264 Robert Platzman and James Franck 



having a great dielectric constant, however, most or all of the initial recombina- 

 tion is inhibited (6), and the chemical effects of ionization stem from interaction 

 with the medium of spatially separated electric charges. This interaction is 

 intimately related to the processes of dielectric absorption characteristic of the 

 medium. It may be noted parenthetically that nonpolar macromolecules, such 

 as many plastics, display utterly different dielectric behavior — in particular 

 showing far less dielectric absorption — and, therefore, that discussions of 

 mechanisms in radiobiology based upon analogies with such polymers, however 

 attractive and despite their current vogue, are perilous, to say the least. 



II. CONSEQUENCES OF IONIZATION IN A POLAR MEDIUM 



Before treating the particular case of biologically important macromolecules 

 it will be useful to consider briefly the general consequences of an ionization 

 act in a condensed polar medium. By polar medium is meant one with a high 

 value of the static dielectric constant e^. The condition e^ ^ 1 implies (with 

 only one exception, which is not germane*) the existence of dipolar molecules, 

 and these must always possess regions of strong dielectric absorption at greater 

 frequencies than those at which the dipoles relax. This dielectric absorption 

 embraces all of the familiar infrared absorption, and more: it includes the 

 region from about 30 /^ to 1 mm, which at the present time is not accessible with 

 commercial instruments and is therefore virtually unexplored (although a few 

 investigations of simple molecules have been made, particularly in recent years, 

 and more are now under way). The dispersive properties of proteins, for 

 example, are almost completely unknown in this spectral region. But it is 

 certain that such absorption must be common and intense among complex 

 polar substances, for only those resonances arising from strong bonds and 

 small masses lie in the 1 to 20-/< region ; weaker bonds and greater masses 

 entail absorption at greater wavelengths. 



The production of an electric charge in a medium will ultimately induce a 

 strong polarization similar to that produced, say, in a condenser filled with the 

 same substance. Tliis polarization will not grow uniformly, however, but rather 

 in several stages, each increase in polarization occurring at a time corresponding 

 in order of magnitude to the reciprocal of its characteristic frequency (7) ; the 

 entire spectrum of dielectric absorption is, of course, involved. The total energy 

 transferred to the medium as the result of an ionization act, excluding the 

 kinetic energy of the ejected electron, can be divided into three parts: that 

 involved in the polarization about the positive ion, a similar quantity released 

 about the electron, or the negative ion which it produces, and finally (and 

 usually much later) the thermal recombination energy of positive and negative 

 ion. The last of these is small (in the case of liquid water, for example, it 

 amounts only to a few per cent of the total) and may be ignored. 



It merits emphasis that an ionization act per se does not usually cause a mole- 

 cular dissociation process. Analysis of results from mass-spectrographic 

 investigations shows that stable parent ions are the rule, and that the dissociation 

 of ions, which is usually the main subject of such studies, results from additional 



* This refers to substances, such as certain semiconductors, in which intense electronic 

 absorption at comparatively low frequencies gives rise to a high value of e^. 



