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Journal of the Physical Society of Japan

m- andl-Changing Collisions of High-Rydberg Atoms with Rare-Gas Atoms

Takashi Yoshizawa, Michio Matsuzawa

Published Mar 15, 1985• 17 citations
Publisher

Abstract

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.

DOI
10.1143/jpsj.54.918

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Physical Review A•Jun 7, 2004•85 citations
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.
The title processes are theoretically investigated based on the model in which an excited Rydberg electron behaves as if it were 'free' and slow, with its interaction with the rare-gas atom playing a decisive role. The cross sections of l-changing collisions of the Rydberg atoms with the rare-gas atoms are evaluated for small angular momentum changes. They are compared with experimental data available at present, under the assumption that there is no selection rule for the distribution over the final angular momentum states of the Rydberg atom, and are found to be consistent with experimental findings. The relation between this approach and some others is examined. The possibility of learning about low-energy electron-rare-gas collisions from refined experimental data on the title processes is briefly discussed.
For $20\ensuremath{\le}n\ensuremath{\le}45$, two types of quasielastic state-changing-collisional (QESCC) processes, i.e., (i) lll-changing and (ii) nnn-and lll-changing processes, have been theoretically investigated based on the free-electron model in which an excited Rydberg electron behaves as if it were "free" and slow with its interaction with a rare-gas atom playing a decisive role. It is shown that a parameter ${u}_{min}[=\frac{\ensuremath{\nu}{\ensuremath{\nu}}^{\ensuremath{'}}\ensuremath{\Delta}E{a}_{0}}{(\ensuremath{\hbar}V)}]$ is quite useful for characterizing both of these QESCC processes in a unified way where $\ensuremath{\nu}(\ensuremath{\simeq}{\ensuremath{\nu}}^{\ensuremath{'}})$ is an effective principal quantum number of the initial (final) state, $\ensuremath{\Delta}E$ is an energy defect, and VVV is a relative velocity. It is theoretically found that higher-angular-momentum states are dominantly populated in the final channel of these QESCC processes with $1<{u}_{min}\ensuremath{\lesssim}\ensuremath{\nu}$ such as in the nnn- and lll-changing collision of Rb∗∗(ns)+He{\mathrm{Rb}}^{**}(\mathrm{ns})+\mathrm{He}Rb∗∗(ns)+He. This is in contrast with the uniform distribution over the final angular-momentum states for the usual lll-changing processes, i.e., ${u}_{min}\ensuremath{\lesssim}1$. The calculated cross sections of these QESCC processes with $1<{u}_{min}\ensuremath{\lesssim}\ensuremath{\nu}$ are found to be in reasonable agreement with experimental findings available at present.
Semiclassical calculations of the quasi-elastic l-mixing and inelastic n,l-changing processes in thermal collisions of a Rydberg sodium atom with rare-gas atoms (Ne, Ar, Kr and Xe) as well as and atoms with Ne are presented in a wide range of principal quantum numbers. The main emphasis is put on the study of the effects in the Rydberg-atom - rare-gas-atom collisions associated with both the short- and long-range (polarization) parts of the electron - perturber interaction. It is shown that the polarization interaction between the highly excited electron and the Ne atom provides the major contribution to the cross section of the l-mixing and n, l -changing transitions. We show that the Ramsauer - Townsend effect in slow electron scattering by the heavy rare-gas atoms significantly affects the values of the cross sections, particularly for the inelastic transitions with large energy transfer. A quantitative explanation of these phenomena is given using the recently developed semiclassical normalized perturbation theory for the Rydberg-atom - neutral collisions combined with the modified effective range theory for scattering of slow electrons by the rare-gas atoms.
We have extensively investigated quasielastic state-changing collisions (QESCC) of high-Rydberg atoms with rare-gas atoms including Kr and Xe based on the free-electron model. Theoretical results are compared with recent experimental findings for 25\ensuremath{\lesssim}n\ensuremath{\lesssim}48. It has been found that they explain almost all existing experimental data on the QESCC for high n available at present consistently and in a unified way. More detailed analyses concerning the higher-order term of the free-electron model show that there is still some discrepancy among the existing experimental data on the QESCC.
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.