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Physical Review A

Radiatively assisted collisions of K Rydberg atoms

D. S. Thomson, Michael J. Renn, Thomas F. Gallagher

Published Jan 1, 1992• 6 citations
Publisher

Abstract

We have observed radiatively assisted collisions between velocity-selected K Rydberg atoms in radio-frequency (rf) fields. The low relative velocity of the atoms leads to long collision times, or equivalently, narrow, 1-MHz, collisional resonances. We have carried out systematic measurements with rf fields in the strong-field regime, and have shown that the cross sections agree quantitatively with the predictions of the strong-field theory. We have also shown experimentally and theoretically that the sum over m of the cross sections for m photon-assisted cross sections is larger than the resonant-collision cross section in the absence of a rf field.

DOI
10.1103/physreva.45.358

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By reducing the collision velocity of Rydberg atoms in resonant energy-transfer collisions, we have observed collisional resonances as narrow as 1 MHz, or collision times as long as 1.0 \ensuremath{\mu}s. Restricting the collision to a time less than the natural collision time results in transform broadening of the resonances, allowing us to know when an individual collision begins and ends. The latter point is verified by observing the dependence of the collision line shape on the phase of an oscillating field of frequency equal to the width of the collisional resonance.

Physical Review Letters•Dec 24, 1990•15 citations
We have observed radiatively assisted collisions between velocity-selected K Rydberg atoms in radio-frequency (rf) fields. The low relative velocity of the atoms leads to long collision times, or equivalently, narrow, 1-MHz, collisional resonances. We have carried out systematic measurements with rf fields in the strong-field regime, and have shown that the cross sections agree quantitatively with the predictions of the strong-field theory. We have also shown experimentally and theoretically that the sum over m of the cross sections for m photon-assisted cross sections is larger than the resonant-collision cross section in the absence of a rf field.
Ramsey interference fringes are observed in the line shape of resonant Rydberg-atom--Rydberg-atom collisions occurring in the presence of a radiation field of frequency comparable to the inverse of the collision time. At low frequencies the fringes are phase dependent, but at high frequencies they are not. We present a theoretical interpretation based on a quasistatic field description which connects the low- and high-frequency regimes. In the high-frequency regime, this model gives results equivalent to those obtained using a ``dressed-atom'' approach.
Resonant collisional energy transfer between atoms with small relative velocity is shown to have such long collision times, \ensuremath{\sim}0.17 \ensuremath{\mu}s, or equivalently such narrow linewidths, 6 MHz, that it may be used to make spectroscopic measurements. Specifically, we report the use of the sharply resonant collisional energy transfer ns+(n-2)d\ensuremath{\rightarrow}np +(n-1)p, between velocity-selected K atoms to determine an improved value, 1.711?5(5), and the K np-state quantum defect.
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.
The observation of radiative collisions in the strong-field regime is reported, specifically the multiphoton-microwave-assisted resonant collisional energy-transfer process $\mathrm{Na}(\mathrm{ns})+\mathrm{Na}(\mathrm{ns})\ensuremath{\rightarrow}\mathrm{Na}(\mathrm{np})+\mathrm{Na}((n\ensuremath{-}1)p)+mh\ensuremath{\nu}$, where mmm is as high as 4. By use of a "dressed-atom" approach in a low-frequency (or adiabatic) approximation, a theoretical interpretation valid for the strong-field regime is given which is in good agreement with the present experimental results.
By reducing the collision velocity of Rydberg atoms in resonant energy-transfer collisions, we have observed collisional resonances as narrow as 1 MHz, or collision times as long as 1.0 \ensuremath{\mu}s. Restricting the collision to a time less than the natural collision time results in transform broadening of the resonances, allowing us to know when an individual collision begins and ends. The latter point is verified by observing the dependence of the collision line shape on the phase of an oscillating field of frequency equal to the width of the collisional resonance.