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AIP conference proceedings

State mixing in collisions involving highly excited barium atoms

M. Allegrini, E. Arimondo, E. Menchi, Charles E. Burkhardt, M. Ciocca, W. P. Garver, Silvia Gozzini, Jacob J. Leventhal, J. D. Kelley

Published Jan 1, 1988• 0 citations
Publisher

Abstract

Using 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.

DOI
10.1063/1.37338

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Journal of the Physical Society of Japan•Mar 15, 1985•17 citations
Using 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.
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.
Shift and broadening parameters for doubly excited states of 5d6d, 5d7d, 5d7s, 5d8s and 6p 2 configurations in neutral barium atoms are presented. A tunable dye laser pumped by an excimer laser was used to populate the Rydberg states of Ba by two-photon absorption from the 6s 2 1 S 0 ground state. Inert gases Ar, Kr and Xe at 100 mbar pressure were used as perturbing gases. The resulting line shapes were fitted to a Voigt line shape function, from which shift and broadening parameters were inferred.
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 lll-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 nnn-mixing and find that its effects are largely in agreement with recent theoretical work on nnn-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-lll Rydberg atoms, which are highly polar and can collide due to static electric-dipole forces between them.
Collisional broadening and shift data for even parity states of neutral barium are presented. A two-photon absorption technique is used to populate Rydberg states of n d 1 D 2 , 3 D 2 and 1 S 0 series. High-resolution Ba excitation spectra were measured using a diode detector while scanning the frequency of an excimer pumped dye laser. Inert gases Ar, Kr and Xe were used as perturbers at pressures ranging from 10 mb to 400 mb. Shifts and broadenings of spectral lines were measured as a function of pressure and principal quantum number n . Several perturbations and deviations from expected results are discussed in this paper.
State changing of high-Rydberg atoms by collisions with rare-gas atoms have been theoretically investigated using the free-electron model. The non-selective character of transitions arising from the short range of the interaction between the high-Rydberg atoms and the rare-gas atoms is found to be strongly modified in the following two cases. First, for the high-Rydberg atoms in the circular states with | m |= l ≃ n , it has been found that transitions take place overwhelmingly between the circular states. This propensity rule originates from a highly anisotropic electron-density distribution of the initial circular states. Second, for the high-Rydberg states with large quantum defects (modulo 1), suppression of the yield of the lower angular momentum states have been observed. This cancels out the fair enhancement of the production of the high-Rydberg states with small | m | seen for the hydrogenic case with small quantum defects. It leads to the m distribution of the final states apparently proportional to their statistical weight.