Rydberg atoms in weak magnetic fields

The quadratic Zeeman effect of Rydberg atoms is treated with the help of first-order perturbation theory and a special WKB approximation. The Bohr-Sommerfeld quantization leads to complete elliptic integrals. As a result we found that the spectrum is doubly degenerate if n²m²>5, this part exhibiting a linear Stark effect in an additional weak electric field; the rest of the spectrum (n²m²<5) is nondegenerate, exhibiting a quadratic Stark effect in an additional weak electric field.