ENERGY EXCHANGE IN PHOTOREACTIONS 16 



generally receive much of the excess energy. In collision processes, on 

 the other hand, there are no rigid restrictions on crossing phenomena, 

 and the energy of both participants can be redistributed in any way 

 consistent with the conservation of energy and momentum and quantum- 

 mechanical restrictions on allowed energy levels. 



Internal conversion may be favored by low field energies of neighbor- 

 ing molecules, especially of ions through their perturbing effect on energy 

 levels (Stark and Zeeman effects) rather than through the less probable 

 destruction of crossing restrictions mentioned previously. Crossing and 

 resonance transfer processes greatly depend on the difference in energy 

 2e between states. Perturbation due to neighboring molecules can pro- 

 duce better matching of electronic states, thus favoring more efficient 

 crossing. Some additional useful papers dealing with the subject are 

 Zener (1933a), Teller (1941), Sponer and Teller (1941), Herzberg and 

 Teller (1933), Franck (1926b), Franck and Eucken (1933), Franck and 

 Rabinowitch (1934), Nordheim (1926), Rosen (1933) and Hirschf elder 

 and Wigner (1939). 



So long as the separation of surfaces at a crossing point is not large, 

 the presence of Planck's constant /i as a factor in the exponential assures 

 a high probabiUty for the process once the crossing point is reached. 

 Furthermore, if such points lie below the total potential energy of the 

 system, crossing should occur rapidly, for in this case less than a total 

 period of a stretching vibration of the nuclei will produce the crossing 

 configuration. The characteristic periods for such motion are of the 

 order of lO-^'-lO"^^ sec. Bending vibrations are considerably slower 

 and probably not generally important in most crossing phenomena. 

 Vibrations involving heavy nuclei will be slower. The time required for 

 crossing along a reaction coordinate from one potential surface to another 

 will vary within wide limits depending on the distribution of crossing 

 points relative to the excitation-energy value and the number of degrees 

 of vibrational freedom effective in the excited state. Crossing is a uni- 

 molecular reaction on excited surfaces and hence subject to the same 

 treatment of energy redistribution to be given in Sect. 3-4 for vibrational 

 degrees of freedom. The difference in energy between the crossing point 

 and the lowest vibrational energy in the excited state will correspond to 

 an activation energy for the unimolecular process (Fig. 1-6). Molecules 

 that never fluoresce must be able to cross in less than about 10^* sec. 

 The minimum time for crossing in most processes cannot be less than 

 about 10-^^ sec, since this is the lower limit of vibrational times. 



2-2. TYPES OF INTERNAL-CONVERSION PROCESSES 



The probability of internal conversion from an excited state will depend 

 on the kind of molecule under consideration. Saturated hydrocarbons 

 lack symmetry and consequently have a very dense energy structure 



