Effects of blackbody radiation on highly excited atoms

Transitions between energetically close-lying Rydberg states are characterized by large electric dipole matrix elements and frequencies low enough that at room temperature the photon occupation number is ≃ 10. Consequently the absorption of blackbody radiation and the stimulated emission produced by it lead to an efficient redistribution of population among nearby levels. In some cases dramatic effects are observed, for example, radiative lifetimes that are shortened to a small fraction of their $T=0\mathrm{}$ values. In addition, the 300-K blackbody field of ∼10 V/cm produces ac Stark shifts of the Rydberg levels. Calculations of the population redistribution effects and ac Stark shifts are presented. Examples are given to illustrate that care must be taken in experiments with highly excited atoms to ensure that only one state is being observed rather than a distribution.