PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 201 



velocity. Slowly moving heavy particles, either charged or uncharged, 

 can also excite atoms in impact. The probability of excitation is, of 

 course, zero unless the relative kinetic energy of a particle exceeds the 

 particular excitation energy of the atom; the kinetic energy must, how- 

 ever, be of order of magnitude 10 ev greater if the probability is to be 

 appreciable. This type of excitation is encountered only infrequently. 

 An atom may also be excited in a collision with a second atom (either the 

 same or a different species) which is already excited. Since this is a 

 collision of the second kind for the second atom, discussion of the process 

 will be deferred to Sect. 3-lc. Finally, excited atoms can be produced by 

 the dissociation of excited molecules (Sects. 3-2e, 3-2f, 3-3c), by the dis- 

 sociation of ionized molecules (Sect. 4-3b), as products of the recombina- 

 tion of positive ions and negative ions or electrons (Sect. 4-3c), as prod- 

 ucts of the attachment of electrons to neutral molecules (Sect. 4-3d), and 

 as products of elementary reactions involving nonexcited atoms and mole- 

 cules. The last mechanism, which is responsible for chemiluminescence, 

 is not directly related to the subject of this article and will not be men- 

 tioned again. 



It is important, in the study of chemical consequences of absorption of 

 either light or high-energy radiation, to follow the excitation energy in 

 an excited atom of a system to its ultimate disposition. Thus, the added 

 energy may be re-emitted, or, if the excited atom suffers an impact, it may 

 cause chemical change, or may be degraded to heat. The physical 

 principles governing these several possibilities are discussed in the follow- 

 ing subsections. 



3-lb. Fluorescence and Phosphorescence.^ If a system which is absorb- 

 ing light re-emits some or all of it, the system is said to be luminescent. 

 Fluorescence is a rapid re-emission; here the excited system has one or 

 more very probable radiative transitions. If the exciting light incident 

 on a fluorescent system is abruptly terminated, the afterglow has only 

 very short duration — of order of magnitude of 10~** second. The fluores- 

 cence light itself may have wave lengths equal to that of the exciting 

 radiation, or different from it ; it may contain one or several or many lines. 

 Phosphorescence, on the other hand, is a slower re-emission. The after- 

 glow of atoms usually has duration of 10~^ to 1 second. (In other types 

 of system it may be very much longer.) Phosphorescence radiation may 

 or may not contain the exciting wave length, and it may contain one or 

 several others. In the case of atoms it arises from the formation, sub- 

 sequent to the initial light absorption act, of metastable atoms. If a 

 metastable atom does not have the possibiUty of losing its energy (or 

 part of it) in an impact, it must perforce radiate it, but the small prob- 

 ability of radiation results in a protracted emission.^ 



®For further details the treatise by Pringsheim (1949) should be consulted. 



^ There exists a considerable confusion as to the definition of and criteria for dis- 



