Lifetimes along perturbed Rydberg series in neutral thallium

Radiative lifetimes of 15 Tl I levels belonging to the 6s(2)ns(2)S(1/2) (n = 7-14) and 6s(2)nd(2)D(3/2) Rydberg series (n = 6-12) have been measured using a time-resolved laser-induced fluorescence technique. All the measured levels have been excited from the ground state 6s(2)6p(2)P(1/2)(0) (odd parity) with a single-step excitation process. The general perturbation of the ns series by the 6s6p(2) configuration and the corresponding modification of the lifetimes are adequately reproduced by a theoretical model including core-polarization effects and combined with a least-squares fit to the observed energy levels. The general behaviour of the lifetime values for the 6s(2)np odd levels along the Rydberg series is also well reproduced. The use of the multiconfiguration quantum defect theory has allowed us to obtain lifetime values along the 6s(2)ns(2)S(1/2) series up to levels with n = 31.