PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 



215 





LU 



LU 



I- 



o 



a. 



He+ Hg* 



4.86 ev 



He +Hg 



excitation of Br 2 by electron impact, for the duration of the impact (order 

 of magnitude 10'^® second) is again too short to permit consequential 

 movement of heavy nuclei. 



An example of a collision process — the impact of He on Hg* — is now 

 considered. Here again a potential curve can be drawn; it presents the 

 potential energy of these two atoms as a function of their separation. 

 At all values of r greater than a few angstroms the curve is horizontal, for 

 no forces act between the atoms, but there is a very shallow minimum in 

 the curve at somewhat smaller r because of the van der Waals force, and 

 at very small values of r there is a 

 steep rise (Fig. 3-5). The potential 

 curve for a collision between He and 

 a normal Hg atom can be drawn 

 below; it is similar to the first except 

 that the minimum is still shallower 

 (smaller van der Waals force because 

 Hg is less polarizable than Hg*) ; at 

 great values of r it is lower, of course, 

 by the excitation energy of Hg*, 

 namely, 4.86 ev. Somewhere at very 

 small r the two curves cross — or at 

 least approach one another closely.' 

 In a collision of Hg* and He the 

 system traces the upper curve from 

 right to left; it moves up the steep, 

 repulsion part of the curve only to 

 a height above the horizontal part 

 equal to the relative translational energy of the two atoms, and then 

 retraces itself. The process 



Hg* + He -^ Hg + He + kinetic energy 



(collision of the second kind, type II) would require the interatomic sep- 

 aration to decrease suddenly during the impact (dotted hne on Fig. 3-5), 

 and the Franck-Condon principle shows this to be impossible." Of 

 course, if the system were to move ''up" the curve to a point where the 

 two curves are only very slightly separated, a transition would be very 

 probable, but this would require a great kinetic energy of the respective 

 atoms. This conclusion can be stated alternatively as: only those colli- 

 sions having very great relative translational energy will result in a transi- 

 tion, or: the process is one with high activation energy. This activation 



" The corresponding deactivation of Hg* by collision with an electron (type I) 

 is not only possible, but rather probable, for the readjustment of the electrons is so 

 quick that it can easily take place within the duration of the collision and is not 

 governed by the Franck-Condon principle. 



.SUM OF "GAS- 

 KINETIC" RADII 



He-Hg INTERATOMIC DISTANCE 

 Fig. 3-5. Potential curves to illustrate 

 the collision of He with Hg and with 

 Hg* (schematic). 



