ULTRAVIOLET ABSORPTION SPECTRA 



183 



motion, which in turn determines the frequency of molecular collision. 

 The latter influences the duration of the excited state and the extent of 

 the distortion of the molecular electronic configurations by the electro- 

 magnetic fields of neighboring molecules. In addition, variation of tem- 

 perature may vary the relative statistical contribution of various possible 

 tautomers (Freed and Sancier, 1951) or resonating states. In general, 

 reduction of the temperature, by reduction of the variety of initial energy 

 levels in an electronic transition induced by absorption and by reduction 

 of the perturbing effect of extramolecular fields, will reduce the width of 

 the individual and fused absorption bands (Fig. 5-13). This effect may 

 be particularly marked if the absorbing substance is in a crystalline 



WAVE LENGTH, A 

 2600 2800 3000 3200 2400 2600 2800 3000 3200 



1— 



1 — I — I — I — r 



THYMINE 

 (FILM) 



— I r 



H 



I I 



rHN^^CHj 



T 



— 1 — I — i — I — I — r 



THYMINE (EPA) 



r/oK 



(b) 



42 4.0 3.8 3.6 



32 3.0 



3.4 3.2 3.0 4.2 4.0 3.8 3.6 3.4 

 WAVE NUMBER, cm"' X 10"^ 



Fig. 5-13. Effect of reduced temperature on the absorption spectrum of thymine. 

 {Sinsheimer et al., 1950b.) 



form so that the molecules have a uniform environment (Scott et al., 

 1952). 



ORIENTATION 



If the molecules of a dichroic substance are uniformly orientated, as 

 might occur in a cellular structure, the absorption spectrum would depend 

 on the plane of polarization of the incident radiation. If unpolarized 

 light is used, as is ordinarily the case, the influence of the molecular 

 orientation on the observed spectrum will depend on the degree of 

 dichroism. In an extreme case, such as might occur if there were no 

 absorption at all of light for which the electric vector lay in a particular 

 plane, the maximum possible light absorption would be 50 per cent, cor- 

 responding to an optical density of 0.3 at that wave length (Commoner 

 and Lipkin, 1949). Obviously, intermediate cases would permit various 

 maximal values of optical density. 



SCATTERING 



If the absorl)ing solution (or living cell) contains objects of dimensions 

 comparable with those of the wave length of light employed, appreciable 

 quantities of light may be lost from the l)eam by scattering out, as well as 



