  We have studied an ionization of alkali-metal Rydberg atoms by blackbody
radiation (BBR). The results of the theoretical calculations of ionization
rates of Li, Na, K, Rb and Cs Rydberg atoms are presented. Calculations have
been performed for nS, nP and nD states which are commonly used in a variety of
experiments, at principal quantum numbers n=8-65 and at the three ambient
temperatures of 77, 300 and 600 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. A Cooper minimum for BBR-induced transitions between
bound Rydberg states of Li has been found. The calculated ionization rates are
compared with our earlier measurements of BBR-induced ionization rates of Na nS
and nD Rydberg states with n=8-20 at 300 K. A good agreement for all states
except nS with n>15 is observed. Useful analytical formulas for quick
estimation of BBR ionization rates of Rydberg atoms are presented. Application
of BBR-induced ionization signal to measurements of collisional ionization
rates is demonstrated.
