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AIP conference proceedings

Radiative collisions of Rydberg atoms

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

Published Jan 1, 1993• 0 citations
Publisher

Abstract

We describe the radiative collisions between Rydberg atoms in which several radio frequency photons are absorbed during the collisions. Specifically, we show the evolution of the process as the photon frequency is raised from much less than the collision time to much greater than the collision time. In addition we show how the theoretical descriptions in these frequency regimes are connected.

DOI
10.1063/1.45236

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Radiatively assisted collisions of K Rydberg atoms

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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.

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A radiative collision is a resonant energy transfer collision in which two atoms absorb or emit photons during the collision. Alternatively, a radiative collision is the emission or absorption of a photon from a transient molecule, and, as shown by Gallagher and Holstein, radiative collisions can also be described in terms of line broadening. In a line broadening experiment there are typically many atoms and weak radiation fields, and in a radiative collision experiment there are few atoms and intense radiation fields. The only real difference is whether there are many atoms or many photons. Due to the short collision times, ∼10 −12 s, simply observing radiative collisions between low lying states requires high optical powers, and entering the regime where the optical field is no longer a minor perturbation seems unlikely. Due to their long collision times and large dipole moments, Rydberg atoms provide the ideal system in which to study radiative collisions in a quantitative fashion. As we shall see, it is straightforward to enter the strong field regime in which the radiation field, a microwave or rf field to be precise, is no longer a minor perturbation. Ironically, while the experiments are radiative collision experiments, with few atoms and many photons, the description of the strong field regime is given in terms of dressed molecular states, which is more similar to a line broadening description.

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Scaling of radiative collisions between Rydberg atoms

Panming Fu, J. D. Newman, D. S. Thomson +1 more

We have measured the n dependence of the microwave field required to make the single-photon radiatively assisted collision process Na(ns)+Na(ns)+kh\ensuremath{\nu}\ensuremath{\rightarrow}Na(np)+Na ((n-1)p)+(k+m)h\ensuremath{\nu}, m=1, 10% as likely as the resonant collision process in which no photons are absorbed, m=0. Experimentally we find an ${n}^{\mathrm{*}}$ dependence of [2.06(15)\ifmmode\times\else\texttimes\fi{}${10}^{3}$]${n}^{\mathrm{*}\mathrm{\ensuremath{-}}2.0}$ (V/cm), where ${n}^{\mathrm{*}}$ is the effective quantum number of the Na ns state. Over the range of n studied, 16\ensuremath{\le}n\ensuremath{\le}24, this field is within 20% of the field calculated using a theory based on resonant dipole-dipole collisional energy transfer of microwave dressed atoms.

Physical Review A•Sep 1, 1989•1 citation
We describe the radiative collisions between Rydberg atoms in which several radio frequency photons are absorbed during the collisions. Specifically, we show the evolution of the process as the photon frequency is raised from much less than the collision time to much greater than the collision time. In addition we show how the theoretical descriptions in these frequency regimes are connected.
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.
A radiative collision is a resonant energy transfer collision in which two atoms absorb or emit photons during the collision. Alternatively, a radiative collision is the emission or absorption of a photon from a transient molecule, and, as shown by Gallagher and Holstein, radiative collisions can also be described in terms of line broadening. In a line broadening experiment there are typically many atoms and weak radiation fields, and in a radiative collision experiment there are few atoms and intense radiation fields. The only real difference is whether there are many atoms or many photons. Due to the short collision times, ∼10 −12 s, simply observing radiative collisions between low lying states requires high optical powers, and entering the regime where the optical field is no longer a minor perturbation seems unlikely. Due to their long collision times and large dipole moments, Rydberg atoms provide the ideal system in which to study radiative collisions in a quantitative fashion. As we shall see, it is straightforward to enter the strong field regime in which the radiation field, a microwave or rf field to be precise, is no longer a minor perturbation. Ironically, while the experiments are radiative collision experiments, with few atoms and many photons, the description of the strong field regime is given in terms of dressed molecular states, which is more similar to a line broadening description.
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.
We have measured the n dependence of the microwave field required to make the single-photon radiatively assisted collision process Na(ns)+Na(ns)+kh\ensuremath{\nu}\ensuremath{\rightarrow}Na(np)+Na ((n-1)p)+(k+m)h\ensuremath{\nu}, m=1, 10% as likely as the resonant collision process in which no photons are absorbed, m=0. Experimentally we find an n∗{n}^{\mathrm{*}}n∗ dependence of [2.06(15)\ifmmode\times\else\texttimes\fi{}103{10}^{3}103]${n}^{\mathrm{*}\mathrm{\ensuremath{-}}2.0}$ (V/cm), where n∗{n}^{\mathrm{*}}n∗ is the effective quantum number of the Na ns state. Over the range of n studied, 16\ensuremath{\le}n\ensuremath{\le}24, this field is within 20% of the field calculated using a theory based on resonant dipole-dipole collisional energy transfer of microwave dressed atoms.