48 



RADIATION BIOLOGY 



typical set of data due to Zemansky (1930) indicates quite clearly a 

 resonance dependence of the quenching efficiency (Fig. 1-16). Other 

 observations on similar processes verify the requirement (Mitchell and 

 Zemansky, 1934), though Laidler (1942b) has proposed that the relation 

 is artifactual. One is faced with the problem of reconciling the distinct 

 chemical nature of quenching reactions with the existence of a resonance 

 requirement. Consideration of the forms of potential surfaces involved 



in quenching reactions suggests that 

 the data may be explained in the fol- 

 lowing manner: Figure l-17a is a pos- 

 sible contour map in configuration 

 space for the energy transfer 



XY -t- Hg* 



electronically 

 excited 



XY* -\- Hg. 



vibrationally 

 excited 



(1-44) 



0.14 0.18 Q22 0.26 0.30 



ENERGY OF VIBRATIONAL QUANTUM, 6 V 



Fig. 1-16. Variation in efficiency of 

 conversion of electronic energy into 

 vibrational energy as a function of the 

 size of vibrational quanta. The ab- 

 scissa values are the energies of the 

 vibrational quantum of each molecule 

 which lies closest to the energy avail- 

 able in electronically excited mercury 

 atoms. {Zemansky, 1930.) 



The equipotential lines shown in this 

 figure apply to a single electronic po- 

 tential surface for the system. Be- 

 neath it one must imagine a very simi- 

 lar surface for the system XY plus Hg 

 (unexcited). At large Hg-XF dis- 

 tances the surfaces will be identical 

 but separated by the excitation energy 

 of Hg. Figure 1-176, which is a verti- 

 cal cross section through I, demon- 

 strates the latter fact. A cross section 

 at II (Fig. l-17c) shows that the sur- 

 faces begin to take on different shapes 

 as the distance of separation Hg-XF 

 becomes smaller. There may even be 

 a crossing point where the line II inter- 



sects the loci of crossing points III. 

 Line III is the line of points common to both surfaces and hence includes 

 all the collision gateways that will allow energy transfer in crossing to the 

 lower surface. A possible projection of this line on a vertical plane such 

 that the abscissa is the length along III is shown in Fig. l-17c^. However, 

 each special case must be examined to determine if line III lies as shown 

 or at a different inclination to the abscissa, perhaps positioned very near 

 the origin, as shown in line IV of Fig. l-17a. In any case, the line prob- 

 ably has one minimum of energy more or less centrally located, so that 

 the most efficient transfer processes find their gateways toward the center 

 of the region of maximum distortion of equipotential lines. For the 

 present example the best gateway from the point of view of collision 



