27 : 4/ Quantum Mechanical Basis of Molecular Spectra 



521 



spectra. Here, the letters start running out and often ones duplicating 

 those of atomic spectra are used. Symbols are also used to indicate 

 whether the bond due to a pair of electrons is symmetric or antisym- 

 metric about the origin, about a plane of symmetry, or about a line of 

 symmetry. 



The symbols used for atomic and molecular electronic levels are 

 summarized in Table IV. 



TABLE IV 

 Some Symbols Used in Spectroscopy for Electronic States 



X-ray shells and subshells — see Table HI (page 519) 

 Molecular levels 



g — gleich "\ + "\ reflection 



u — ungleich 



^parity 



/in 



a plane 



«} 



symmetry about 



an axis 



For molecular spectra resulting from dipole transitions, there are a 

 set of selection rules which include 



AS = 

 AA = ±1,0 



