton is a Coulomb-correlated electron-hole pair. Frenkel treated the crystal potential as a perturbation to the Coulomb interaction between an electron and a hole which belong to the same crystal cell. This method is most effective in organic molecular crystals. The binding energy of Frenkel exciton (i.e. the energy of its ionisation to a non-correlated electron hole couple) can be of the order of 100-300 meV. Frenkel excitons have been searched for and observed in alkali halides. At present, they are widely studied in organic materials where they dominate the optical absorption and emission spectra. In the end of 1930s the Swiss physicist Gregory Hugh Wannier (1911-1983) and English theorist Sir Nevil Francis Mott have developed a concept of exciton in semiconductor crystals, where the rate of electron and hole hopping between different crystal cells much exceeds the strength of their Coulomb coupling with each other. Unlike Frenkel excitons, Wannier-Mott excitons have a typical size of the order of tens lattices constants and a relatively small bidning energy (typically, a few meV).

b) When interpreting the absorbance and fluorescence spectra of a given molecule, compound, material, or an elemental material, understanding the possible electronic transitions is crucial. Assigning the peaks in the absorption spectrum can become easier when considering which transitions are allowed by symmetry, the Laporte Rules, electron spin, or vibronic coupling. Knowing the degree of allowedness, one can estimate the intensity of the transition, and the extinction coefficient associated with that transition.

Knowing whether a transition will be allowed by symmetry is an essential component to interpreting the spectrum. If the transition is allowed, then it should be visible with a large extinction coefficient. If it is forbidden, then it should only appear as a weak band if it is allowed by vibronic coupling. In addition to this, a transition can also be spin forbidden.

The molar extinction coefficients for d-d transition hover around 100

The molar extinction coefficients for LMCT transitions are around 10^4

genius_generous answered 1 year ago

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