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Physical Review Letters

Resonant Rydberg-Atom-Rydberg-Atom Collisions

Kambiz Ali Safinya, J. F. Delpech, F. Gounand, Wolfgang Sandner, Thomas F. Gallagher

Published Aug 10, 1981• 88 citations
Publisher

Abstract

We have observed the sharply resonant enhancement of the cross section of the collision process $ns+ns\ensuremath{\rightarrow}np+(n\ensuremath{-}1)p$ between two $\mathrm{Na}ns$ ($16<~n<~27$) atoms which occurs when the levels are shifted by an electric field so that the $ns$ level lies midway between the two $p$ states. The widths of the collisional resonances scale as ${n}^{\ensuremath{-}2}$ and the magnitudes of the cross sections as ${n}^{4}$, with values of 0.5 GHz and 8\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{\AA{}}}^{2}$ for the $20s$ state. These results are in good agreement with calculations based on the long-range dipole-dipole interaction between the two atoms.

DOI
10.1103/physrevlett.47.405

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We have observed the sharply resonant enhancement of the cross section of the collision process ns+ns\ensuremath→np+(n\ensuremath−1)pns+ns\ensuremath{\rightarrow}np+(n\ensuremath{-}1)pns+ns\ensuremath→np+(n\ensuremath−1)p between two Nans\mathrm{Na}nsNans (16< n< 2716<~n<~2716< n< 27) atoms which occurs when the levels are shifted by an electric field so that the nsnsns level lies midway between the two ppp states. The widths of the collisional resonances scale as n\ensuremath−2{n}^{\ensuremath{-}2}n\ensuremath−2 and the magnitudes of the cross sections as n4{n}^{4}n4, with values of 0.5 GHz and 8\ifmmode\times\else\texttimes\fi{}108{10}^{8}108 A˚2{\mathrm{\AA{}}}^{2}A˚2 for the 20s20s20s state. These results are in good agreement with calculations based on the long-range dipole-dipole interaction between the two atoms.
The experimental investigation of the resonant collision process $\mathrm{Na}ns+\mathrm{Na}\mathrm{ns}\ensuremath{\rightarrow}\mathrm{Na}np+\mathrm{Na}(n\ensuremath{-}1)p$ which occurs when the levels are tuned with an electric field so that the ns\mathrm{ns}ns level lies midway between the two ppp levels is described in detail. The observations are shown to be in good agreement with a long-range resonant dipole-dipole interaction.
A large enhancement in the resonant collisional energy-transfer process $\mathrm{Na}(\mathrm{ns})+\mathrm{Na}(\mathrm{ns})\ensuremath{\rightarrow}\mathrm{Na}[(n\ensuremath{-}1)p]+\mathrm{Na}(\mathrm{nl})$ or $\mathrm{Na}(\mathrm{ns})+\mathrm{Na}(\mathrm{ns})\ensuremath{\rightarrow}\mathrm{Na}(\mathrm{np})+\mathrm{Na}[(n\ensuremath{-}1)l]$ is observed when the initial ns\mathrm{ns}ns state lies midway between $(n\ensuremath{-}1)p$ and an nl\mathrm{nl}nl state or between np\mathrm{np}np and an $(n\ensuremath{-}1)l$ state. This resonance condition, achieved by the electric field tuning of the levels, leads to large cross sections \ensuremath{\sim} 109{10}^{9}109 A˚2{\mathrm{\AA{}}}^{2}A˚2 and narrow linewidths \ensuremath{\sim} 1 GHz. These cross sections and linewidths are comparable to the previously observed resonant process $\mathrm{Na}(\mathrm{ns})+\mathrm{Na}(\mathrm{ns})\ensuremath{\rightarrow}\mathrm{Na}[(n\ensuremath{-}1)p]+\mathrm{Na}(\mathrm{np})$. We have also observed a similar collisional energy transfer starting from a completely different initial state, i.e., $\mathrm{Na}(15d)+\mathrm{Na}(15d)\ensuremath{\rightarrow}\mathrm{Na}(16f)+\mathrm{Na}(15p)$. In this case the enhancement in the cross section (\ensuremath{\sim} 108{10}^{8}108 A˚2{\mathrm{\AA{}}}^{2}A˚2) is observed at a relatively low electric field \ensuremath{\sim} 17 V/cm.
Microwave-assisted resonant collisional energy transfer process $\mathrm{Na}(\mathrm{ns})+\mathrm{Na}(\mathrm{ns})+h\ensuremath{\nu}\ensuremath{\rightarrow}\mathrm{Na}(\mathrm{np})+\mathrm{Na}((n\ensuremath{-}1)p)$ is observed between two colliding Na(ns)\mathrm{Na}(\mathrm{ns})Na(ns) atoms. The collisional resonance occurs when the microwave frequency $\ensuremath{\nu}$ equals the collisional energy defect $\frac{\ensuremath{\Delta}W}{h}$. The observed cross sections are very large (\ensuremath{\sim}108{10}^{8}108 A˚2{\mathrm{\AA{}}}^{2}A˚2) and require very low microwave power, \ensuremath{\sim}10 W/cm2{\mathrm{cm}}^{2}cm2. These observations are in good agreement with an intuitive two-photon absorption model for these collisions.
Resonant collisional energy transfer can be studied in uprecedented detail using Rydberg atoms. Using the dependence of the energy level spacing on principal quantum number it is possible to examine resonant energy transfer in collisions with molecules. Finer control of the level spacing is possible by varying the level spacings with an electric field. Using this approach it is possible to observe sharply resonant collisions between Rydberg atoms. They are resonant dipole-dipole collisions which have cross sections orders of magnitude larger than the geometric cross sections of the colliding Rydberg atoms. These collisions are interesting in themselves; for example, they exhibit intracollisional interference, and it is possible to know when individual collisions begin and end. Perhaps the most interesting aspect of these collisions is that, because of the large dipole moments of the Rydberg atoms, they are easily controlled by microwave and radio frequency fields.
We have observed resonant-energy-transfer collisions between K Rydberg atoms. We have tuned the ns1/2{\mathrm{ns}}_{1/2}ns1/2​+(n-2)dj{\mathrm{d}}_{\mathrm{j}}dj​\ensuremath{\rightarrow}${\mathrm{np}}_{\mathrm{j}\ensuremath{'}}$+(n-1)${\mathrm{p}}_{\mathrm{j}\mathrm{\ensuremath{''}}}$ collisional process into resonance with a static electric field. The measured resonance field positions from n=24 to 29 agree well with the calculated positions. We show that the dipole-dipole selection rule, \ensuremath{\Delta}mj{\mathrm{m}}_{\mathrm{j}}mj​=0,\ifmmode\pm\else\textpm\fi{}1, is valid for these resonances but that \ensuremath{\Delta}J=0,\ifmmode\pm\else\textpm\fi{}1 is not, due to the presence of the electric field. In addition, the polarization of the lasers used to excite the Rydberg states can suppress many of the resonances.
Rydberg states of atoms have intrinsic properties that should allow their use as highly sensitive and tunable detectors at far infrared wavelengths. The intrinsic limit of such atomic detectors are collisional processes. This report describes the results of a research program to better understand an important class of collisions--namely, the resonant collision of two Rydberg atoms. We have extensively measured the linewidths and the collision cross sections and have determined thier dependence on parameters such as the principal quantum number and the Rydberg state angular momentum. The resonant collisions are shown to occur for a wide variety of initial states with cross sections of the order of 10 to the 9th power square-Angstroms and scaling as (n*O to the 4th order).