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 rim and THG 355 nm of a Nd:YAG laser in conjunction with a thermionic diode ion detector. The Rydberg series 6s(2) np P-2(1/2) (15 less than or equal to n less than or equal to 31) and np P-2(3/2) (15 less than or equal to n less than or equal to 56) have been studied via the 6s(2) 7s S-2(1/2) 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 P-2(1/2.3/2) series decrease monotonically and follow the 1/n(2.3) scaling. (C) 2004 Elsevier B.V. All rights reserved.