Crystals, electron excitation migration

Because of periodic symmetry, the electronic excitation states in a molecular crystal are also of collective nature. These are the well-studied exciton states.81 82 Their energy is close to that of discrete electronic states of isolated molecules (4-8 eV), but the excitation envelops a large group of molecules, migrating efficiently up to 100 nm along the crystal.82 In the same manner, because of efficient migration, the excitation of a fragment of a polymer chain rapidly spreads over the whole molecule.37... 

An even longer-range transfer, showing a 1/r3 dependence, may occur in crystals, solid solutions, and some fluids, as a result of exciton migration. The concept of the exciton was introduced by Frenckel to interpret certain crystal spectra an electron-hole pair was looked upon as an entity that could move about the crystal as a result of interactions between lattice sites. For the present purposes, the electronic excitation in an irradiated species can be regarded as an exciton that is free to wander over a considerable number of lattice sites. 

In Sections 7.3 and 7.4, the temperature dependence of radiationless transitions and the effect of deuteration on the lifetimes of excited electronic states are examined. In Section 7.5, a contribution to time-resolved spectroscopy is presented. In that section, we will discuss a problem dealing with transport phenomena of electronic excitations in doped molecular crystals. The theory of singlet excitation energy transfer uses an effective Hamiltonian to account for intramolecular excited-state depopulation and energy transfer by multistep migration among guest molecules. 

The other salient point of the interaction of 02 with stepped Ag surfaces is the direct observation of the subsurface migration mechanism, witnessed by the disappearence of the loss at 56meV in the HREEL spectra of 0/Ag(2 1 0) [97]. This vibration is ascribed to the metastable occupation by oxygen atoms of the tetrahedral interstitials, where the O/Ag mode can still be efficiently excited by HREELS. When heating the crystal to 300 K the loss disappears because the oxygen atoms move to the octahedral interstitials, which are too deep and well screened to be monitored by low energy electrons. 

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