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Hello friends welcome once again on our blog.. today I’m going to explain Fine Structure of Alkali Spectra which is also an important topic of Atomic Physics. So let us start…. 

When the spectral lines of alkali atom are seen through high resolving power spectroscope, each of them is found to consists of two close components. 
e.g. the yellow D-lines of sodium consists of two close lines of wavelength 5893 A° and 5895 A° ,this is called fine structure.
 The concept of fine structure was explained by introducing electron spin. The electron performs orbital & spin motion.
 Therefore it has orbital angular momentum given by :

Since, we’ve to calculated the total angular momentum. Now,

Due to electron spin 3p level consists of two sub-levels 3p(1/2) & 3p(3/2) corresponding to two values of j i.e. 1/2 and 3/2 respectively, while 3s level remains unsplitted. 

Therefore, the transition 3p(3/2)»»3s(1/2) corresponds to Sodium D2 line, and transition 3p(1/2)»»3s(1/2) corresponds to sodium D1 lines. 

These two transitions are called doublets. 
So In this post I’ve explained Fine Structure of Alkali Spectra.. Stay tuned in my next posts I’ll explain other important topics..thanks..