Added a README to the Zeeman constants by Jana's request.

git-svn-id: https://radiativetransfer.org/svn/rt/arts-xml-data/trunk@8587 aaf1aab0-4228-0410-ad68-8dceda47f409

ChangeLog

+2013-08-14  Richard Larsson  <ric.larsson@gmail.com>
+
+	* ARTS-XML-DATA-2-1-55
+	
+	* spectroscopy/QuantumConstants/README: Now explains the other file.
+
 2013-08-14  Richard Larsson  <ric.larsson@gmail.com>
 
 	* ARTS-XML-DATA-2-1-54

spectroscopy/QuantumConstants/README

+Format of zeeman_constants.xml:
+
+@ Isotope GS Spin Hund_case
+
+where GS is a constant for the molecule that calculates the effective g-value,
+Spin is a molecular constant, and Hund_case tells the program to estimate the
+calculations with a specific Hund case.
+
+The Zeeman split is calculated with changes in energy levels. Essentially, 
+dE = g M C_Z,
+where 
+g = GS/2 * g_H,
+and g_H is a function of the quantum numbers for a certain transition and 
+certain Hund case, M is the quantum number projection of the total angular 
+momentum on the magnetic field, and C_Z is a function of physical constants and
+the magnetic field.
+
+The change in frequency is then calculated from 
+df = dE'-dE'',
+where the primes tells you that the quantum numbers are before and after the 
+transition.
+
+Hund case a is number "0", and 
+Hund case b is number "1".
+
+GS = 2.0023193043622 means that the electron is free, and is used when we had no
+data for GS, but rather just guessed.
+
+The GS values for O2, NO, OH, ClO, and SO are from 
+
+H. Christensen, and L. Veseth, On the High-Precision Zeeman Effect in 02 and SO.
+Journal of Molecular Spectroscopy 72, 438-444, 1978.
+
+L. Veseth, Relativistic Corrections to the Zeeman Effect in Diatomic Molecules.
+Journal of Molecular Spectroscopy 66, 259-271, 1977.