A new branch of atomic physics -~the interactions, dynamics and collisions in ultracold systems-~ has naturally evolved from recent advances in the cooling and trapping of neutral gases. Cold Rydberg plasmas, wherein electrons and ions coexist with atoms Ry(n,l) in highly excited Rydberg states n,l have been recently produced by direct laser excitation or by laser ionization of atoms initially prepared at sub-millikelvin temperatures. When atoms, initially prepared in the ground state at sub-millikelvin temperatures, are laser excited to highly excited Rydberg levels, a gas of slowly moving Rydberg atoms is produced. The sufficiently dense sample of highly excited atoms then ionizes spontaneously with very high efficiency producing extremely cold plasma containing Rydberg atoms, electrons and ions. The ionization is originally caused by collision with the small number of Rydberg atoms at room temperature or by absorbtion of background radiation. Most of the electrons are trapped by the ions and collide, de-excite and ionize the Rydberg gas creating an electron avalanche. The plasma state can also revert back to Rydberg atoms via three-body recombination between electrons and ions. The fact that the plasma can be sustained is evidence of the importance of ``super-elastic" (de-excitation) collisions which provides free electrons with sufficient energy needed for ionization. Electron-impact ionization of Rydberg atoms is therefore of key significance and has been recently observed for sodium Rydberg atoms in the $ns$ and $nd$ levels originally produced by laser excitation.

In this paper, we discuss the interactions, collisions and radiative cascade involved in Rydberg plasmas. In particular, a classical theory of radiative cascade is presented. Cross sections for ionization of Rydberg atoms in n,l states by collision with electrons are determined from new theory. The full dependence of the cross sections on the initial angular momentum l of the target is revealed, with interesting physical characteristics. Analytical curve-fits for the cross sections are then provided. The rates for three-body capture into state n,l are deduced from detailed balance. Analytic expressions for the ionization cross section are also derived within the classical impulse approximation and compared with the classical trajectory results. Rydberg-Rydberg long-range interactions are also provided and discussed.