Chemistry and Biochemistry at Low Temperatures 



173 



more sharply defined environmental fields, there are fewer kinds of pertur- 

 bations. 



An especially vivid example of a solution showing somewhat similar pheno- 

 mena is given by the fluorescence spectrum of solutions of europium chloride 

 in ethanol at various temperatures (1), The spectra were taken to discover the 

 discrete number of lines in the three separate sets which may furnish the point 



4000 



4500 



5000 



Fig. 2. Absorption spectra of carotene (90% alpha and 10% beta). A — In hep- 

 tane at room temperature; B — In equal volumes of liquid propane and propene 



at 77°K. 



group symmetry of the electrical fields about europium ion in the solution. 

 It is clear that at room temperature the continuous noise is so great as to make 

 enumeration impossible. As the temperature is lowered a few discrete Hnes 

 can be resolved with such definiteness that they serve to eliminate some of the 

 possible point group symmetries. At the temperature of liquid nitrogen and 

 even at the temperature of dry ice adequate resolution is clearly achieved and 

 the number of possible symmetries of the environmental fields is reduced to 

 one only. 



Figure 2 gives the absorption spectrum of a substance of some biological 

 interest, /9-carotene, and illustrates the increased contrast between absorption 

 and transmission at the lower temperature, that is, the increased signal to noise 

 ratio. 



Figure 3 is presented to illustrate the resolution into components of what 



