Alkali Doublet Method
(Varshalovich, Potekhin, Ivanchik, et al)
•Fine structure interval
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•DFS = E(p3/2) - E(p1/2) = A(Za)2 
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•If DZ is observed at red shift Z and D0 is FS measured on Earth then
Ivanchik et al, 1999:   Da/a = -3.3(6.5)(8) x 10-5.
Murphy et al, 2001:   Da/a = -0.5(1.3) x 10-5.