42 D. SHUGAR 



2. The Primary Photochemical Process 



The absorption of a light quantum by a molecule leads to its being raised 

 from its normal, or ground, state to one of a possible number of discrete 

 higher energy levels, or excited states. For absorption in the visible and ul- 

 traviolet regions of the spectrum, the excitation energy is mainly electronic. 

 The resulting excited molecule is no longer in thermal equilibrium with its 

 surroundings and must therefore give up its excitation energy in one of sev- 

 eral ways. 



(a) By the reemission of radiation at wavelengths to the red of that ab- 

 sorbed (i.e., of lower energy) in which case we are dealing with fluorescence 

 (if the lifetime of the emission is less than 10" 4 seconds) or phosphorescence 

 (if the lifetime of emission is greater than 10" 4 seconds and may even last 

 as long as several seconds). 4 



(b) By a process called internal conversion, resulting from the fact that 

 each energy level of a molecule is the resultant of its electronic, vibrational, 

 and rotational energies. It is therefore possible for a transition to occur from 

 a low vibrational level of a high electronic state to a high vibrational level 

 of a lower electronic state, the total energy being the same in both states, 

 so that the molecule does not reemit energy in the form of radiation. Conse- 

 quently a portion of the electronic excitation energy is "converted" to vibra- 

 tional energy and it becomes a "hot" molecule. It may then lose this vibra- 

 tional energy by successive collisions with neighboring unexcited molecules, 

 the excitation energy being thus dissipated as heat. 



(c) Or it may rupture into two stable molecules or into free radicals, or 

 undergo a rearrangement. If this happens very rapidly the process is re- 

 ferred to as optical dissociation or predissociation . 



(d) The absorbing molecule may also initiate a chemical reaction in an- 

 other molecule by loss of its energy to the latter. Reactions of this type are 

 known as sensitized reactions. 



3. Excited States of Molecules 



At least in the case of phosphorescence, and what is sometimes referred 

 to as long-lived fluorescence, we are dealing with molecules which remain 

 in an excited state for an appreciable length of time by comparison with 

 that required for many chemical reactions. In solution at room temperature, 

 of course, deactivation by collisions competes with, and frequently com- 

 pletely eliminates, phosphorescence. But, by dissolving the substance in a 

 solid medium (such as fused boric acid which sets to a rigid glass, or in a 

 fluid medium which freezes at a low temperature), so that the excited mole- 

 cules are "trapped" and deactivation by collisions is reduced considerably, 

 both the fluorescence and phosphorescence phenomena may be readily in- 

 vestigated. 47 



