State mixing in collisions involving highly excited barium atoms

With the use of two-step laser excitation, it is shown that the state distributions of highly excited barium atoms are substantially altered by inelastic collisions with barium atoms. For ''pure'' Rydberg states the initially produced ns or nd eigenstates are rapidly mixed with nearly degenerate n'l' states. For initially produced states that may be viewed as admixtures of Rydberg and doubly excited independent-electron eigenstates, collisions convert the distribution to one with nearly pure, albeit state-mixed, Rydberg character. Broadening of the absorption profiles of the highly excited states is also observed. This effect results from quasistatic interaction between two highly excited atoms during the excitation. Beyond a critical excited-atom density, avalanche effects lead to an apparent narrowing of these profiles which subsequently broaden at still higher atom density