820 LIGHT AND LIFE 



singlet state ol the same electronic configuration, which usually 

 has a somewhat higher energy level. 



The decay of an excited singlet state to the ground state is known 

 as fluorescence. It is temperature-independent and is commonly rapid 

 (producing an afterglow of the order of 10-^ second) . If the excited 

 singlet state undergoes a transition to the related triplet state, which 

 then decays to the ground state, phosphorescence occurs. This is 

 temperature-dependent, and is marked by a considerably slower after- 

 glow (10-4 up to 1 second) . Or, if the triplet experiences a new 

 transition to the excited singlet state, which then decays to the 

 ground state, this too may yield phosphorescence. 



The spectroscopic detection of the first excited singlet and triplet 

 states of atoms and molecules is by virtue of their emission of ultra- 

 violet, visible, or infrared radiation following absorption of radiation 

 also belonging to those portions of the spectrum. The emission is 

 largely at longer wavelengths, but may include the same wavelengths 

 as that absorbed.) The photochemical consequences of the elevat- 

 tion of atoms or molecules to higher energy states reside, however, 

 more importantly in the radiationless transitions back to the ground 

 state, for these more usually involve chemical reactions with other 

 systems. These result from "collisions of the second kind" — "collisions 

 of the first kind" being those in which translational energy is con- 

 verted into excitation energy of an atom or molecule. Collisions of 

 the second kind, among other consequences, may produce chemical 

 changes of the model A* + BC ^ AB + C (an exchange reaction) 

 or A* + BC ^. A + B -f C, where A* represents the activated atom 

 or molecule and BC a diatomic or polyatomic molecule. 



As a preliminary to understanding the electronic excited states of 

 large molecules, which is made difficult by interactions between parts 

 of the molecule, G. Wilse Robinson discusses the spectra and under- 

 lying electronic transitions of two relatively simple molecules, for- 

 maldehyde and pyridine. 



The energy of a molecule is divisible into its electronic orbital and 

 electronic spin components, as well as its translational, vibrational, 

 rotatory, and nuclear spin motions. To treat these separately is to 

 ignore the important cross terms between the electronic orbital and 

 spin and the molecular vibration, which are particularly significant 

 in the appearance of weak but indicative spectral lines in the visible 

 and near idtraviolet regions. 



Quantum mechanics makes possible the description of the electronic 



