BENT LEY GLASS 819 



is a knowledge of the excited states of molecules producible by the 

 absorption of energy, as well as the subsequent reactions of the 

 energy-rich molecules at ordinory temperatures. For the most strik- 

 ing difference between photochemical reactions and thermal reactions, 

 as Franck and Platzman have pointed out (Radiation Biology, A. 

 Hollaender, ed., Vol. I, p. 193, 1954), is that the former involve 

 "in intermediate stages electronically excited atoms, radicals, or mole- 

 cules which are virtually never excited in a system in thermodynamical 

 equilibrium at room temperature." 



Transitions between different energy states of a molecule may 

 occur by the absorption or emission of radiation, or in a radiation- 

 less transfer upon impact with another system. In a very qualitative 

 way we can picture the electronic distribution of a molecule as fol- 

 lows. Electrons move only in certain orbitals, or paths around the 

 atomic nuclei of the atoms in a molecule. The energy of an electron 

 depends on the orbital which it occupies. When a molecule is elec- 

 tronically excited, one of its electrons is driven to an orbital of higher 

 energy. The transition of an electron to a new orbital results in a 

 redistribution of the electronic charge over the entire molecule. Inas- 

 much as the chemical properties of a molecule depend not only on 

 the energy but likew^ise on the charge density at various sites, electronic 

 excitation results in the formation of an essentially novel molecule, 

 one commonly more chemically reactive than the ground state. The 

 transitions which are detected spectroscopically are almost exclusively 

 those involving the most weakly bound (i.e., the valence) electrons. 

 Particularly significant in respect to such transitions are the "lone- 

 pair" electrons of an atom, that is to say, those which are not in- 

 volved in actual bonding. Such "lone-pair" electrons are found in 

 molecules with nitrogen and oxygen atoms. Of the two electrons mak- 

 ing up such a pair and occupying the same orbit, one of them, upon 

 becoming excited with a requisite amount of energy undergoes an 

 abrupt transition to a higher energy state in a different orbital. Now 

 in general two electrons occupying the same orbital must be "paired," 

 that is, must have opposite spins, according to the Pauli exclusion 

 principle. But following a transition of one electron of an original 

 pair to a higher energy state, the spins may be either opposite or alike. 

 The former is known as a singlet state; the latter is a triplet state. 

 The probability of different possible transitions to higher energy 

 states differs, being in some cases so low as to be "forbidden." Tran- 

 sitions from the ground state to a triplet state are usually such; and 

 in actuality the triplet state is generally achieved by decay from the 



