230 RADIATION BIOLOGY 



spectral lines (effect on the transition of the electric field engendered by 

 the environment), of "pressure broadening" (an enhanced case of the 

 broadening of spectral lines of gases at high pressure arising from the fact 

 that an appreciable fraction of transitions occurs while the atom or 

 molecule is more or less closely associated with another during an impact) , 

 and finally of the fact that the solvent always participates to the extent of 

 being coupled to the solute in vibrational modes. (In a sense, all solutes 

 are polyatomic molecules.) The difference between a particular absorp- 

 tion spectrum for the gaseous and dissolved states derives from the effect 

 of the solvent on the electronic transition or transitions involved. Quali- 

 tatively, the more "exposed" a transition is (in the orbital picture, the 

 larger the electron orbit of the excited state), the greater the modification. 

 And the stronger the interaction of the solvent and a solute atom or 

 molecule, the greater is the modification. Furthermore, a strong inter- 

 action often destroys the ability of a solute to fluoresce. Optical transi- 

 tions of rare earth atomic ions dissolved in water involve internal electron 

 orbits which are only slightly influenced by the solvent ; thus the lines are 

 often very sharp and fluorescence is strong. Optical properties of a great 

 many dye molecules have been studied in detail (cf. the book of Forster, 

 1951; Pringsheim, 19-49; and, for an important example of a group of 

 molecules which has been studied with particularly great thoroughness — 

 namely, the photosynthetic pigments — Rabinowitch, 1951). Because 

 the electronic transitions are rather well protected, often because they 

 involve conjugated double-bond systems, the absorption spectrum of a 

 given dye is usually very much the same in different solvents, and fluor- 

 escence is a common phenomenon. Somewhat greater interaction is 

 shown by measurements of the 2537 A resonance hne of Hg, which can be 

 observed as absorption by mercury dissolved in a number of solvents, 

 despite the very low solubility. The line is broadened into two continu- 

 ous bands, the broadening depending on the interaction of an Hg atom 

 with molecules of the solvent and increasing with the polarity of the 

 latter. Thus the broadening increases for the series of solvents : hexane, 

 methyl alcohol, water. 



When understood in detail, the absorption spectra can furnish invalu- 

 able information on the physical nature of a solution. 



It should be emphasized that, even if a particular solute has electronic 

 transitions which are only slightly modified by the presence of a solvent, 

 there must always be "higher" transitions, following greater energy 

 transfer, which bring the electron to an "unprotected" state that is 

 strongly affected by the environment. 



If an atom or molecule of a solute or solvent (or of a pure liquid) is 

 excited, the usual result is deactivation without luminescence or dissocia- 

 tion, the excitation energy being dissipated to heat. This can be viewed 

 as arising from internal conversion in a polyatomic pseudo-molecule con- 



