27 : 4/ Quantum Mechanical Basis of Molecular Spectra 



519 



Saunders coupling. The reader may readily extend these concepts to 

 more than two electrons. 



Selection rules for atomic spectra are very similar to those for single 

 electrons, namely 



AL = ±1 



AS = 



AJ = ±1,0 0->C 



In a weak magnetic field 



AM, = ±1, 0->0 



and in strong magnetic fields 



AM L = ±11 

 AM S = 0> J 



The letters used for different L values are the same as those for different 

 / values except that capitals are used. For a given value of L and S, 

 there are at most 2S+ 1 values of J; this is sometimes called the multi- 

 plicity. The quantum state of an atom due to its electronic configuration 

 is often represented by symbols such as 3 P Q . The superscript 3 is the 

 value of 2*9+1; in this example, it tells one that S — 1. The letter P 

 is the value of L, namely 1. The subscript is the value of J. 



(So many letters having been introduced to describe the state of the electrons 

 within an atom, a few more will be included for completeness. In X-ray studies, 

 photons are emitted when a free electron falls into an atom. These photons are 

 absorbed when an electron is raised from a lower energy level to a much higher 

 level. The X-ray researchers have their own letter scheme for representing 

 quantum numbers. They refer to the different values of n as shells, and of / as 

 subshells. Each shell is designated by a letter starting with A, as shown in 

 Table III.) 



TABLE III 

 X-ray Terminology for Electron-Energy Levels 



(As another aside, attention should be drawn to the Pauli exclusion principle 

 which states that no 2 electrons within the same atom may have the same 5 

 quantum numbers. Because s is always 1/2, it is sometimes not counted; then the 



