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 $6^{2}P_{1/2}\to6^{2}D_{3/2}\to\mathrm{Tl}^{**}$ in the presence of an electric field with a strength of up to $1.5$ kV/cm near the level $16 F_{5/2}$. The optical transitions $6D_{3/2}\to18D_{3/2}$ и $6D_{3/2}\to16G_{7/2}$, which were induced by an external electric field and dipole-forbidden, have been studied experimentally. The values for the scalar polarizabilities (in units сm$^{-1}$/(kV/сm)$^2$) $\alpha_{0}(16F_{5/2})=3.71\pm0.3$, $\alpha_{0}(18D_{3/2})=11.70\pm0.25$, and $\alpha_{0}(16G_{7/2})=44.1\pm0.9$, 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.