Description

In the present work, we present extensive new data on thallium employing multi-photon and multi-step excitation techniques in conjunction with a thermionic diode ion detector. A number of different excitation paths have been used. The first experiment addresses the observation of odd- parity levels np 2P1/2,3/2 and np 2F5/2 using two- photon excitation from the ground state. In the second set of experiments we employed a two-step excitation scheme using 6s2 7s 2S1/2 intermediate level and observed the np 2P1/2,3/2 levels upto n = 56. The even-parity levels have also been excited using three-photon excitation from the ground state revealing ns 2S1/2 and np 2P3/2,5/2 levels. In the subsequent experiments, we used a two-color three- photon excitation technique to approach the ns 2S1/2 and nd 2D3/2 levels using 7p 2P1/2 as an intermediate level. The nd series have been extended upto n = 65 and it is noticed that the intensity of the high Rydberg series scales as 1/n2.8. However, the np 2P1/2,3/2 series, which have been observed upto n = 56 scale as 1/n2.3 in contrast to the scaling law of 1/n3 for an unperturbed Rydberg series.