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Physical Review A

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 Oct 1, 1988• 8 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.38.3271

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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.
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.
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.
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.
We apply two-photon resonant nondegenerate four-wave mixing with a resonant intermediate state for observing the broadening and the shifting of the barium Rydberg 6snd 1D2 series by collision with argon. The collision broadenings and the collision shifting cross sections are measured for n=1633. This technique is a purely optical means, and can achieve Doppler-free resolution of narrow spectral structures of Rydberg levels when the incident lasers have narrow bandwidths. Different from other experimental methods of studying the pressure dependence of the longitudinal relaxation rate of Rydberg states, our method is to investigate the pressure dependence of the transverse relaxation rate of the transition.
In rubidium Rydberg states, the collision nD_(5/2)+nD_(5/2)-->(n-2)F_(7/2)+(n+2)P_(3/2) is nearly resonant in the vicinity of n=43. As a result, over a short range of n centered around n approximately 43 the Rydberg-Rydberg interaction potential is quite large and turns from repulsive to attractive [Phys. Rev. A 75, 032712 (2007)10.1103/PhysRevA.75.032712]. We use state-selective field ionization to investigate the effect of this resonance on instantaneous excitation of mixed two-particle states, state-mixing collisions, and Penning ionization. We find that these processes depend on the magnitude and sign of the two-particle interaction potential, and thus on n near the resonance. The large magnitude of the observed state mixing provides evidence for many-body effects.
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.