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Physical Review A

Collisional angular-momentum mixing of Na Rydberg states with Xe

Ravinder Kachru, Thomas F. Gallagher, F. Gounand, Kambiz Ali Safinya, Wolfgang Sandner

Published Feb 1, 1983• 28 citations
Publisher

Abstract

The collisional angular-momentum mixing of the $\mathrm{Na}\mathrm{nd}$ Rydberg states into states of higher angular momentum with Xe as the collisional partner has been studied in zero field and in the presence of a small electric field. The collisional $l$-mixing cross section in zero field has been measured for $9\ensuremath{\le}n\ensuremath{\le}23$ using a vapor-cell-fluorescence method and a crossed-beam technique with a selective-field-ionization (SFI) scheme as the state detector. Using the $|{m}_{l}|$ selectivity of the SFI, we have also investigated the final-$|{m}_{l}|$-state distribution of the collisionally populated states of $l>2$. Our measurements suggest that the collisionally populated states have predominantly low $|{m}_{l}|$ values ($|{m}_{l}|\ensuremath{\le}2$). An application of a low electric field (\ensuremath{\sim} 50 V/cm) substantially reduces the collisional population of the low-$|{m}_{l}|$ states, thus reducing the total cross section as well.

DOI
10.1103/physreva.27.795

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The collisional angular-momentum mixing of the Nand\mathrm{Na}\mathrm{nd}Nand Rydberg states into states of higher angular momentum with Xe as the collisional partner has been studied in zero field and in the presence of a small electric field. The collisional lll-mixing cross section in zero field has been measured for 9\ensuremath≤n\ensuremath≤239\ensuremath{\le}n\ensuremath{\le}239\ensuremath≤n\ensuremath≤23 using a vapor-cell-fluorescence method and a crossed-beam technique with a selective-field-ionization (SFI) scheme as the state detector. Using the ∣ml∣|{m}_{l}|∣ml​∣ selectivity of the SFI, we have also investigated the final-∣ml∣|{m}_{l}|∣ml​∣-state distribution of the collisionally populated states of l>2l>2l>2. Our measurements suggest that the collisionally populated states have predominantly low ∣ml∣|{m}_{l}|∣ml​∣ values (∣ml∣\ensuremath≤2|{m}_{l}|\ensuremath{\le}2∣ml​∣\ensuremath≤2). An application of a low electric field (\ensuremath{\sim} 50 V/cm) substantially reduces the collisional population of the low-∣ml∣|{m}_{l}|∣ml​∣ states, thus reducing the total cross section as well.
The collisional angular-momentum mixing of $n=5\ensuremath{-}15$ Na ddd states with the l>2l>2l>2 states of the same nnn has been studied using He, Ne, and Ar as collision partners. At rare-gas pressures of \ensuremath{\sim}1 Torr, the $l\ensuremath{\ge}2$ states are thoroughly mixed and the mixture decays at the average radiative lifetime of all the $l\ensuremath{\ge}2$ states of the same nnn. Using \ensuremath{\sim}1 Torr of neon to collisionally mix the $l\ensuremath{\ge}2$ states, the average lifetime for $l\ensuremath{\ge}2$ was measured for each nnn and compared to the theoretical prediction for hydrogen, with which it is in excellent agreement. The cross section for lll mixing by each of the gases is found to increase sharply from n=5to10n=5 \mathrm{to} 10n=5to10 and decrease above n=10n=10n=10. The magnitudes of the lll-mixing cross sections are found to correlate with the low-energy scattering lengths of the collision partners.
In Na, the energy gap $\ensuremath{\Delta}E$ between the nnn, ddd state and the nnn, l>2l>2l>2 states is ten times larger than $\ensuremath{\Delta}E$ between nnn, fff and the nnn, l>3l>3l>3 states. To investigate the effect of this difference on the ddd- and fff-state lll-mixing cross sections, ${\ensuremath{\sigma}}_{d}$ and ${\ensuremath{\sigma}}_{f}$, we have recently carried out experiments to measure ${\ensuremath{\sigma}}_{d\ensuremath{-}f}$, the lll-mixing cross sections for a microwave-induced mixture of ddd and fff states. Using the previously measured values of ${\ensuremath{\sigma}}_{d}$, we have derived values of ${\ensuremath{\sigma}}_{f}$ which have the same nnn dependence as ${\ensuremath{\sigma}}_{d}$. ${\ensuremath{\sigma}}_{f}$ is larger than ${\ensuremath{\sigma}}_{d}$ for Ne, but smaller for He and Ar. In addition, we have analyzed the final states after one collision with Ar using selective field ionization of the 17l17l17l states and have observed that after one collision all the lll states are equally populated indicating that there are no $\ensuremath{\Delta}l$ selection rules for the process. This confirms the earlier suggestion that lll mixing is caused by a short-range interaction.
The degree of dipole-forbidden l change in Na(26d) and Na(28d) Rydberg atoms caused by slow Na+{\mathrm{Na}}^{+}Na+ ion impact at reduced velocity v\ifmmode \tilde{}\else \~{}\fi{}=0.1--0.9 has been measured. Na+{\mathrm{Na}}^{+}Na+ beam energies 7.3--684 eV were used. Previous measurements were greatly improved by reducing stray electric fields in the Rydberg collision region to about 10 mV/cm. The distributions of final-angular-momentum states were analyzed using directly measured diabatic selective-field-ionization profiles for l=3, 4, and 5. l-change results in Na(26d) and Na(28d) targets are very similar and clearly show an increasing amount of dipole-forbidden behavior for decreasing reduced velocities v\ifmmode \tilde{}\else \~{}\fi{} and partial agreement with the coupled-channel calculation of Beigman and Syrkin (Zh. Eksp. Teor. Fiz. 89, 400 (1985) [Sov. Phys. JETP 62, 226 (1985)]). The dipole-forbidden process is the main channel for angular-momentum change in ion--Rydberg-atom collisions at v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.7. At very low energy (v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.3) the measurements demonstrate that the final-state distributions are dominated by very high angular-momentum states.
Collisions with rare-gas atoms are observed to produce a lengthening of the lifetime of the highly excited ($n=5\ensuremath{-}10$) ddd states of Na. The effect is interpreted as collisional angular momentum mixing of the ddd state with l>2l>2l>2 states; thus the average lifetime of all states for which l> 2l>~2l> 2 is observed. The cross section for the process appears to increase as the geometrical cross section of the excited atom.
New experimental results have been obtained in collisions of singly charged ions with Na Rydberg atoms. (a) The distribution of final n states populated by electron capture shows non-monotonic variations of width with incident energy over the range 60-2100 eV. (b) The total capture cross section for alkali ions on Rydberg targets shows peaks and dips over the same energy range, correlated with particular reduced velocities v and the polarization of the exciting laser. (c) {ell}-Changing collision of Na{sup +} on Na(nd) targets, n=26 and 28, at low v preferentially transfer multiple units {h_bar} of orbital angular momentum under single-collision conditions, despite very large impact parameters. (d) Inelastic impacts of Na{sup f+} on Na 36s and 30p states, perturbed by electric fields, populate a strongly field-dependent mixture of nearby Stark levels. A synopsis of recent progress will be given.
An approximate scaling formula has been determined that permits the rapid estimation of cross sections for angular-momentum-changing collisions of Rydberg atoms with a variety of targets, using information about low-energy-electron scattering from the target. The formula is obtained by fitting the results of coupled-channel and Born-approximation calculations to functions of reduced parameters. Application to Na(nd)\mathrm{Na}(\mathrm{nd})Na(nd) + He, Ne, Ar, N2{\mathrm{N}}_{2}N2​, CH4{\mathrm{H}}_{4}H4​, C3{\mathrm{C}}_{3}C3​H8{\mathrm{H}}_{8}H8​; Rb(nf)\mathrm{Rb}(\mathrm{nf})Rb(nf) + He, Ar, Xe; and Xe(nf)\mathrm{Xe}(\mathrm{nf})Xe(nf) + CO2{\mathrm{O}}_{2}O2​, suggests that the accuracy is about a factor of 2.