36 RADIATION BIOLOGY 



Molecules of the first-class reactions seldom fluoresce, so that internal 

 conversion usually occurs in less than 10~* sec. Most small molecules 

 are of this type when irradiated with wave lengths of energy greater than 

 bond energies. The absorption spectrum for such wave lengths is a con- 

 tinuum. At longer wave lengths a diffuse rotational fine structure is 

 frequently observed, though the vibrational lines are sharp. These dif- 

 fuse spectra, termed " predissociation " spectra, give some estimates of 

 the times required for the crossing conditions to be met in upper surfaces. 

 The excited molecule remains on the upper surface long enough to allow 

 development of the vibrational structure but crosses before rotation 

 becomes fully realized. The calculation of the time before crossing 

 requires the use of the Heisenberg uncertainty principle after the manner 

 of Bonhoeffer and Harteck (1933) . The form of the principle useful here 



is 



A. • At - 1, (1-28) 



in which Ai' is the uncertainty in freciuency and At is the uncertainty in 

 lifetime. Rotational lines are on the average 100 cal apart, correspond- 

 ing to a frequency of 10 ^^ secK To produce the observed diffuse band 

 structure, Ap must be of the same order as the separation of lines in a 

 sharp spectrum. Hence 



At = 1/Ai/ = 10-11 sec, (1-29) 



where A^ is now the average lifetime in the excited vibrational state. It 

 varies from case to case, but this value may be taken as a suitable average 

 (Rice, 1933; Burton and Rollefson, 1938; Noyes and Henriques, 1939; 

 Rosen, 1933; Kimball, 1937). The lifetime is long with respect to the 

 interval between collisions with solvent molecules which will remove the 

 extra energy. It is short with respect to the time between collisions with 

 other solute reactants. Hence we see that a second molecule different 

 from the solvent must be very near the excited molecule at the time of 

 excitation to receive any of the energy. Such a molecule must further 

 compete with the very rapid internal processes of the excited molecule 

 and will generally be the loser. 



In the second class are included those reactions in which crossing is 

 induced by a quencher in a molecule which would otherwise fluoresce 

 and in which the quencher receives some portion of the extra energy. 

 These reactions, termed "sensitizations," form a major part of the domain 

 of photoreactions. The second class may be further divided into two 

 subclasses: those reactions in which the quencher receives its energy in 

 vibrational, rotational, and translational forms; and those in which the 

 quencher undergoes electronic excitation. Only infrequently for small 

 molecules are the two classes likely to overlap in the sense that vibra- 

 tional or translational energy is reconverted in the quencher to electronic 

 energy. 



