Two-step laser spectroscopy of the np 2P1/2,3/2 Rydberg levels in thallium

New data on the energies and quantum defects of the highly excited Rydberg levels of thallium are presented using the two-step resonant excitation spectroscopic technique. The spectra were observed using two dye lasers simultaneously pumped by the SHG 532 nm and THG 355 nm of a Nd:YAG laser in conjunction with a thermionic diode ion detector. The Rydberg series 6s2np 2P1/2 (15 ⩽ n ⩽ 31) and np 2P3/2 (15 ⩽ n ⩽ 56) have been studied via the 6s2 7s 2S1/2 intermediate state. The first ionization potential is derived as 49266.66(5) cm−1, which is in good agreement with the earlier work. The relative intensities of the np 2P1/2,3/2 series decrease monotonically and follow the 1/n2.3 scaling.