Fine structure of hydrogen: 2p → 2p₁/₂, 2p₃/₂ via relativistic ℓ·s interaction
Spin-orbit coupling arises from the relativistic interaction H_SO = (1/2m²c²)(1/r)(dV/dr) ℓ·s. For hydrogen 2p: ΔE = (α²/2) × E₁ × (1/(2·2·3)) splitting 2p into j=1/2 (lower) and j=3/2 (upper) with ratio 1:2 degeneracy. In 2D systems, Rashba coupling (H_R = λ_R(σ×k)·ẑ) breaks inversion asymmetry, while Dresselhaus coupling (H_D = λ_D(σₓkₓ−σᵧkᵧ)) arises from bulk inversion asymmetry — key for spintronics.