Three-photon resonance enhanced excitation spectrum of atomic thallium

We present first measurements on the resonance enhanced three-photon excitation in thallium, using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. The even-parity 6s2ns2S1/2 (15 ⩽ n ⩽ 31) and nd 2D5/2 (13 ⩽ n ⩽ 42) Rydberg states have been observed. The measured level energies reveal a dynamic shift from the photoabsorption values, which is decreasing with increasing n, while the asymmetry in the line profile is observed to be increasing with increasing n. In addition, an autoionising level (sp24P3/2) adjacent to the ionization threshold has been observed and quantitatively analyzed using the Fano's photoionization cross-section relation for an isolated autoionising resonance.