202 RADIATION BIOLOGY 



Resonance radiation is a term given to fluorescence radiation having 

 wave length identical with that of the exciting light. If a photon of this 

 wave length is incident on the atomic gas or vapor at suitable pressure, it 

 may be absorbed and re-emitted many times before ultimately escaping; 

 in a sense, the energy migrates or diffuses through the gas. Only very 

 special absorption lines of an atomic gas can be resonance lines. Since 

 an atom excited to a high level usually has several possible radiative 

 transitions to lower levels, it is customary to consider only those absorp- 

 tion lines of a given element having longest wave length (for a particular 

 spectral series) as resonance lines. Very few elements have more than 

 one or two of them. 



Mercury vapor provides excellent illustrations of each of the processes 

 just discussed. The two absorption lines of wave length 1850 A and 

 2537 A (Fig. 3-1) are resonance lines. An atom brought to a high state 

 (e.g., 7^Pi) will emit fluorescence radiation having several wave lengths. 

 If an atom is raised to the 6^Pi state, either by light absorption or by 

 electron impact, and then brought to the 6^Po by impact (say, with an 

 N2 molecule; cf. Sect. 3-le) before it emits a photon of resonance radiation, 

 it will not undergo a direct radiative transition because the ^Pq state is 

 metastable. Instead it will have a prolonged existence in the ^Po state, 

 and not until a subsequent collision brings it back to the ^Pi state will it 

 radiate a photon of 2537 A, which would then be called phosphorescence. 



Emission of radiation by an excited atom can occur only if this process 

 competes favorably with (crudely, is not much slower than) other 

 de-excitation processes. Thus, the fluorescence or phosphorescence of a 

 gas may be quenched by the addition of another gas, the atoms of which 

 remove the excitation energy in collisions of the second kind. The study 

 of quenching is a valuable source of information on the laws governing 

 energy transfer in impacts of excited with normal atoms or molecules. 



3-lc. Collisions of the Second Kind.^ The designation "collision of the 

 second kind" as used here embraces all cases in which an excited atom 

 . / 



tinguishing between fluorescence and phosphorescence. The above definition was 

 chosen by virtue of its prevaiHng use. However, it would be advantageous if the 

 more refined definitions recommended by Pringsheim (1949) were adopted. Prings- 

 heim calls any luminescence which is protracted merely because of an intrinsically 

 small radiative transition probability a sloiv fluorescence. Thus direct radiation 

 from a metastable state is not phosphorescence. The term phosphorescence is 

 reserved for luminescence for which an external influence is required in order that the 

 activated system (metastable atom, etc.) be able to radiate. Thus, in the case of 

 metastable atoms, the direct radiative transitions are called slow fluorescence; if the 

 metastable atoms are brought to normal (i.e., not metastable) states by impacts, the 

 radiation is called phosphorescence. The necessity of impacts here leads to a depend- 

 ency of the duration of afterglow on the pressure and temperature of the gas. 



^ For detailed treatment and a wealth of examples, see Massey and Burhop (1952); 

 Willey (1937); and Massey (1949). 



