Fine structure and Zeeman effect in the b(1)3Πu state of 6Li2 by Doppler-free polarization spectroscopy

The fine structure of the b(1)3Πu state of the lithium dimer was observed for the first time and studied using Doppler-free polarization spectroscopy. The triplet state was accessed by excitation of the A 1Σ+u(v=2,J=33)∼b(1)3Πu (v=9,N=32,F1,e) spin–orbit perturbed levels. The remaining two fine structure levels were excited by using the Zeeman interaction to mix levels of different J. Two polarization geometries, π pump and σ pump, were used to study separately different groups of MJ levels in magnetic fields up to 3 kG. An irreducible tensor derivation of the fine structure and Zeeman Hamiltonian matrix in Hund’s case (b) symmetrized basis functions was carried out and used to determine the perturbation parameters and the spin–spin and spin–rotation fine structure constants by a fit to the Zeeman splitting pattern. The spin–orbit contribution to the fine structure is too small in the high J states to be found accurately. The electronic spin–orbit coupling perturbation for the interaction between the singlet and triplet states was also determined to be Hel(A∼b)=0.1172±0.0008 cm−1.