Results of theoretical calculations of ionization rates of Rb and Na Rydberg atoms by blackbody radiation 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 K. In the calculations we have taken into account the contributions of BBR-induced redistribution of population between Rydberg states prior to photoionization and .eld ionization by extraction electric .eld pulses.
Figure 1: A comparison of the calculated BBR photoionization rates of Na nD Rydberg atoms with the experimental data.
A semi-classical model  was used to calculate the bound-bound and bound-free matrix elements. The obtained results show that these phenomena affect both the magnitude of total ionization rates and shapes of their dependencies on the principal quantum number n. The theoretical 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 K , and good agreement for all states except nS with n > 15 is obtained. Useful analytical formulae allowing quick estimates of ionization rates are also presented .