EXCITATION 63 



process of exciton migration probably plays an important role in ionic 

 crystals, but is of less importance in crystals made up of organic mole- 

 cules. Crystals in which a strong exciton migration occurs have absorp- 

 tion spectra which differ typically from the spectra (13) of the compound 

 in the gas phase. The normal molecular spectrum will be replaced by a 

 much narrower, strong absorption region. This criterion leads to the 

 conclusion (14) that transfer of energy of excitation by exciton migration 

 is important in the micelles of cyanine dyes but is of little consequence 

 in crystals like naphthalene. The well-known fact (14) that energy 

 given to the lattice of naphthalene by absorption of radiation is largely 

 re-emitted by naphthacene (present as a trace impurity) has probably 

 little to do with exciton migration. It may be better explained by the 

 process of sensitized fluorescence or, to use a more general term, "classical 

 resonance" (15). This process allows transfer of excitation energy be- 

 tween molecules separated by distances which are great relative to their 

 collision diameters. It has been discussed chiefly in connection with the 

 self-quenching and the depolarization of the fluorescence of solutions of 

 dyes. Its occurrence is most probable when the emission spectrum of 

 the excited molecule overlaps the absorption spectrum of the receiver. 

 However, there is a finite probability of its happening even when the 

 excited molecule is non-fluorescent. It appears to be of importance (14) 

 for intramolecular, as well as for intermolecular, transfer of excitation 

 in complex molecules. 



From this viewpoint of radiation chemistry, the most important mode 

 of excitation is the impact of charged particles upon molecules. The 

 specific characteristics of the excitation by the several charged and 

 uncharged particles have been discussed by the physics panel. Direct 

 excitation by impact is in many respects similar to photoexcitation. 

 The Franck-Condon principle still applies, but some of the selection 

 rules are different. The direct transition from a ground singlet state to 

 a repulsive triplet level due to the absorption of a photon is forbidden, 

 but the corresponding transition may be produced by some types of 

 impacts of charged particles. Impact excitation can produce this type 

 of direct dissociation in addition to the other methods of dissociation 

 which are also produced by the absorption of a photon. In some cases 

 the impact excitation of the molecule may be followed by light emission 

 together with a dissociation process. The continuous emission spectrum 

 of a hydrogen arc is an example of this kind (16). The hydrogen mole- 

 cule, normally in a singlet state, is raised to an excited stable triplet 

 state by electric impact and then falls to a lower repulsive triplet state, 

 emitting a photon and dissociating into atoms. Either predissociation 

 or internal conversion may follow excitation by impact just as it follows 



