Results of theoretical calculations of ionization rates of Rb and Na Rydberg atoms by blackbody radiation (BBR) are presented. Calculations have been performed for $nS$, $nP$, and $nD$ states of Na and Rb, which are commonly used in a variety of experiments, at principal quantum numbers $n=8–65$ and at three ambient temperatures of 77, 300, and $600\mathrm{K}$. A peculiarity of our calculations is that we take into account the contributions of BBR-induced redistribution of population between Rydberg states prior to photoionization and field ionization by extraction electric field pulses. The obtained results show that these phenomena affect both the magnitude of measured ionization rates and shapes of their dependences on $n$. The calculated ionization rates are compared with the results of our earlier measurements of BBR-induced ionization rates of Na $nS$ and $nD$ Rydberg states with $n=8–20$ at $300\mathrm{K}$. A good agreement for all states except $nS$ with $n>15$ is observed. We also present the useful analytical formulas for the quick estimation of BBR ionization rates of Rydberg atoms.

We report results from studies of the Autler-Townes (AT) effect observed in sodium molecules from a molecular beam. A relatively weak laser field $P$ couples an initially populated rovibronic level $g$ in the electronic ground state (here $X{}^1\Sigma _g^{+},v^{″}=0$, $J^{″}=7$) to a selected excited rovibronic level $e$ (here $A{}^1\Sigma _u^{+},v^{\prime }=10$, $J^{\prime }=8$), which in turn is coupled by a relatively strong laser field $S$ to a more highly excited level $f$ (here $5{}^1\Sigma _g^{+},v=10$, $J=9$), a scheme we idealize as a three-state ladder. The AT effect is seen by scanning the frequency of the $P$ field while recording fluorescence from both the $e$ and $f$ levels in separate detection channels. We present qualitative theoretical considerations showing that, when the $P$ field is weak, the ratio of doublet component areas in the excitation spectrum from level $f$ can be used to determine the lifetime of this level. We obtain a value of $17\pm 3\mathrm{ns}$. When the $P$ field is stronger, such that its Rabi frequency is larger than the decay rate of level $e$, the fraction of $f$-level population that decays to the intermediate electronic state $A{}^1\Sigma _u^{+}$ can be deduced from the AT spectrum. When supplemented with values of Franck-Condon and Hönl-London factors, our measurements give a value for the branching ratio (the fraction returning to level $e$) of $r_e=0.145$ with a statistical error of $\pm 0.004$. The use of a strong $P$ field on the $g\text{−}e$ transition and a weak $S$ field as a probe on the $e\text{−}f$ transition results in complex line shapes in the excitation spectrum of level $f$, not showing the familiar Autler-Townes doublet structure.