Hyperfine structure and multichannel quantum defect theory analysis of the 6sng rydberg series of barium

High resolution laser-atomic-beam spectroscopy has been applied to study the hyperfine structure of the barium 6sng Rydberg series forn=6?40. The singlet-triplet mixing of the two 6sng J=4 series is deduced through a diagonalisation of the total Hamiltonian. Multichannel Quantum Defect Theory (MQDT) has been used to reproduce both experimental level energies of the 6sng J=3,4 and 5 series and the extracted singlet-triplet mixing. The close-coupling channels of the 6sng Rydberg series turn out to be nearly purejj-coupled channels in contradistinction to lowerl Rydberg series. Furthermore MQDT is used to analyse the observed isotope shifts and peak intensities of the 6sng Rydberg series.