## Tuesday, 3 May 2016

### Effective Lande g-factor (in IDL)

The effective Lande g-factor (Shenstone and Blaire, 1929) is defined for a spectral line. It shows how much the $\sigma$ components of the normal Zeeman triplet are separated from the line center.  The higher the Lande factor is, the line is more sensitive to the magnetic field. It is computed as a combination of g-factors for each of the involved atomic levels. Derivation of the equation can be found, for example, in Landi Degl'Innocenti (1982SoPh...77..285L, see it for more details and for an alternative expression useful in the polarized radiative transfer).

For each of the levels (u for upper, l for lower) in LS coupling holds:
$$g_\mathrm{LS} =\frac{3}{2}+\frac{S (S+1) - L (L+1)}{2J(J+1)},$$
where $L$, $S$ and $J$ are the orbitatl, spin and total angular momentum quantum numbers of the atomic level (note that it is undefined for $J=0$). The effective Lande g-factor is then defined as:
$$\bar{g} = \frac{1}{2}(g_\mathrm{l} +g_\mathrm{u}) + \frac{1}{4}(g_\mathrm{l} - g_\mathrm{u})(J_\mathrm{l}(J_\mathrm{l}+1) - J_\mathrm{u}(J_\mathrm{u}+1)).$$

Here is my IDL code for computing the effective Lande g-factor of a spectral line in LS coupling. At the input the code takes the quantum number of the lower and the upper level of a transition. The numbers may be specified either as an array [L, S, J] or in spectroscopic notation.

Example:
IDL> PRINT, LANDE_FACTOR(lower  = [3., 1, 2.], upper = '3p1.0')
Upper term: 3p1.0 => l = 1.00000s = 1.00000j = 1.00000
0.250000

IDL> PRINT, LANDE_FACTOR(lower  = '5F1.0', upper = '5d0.0')
Lower term: 5f1.0 => l = 3.00000s = 2.00000j = 1.00000
Upper term: 5d0.0 => l = 2.00000s = 2.00000j = 0.00000
0.00000