Paschen-Back effect

In the Paschen-Back effect, the external magnetic field interaction becomes large compared with the spin-orbit interaction that holds $\mathbf{L}$ and $\mathbf{S}$ coupled. Then $J$ ceases to be a good approximate quantum number; $m_L,m_S$ become more useful, with leading electronic shifts $\Delta E\simeq {\mu }_{B}B(m_L+g_s{m}_S)$. In the ideal complete-decoupling limit an electric-dipole interaction preserves $m_S$, so its spin contribution cancels between the levels and orbital triplet spacing can reappear. Intermediate fields require mixing the coupled states. This is a change of coupling regime, distinct from the weak-field anomalous Zeeman effect; still larger fields require additional terms beyond this linear model.

For orbital angular momentum $\ell =1$ and spin $s=1/2$, considerThe quantum number $m_j=m_{\ell }+m_s$ is conserved. The stretched states with $m_j=\pm 3/2$ have energies $K\pm 2a$. The remaining eigenvalues follow from two $2\times 2$ blocks with off-diagonal entries $\sqrt{2}K$:The subscripts denote $m_j=\pm 1/2$. For $K>0$, the zero-field multiplets are $j=3/2$ at $K$ and $j=1/2$ at $-2K$. Their weak-field Zeeman splitting is $g_{j}a{m}_j$, with Landé g-factors $4/3$ and $2/3$, respectively. When $a\gg |K|$, the Paschen-Back effect gives approximate product states $(m_{\ell },m_s)$ and energies $a(m_{\ell }+2m_s)+2K{m}_{\ell }{m}_s$. Mixing within a fixed $m_j$ causes an avoided crossing; different $m_j$ sectors can cross because the Hamiltonian does not couple them.