Two-color three-photon excitation studies of thallium Rydberg states

We present new data on the even-parity Rydberg states of atomic thallium using two-step three-photon laser excitation technique in conjunction with a thermionic diode ion detector. Atoms are excited from the 6p 2P1/2 ground state to the 7p 2P1/2 intermediate state via two-photon excitation and subsequently promoted to the high lying ns 2 S1/2 and nd 2D3/2 Rydberg states. The first ionization potential of thallium is determined as 49,266.66(1) cm-1 using data for the ns 2 S1/2 (25 ≤ n ≤ 54) and nd 2D3/2 (24 ≤ n ≤ 65) Rydberg series. This value is believed to be more accurate because the contribution due to the hyperfine structure splitting of the 7p 2P1/2 state (0.07185 cm-1) is much smaller as compared to that of the 6p 2P1/2 ground state (0.711 cm-1).