Cold-Rydberg-gas dynamics

Using state-selective field ionization, the state distributions of Rydberg atoms in cold Rydberg gases are measured for various initially excited Rydberg levels, populations, and evolution times. We provide direct experimental evidence for l-changing collisions that we previously observed indirectly [ S. K. Dutta, D. Feldbaum, A. Walz-Flannigan, J. R. Guest and G. Raithel Phys. Rev. Lett. 86 3993 (2001)]. We also observe n-mixing and find that its effects are largely in agreement with recent theoretical work on n-changing collisions between electrons and Rydberg atoms, thus enabling an estimation of the electron temperature. Unexpectedly large populations of atoms are found in states with principal quantum numbers much lower than that of the initially excited atoms. We explain this observation by collisions between high-l Rydberg atoms, which are highly polar and can collide due to static electric-dipole forces between them.