Rydberg atoms in far-infrared radiation fields. I. Dipole matrix elements of H, Li, and Rb

The radial electric dipole moments of bound-bound, bound-free, and free-free transitions of high-lying (n>15) Rydberg states and low-lying continuum states (e<400 cm^-1) are calculated for hydrogen and the alkali-metal atoms. All dipole matrix elements for the transitions between low-l (s, p, d, and f) states of H, Li, and Rb are presented. Results of similar calculations for the other alkali-metal atoms (Na, K, and Cs) are summarized. These numerical results, using quantum defect theory, are compared with semiclassical approximations. The dipole matrix elements cross zero at certain combinations of initial and final states. The matrix elements at these so-called Cooper minima in Li, K, Rb, and Cs are presented in detail.