The Excitation of Rydberg Atoms of Thallium in an Electric Field

The spectrum of excitation of Rydberg states of thallium atoms has been investigated using a collimated atomic beam in a two-step isotope selective laser scheme 6(2) P (1/2) -> 6(2) D (3/2) -> Tl** in the presence of an electric field with a strength of up to 1.5 kV/cm near the level 16F (5/2). The optical transitions 6D (3/2) -> 18D (3/2) and 6D (3/2) -> 16G (7/2), which were induced by an external electric field and dipole-forbidden, have been studied experimentally. The values for the scalar polarizabilities (in units Nm(-1)/(kV/Nm)(2)) alpha(0)(16F (5/2)) = 3.71 +/- 0.3, alpha(0)(18D (3/2)) = 11.70 +/- 0.25, and alpha(0)(16G (7/2)) = 44.1 +/- 0.9, which are compared with the calculated one, have been obtained. The new values of energy parameters for the states 18D (3/2) and 16G (7/2) have been determined.