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Physical Review A

Thermal-energy collisions of rubidium Rydberg states with N2 molecules

L. Petitjean, F. Gounand, P. R. Fournier

Published Aug 1, 1984• 23 citations
Publisher

Abstract

The total depopulation cross sections by ${\mathrm{N}}_{2}$ of high rubidium Rydberg states, $\mathrm{Rb}(\mathrm{ns}) (n=25\ensuremath{-}46)$ and $\mathrm{Rb}(\mathrm{nd}) (n=23\ensuremath{-}44)$, have been measured using time-resolved field ionization technique and compared to previous results obtained on the quenching of $\mathrm{Rb}(\mathrm{nl})$ by CO. It is shown that the $\mathrm{Rb}(\mathrm{nl})$ depopulation by ${\mathrm{N}}_{2}$ is much less efficient than the $\mathrm{Rb}(\mathrm{nl})$ depopulation by CO and leads mainly to the quasielastic process of angular momentum mixing via the short-range interaction between the quasifree Rydberg-state electron and the molecule. It is clearly demonstrated that the presence of a permanent dipole in CO changes the collisional processes drastically. The agreement between experimental and theoretical predictions based on the impulse approximation is quite satisfactory.

DOI
10.1103/physreva.30.736

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The total depopulation cross sections by N2{\mathrm{N}}_{2}N2​ of high rubidium Rydberg states, Rb(ns)(n=25\ensuremath−46)\mathrm{Rb}(\mathrm{ns}) (n=25\ensuremath{-}46)Rb(ns)(n=25\ensuremath−46) and Rb(nd)(n=23\ensuremath−44)\mathrm{Rb}(\mathrm{nd}) (n=23\ensuremath{-}44)Rb(nd)(n=23\ensuremath−44), have been measured using time-resolved field ionization technique and compared to previous results obtained on the quenching of Rb(nl)\mathrm{Rb}(\mathrm{nl})Rb(nl) by CO. It is shown that the Rb(nl)\mathrm{Rb}(\mathrm{nl})Rb(nl) depopulation by N2{\mathrm{N}}_{2}N2​ is much less efficient than the Rb(nl)\mathrm{Rb}(\mathrm{nl})Rb(nl) depopulation by CO and leads mainly to the quasielastic process of angular momentum mixing via the short-range interaction between the quasifree Rydberg-state electron and the molecule. It is clearly demonstrated that the presence of a permanent dipole in CO changes the collisional processes drastically. The agreement between experimental and theoretical predictions based on the impulse approximation is quite satisfactory.
The quenching cross sections of highly excited Rb $ns(25\ensuremath{\le}n\ensuremath{\le}45)$ and Rb $nd(23\ensuremath{\le}n\ensuremath{\le}44)$ states by CO, a weak polar molecule, are measured, and these experimental data are compared to theoretical predictions where the main process responsible for the quenching is taken to be the Rydberg-electron-CO interaction. It is shown that the long-range Rydberg-electron-molecular-dipole interaction is often dominant leading mainly to nnn-changing and, to a lesser extent, to Rydberg ionization via quasiresonant molecular rotational transitions. It is also shown that the Rydberg-${e}^{\ensuremath{-}}$ ---molecular-quadrupole interaction is negligible and the short-range interaction leading to lll-mixing processes is less efficient. The agreement between experimental and theoretical values is quite satisfactory.
We have measured total cross sections for the collisional depopulation of high-lying nP\mathrm{nP}nP states ($12\ensuremath{\le}n\ensuremath{\le}22$) of rubidium induced by thermal collisions with ground-state rubidium or rare-gas atoms. It is observed that for $n\ensuremath{\ge}14$ the rubidium-rare-gas cross sections do not vary with the principal quantum number, contrary to previous hypotheses. Our results are discussed using the simple argument of quasifree behavior of the valence electron involving either an elastic-scattering or a binary-encounter approach. However, the disagreement observed between the experimental and the computed cross sections suggests that a three-body approach is generally needed for a good understanding of collisional processes involving highly excited states.
The cross sections for the quenching of highly excited Rb(nS) (32<or=n<or=45) and Rb(nD) (30<or=n<or=41) states have been measured for the different rare gases. In most cases studied, the quenching cross sections seem rather constant or slightly decreasing with n for a given series and a given rare gas. The experimental data are compared with the results of calculations using various theoretical approaches. It is shown that in this range of large n values the main process responsible for the quenching is the Rydberg-electron-rare-gas interaction, as previously stated for lower n values (10<or=n<or=20).
Total depopulation cross sections of Rb(ns) and Rb(nd) levels by collisions with NH3{\mathrm{NH}}_{3}NH3​ have been measured over a wide range of n values (24\ensuremath{\le}n\ensuremath{\le}49). The analysis of the n-dependent behavior of the cross sections reveals that NH3{\mathrm{NH}}_{3}NH3​ inversion can produce a very efficient l-mixing process via a quasiresonant mechanism. This behavior appears qualitatively well described by theoretical calculations based on the impulse approximation (IA). A quantitative comparison between theory and experiment appears difficult, but it seems that the IA should provide reasonable estimates (within a factor of 4), though its use for large dipole moment (1.47 D for NH3{\mathrm{NH}}_{3}NH3​) is questionable from a theoretical point of view.
Thermal collisions between laser excited Xe(nf) Rydberg atoms and a variety of neutral atomic and molecular targets have been investigated. In Xe(nf)-HF collisions, n-changing, 4-changing and collisional ionization were observed. For n = 26 and 27, absolute collisional cross sections for these processes have been determined using time-resolved selective field ionization. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented. In Xe(nf) - Xe, Kr, Ar, CO2, CO and N2 collisions the dominant mechanism was observed to be 4-changing: no evidence of n-changing or collisional ionization was apparent. Absolute collisional cross sections have been determined for a range of n between 24 and 37. Comparison of these cross sections with those obtained using a theoretical model in which effects due to the Rydberg core are ignored appears to lead to satisfactory results only for Xe, Kr and CO2, whereas when core effects are included satisfactory agreement between experiment and theory is found in all cases.
Collisional transfers induced in the n=6--13 Li Rydberg states by H2{\mathrm{H}}_{2}H2​, N2{\mathrm{N}}_{2}N2​, and CO molecules have been experimentally studied in a heat-pipe oven. The l-mixing cross sections have been deduced from the fast component of the population relaxations following pulsed laser excitation. The order of magnitude of these cross sections (103{10}^{3}103 A˚2{\r{}}^{2}˚2) is well predicted by the existing theories. For large n, the cross sections stay approximately constant. This can be partly explained by the influence of inelastic processes such as n changing. In the particular case of H2{\mathrm{H}}_{2}H2​, n-changing collisions have been identified by the fluorescence starting from the n'=n\ifmmode\pm\else\textpm\fi{}1, n-2 levels. Cross sections of the order of 10 A˚2{\r{}}^{2}˚2 have been measured. The systematic study of the n-changing cross sections as a function of the transferred energy shows some evidence of resonance phenomena corresponding to an energy exchange between the Rydberg electron and the molecular rotation. This effect is also observed when H2{\mathrm{H}}_{2}H2​ is replaced by D2{\mathrm{D}}_{2}D2​. These resonances are well described by the theory of Petitjean and Gounand based on the e-quadrupolar interaction.