ULTRA VKJLKT ABSOKPTION SPKCTRA 170 



The Lambert or Boiiguer law is valid under all foiiditions of normal use. 

 The modification due to Beer is accurate as long as the condition for its 

 validity is met — that the absorbing molecules act independently. In 

 concentrated solutions there is often a tendency toward dimerization or 

 other forms of molecular association. In these instances the nature of the 

 absorbing entity really changes with changing concentration, so that 

 Beer's law will not apply. 



Another method of indication of the absorbing power of a substance is 

 the specification of the "absorption cross section" (a) of molecules of the 

 substance as a function of wave length. The absorption cross section is a 

 measure of the probability of absorption of a photon, known to be crossing 

 a unit area transverse to the beam, by a single molecule known to be con- 

 fined within that area. This quantity is useful in calculations concerning 

 the possibility of radiative-energy transfer from the primary receptor to 

 other receptors within a cell (Arnold and Oppenheimer, 1950). 



The absorption cross section is related to e by the following formula: 



a = 3.83 X 10-21^ 



where a is in square centimeters. The cross section a does not necessarily 

 bear simple relation to the physical cross-sectional area of the molecule, 

 although in certain instances a good correlation has been demonstrated 

 between an "effective" geometrical cross section and absorptive power 

 (Braude, 1950). 



SPECTRAL POSITION 



The position of an absorption band in the electromagnetic spectrum is 

 usually defined by the position of the absorption maximum. This posi- 

 tion may be expressed in terms of (1) wave length, in Angstroms (1 A = 

 10-« cm) or millimicrons (1 m^ = lO"" cm); (2) wave number,^ in cm-^ 

 or mm-i; or (3) frequency, in vibrations per second or in fresnels (1 f = 

 10'2 vps). 



Thus the longer wave maximum in the absorption spectrum of methyl- 

 cholanthrene at 77°K occurs at 2995 A. 299.5 m/x, 33,390 cm-', 10.02 X 

 101^ vps, or 10,020 f. 



BAND WIDTH 



The width of an absorption band is usually considered to be the spectral 

 separation between the points of half-maximal absorption. This separa- 

 tion may be expressed in any of the units used to express the position of 

 the absorption maximum. 



'• The wave number is defined as the number of wave lengths per centimeter (or 

 millimeter) of path in vacuo. 



