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Physical Review A

Frequency evolution of radiatively assisted collisions of K Rydberg atoms

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

Published Jan 1, 1994• 6 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.49.409

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Cambridge University Press eBooks•Sep 15, 1994•0 citations

Radiative Rydberg-atom–Rydberg-atom collisions in the strong-field regime

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Physical Review A•Aug 1, 1987•37 citations
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.
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.
It is shown theoretically that Ramsey fringes can be produced when a laser-assisted reaction of the form A2{\mathrm{A}}_{2}A2​+${\mathrm{A}}_{1}^{\ensuremath{'}}$+\ensuremath{\Elzxh}\ensuremath{\Omega}\ensuremath{\rightarrow}A1{\mathrm{A}}_{1}A1​+${\mathrm{A}}_{2}^{\ensuremath{'}}$ is driven by a pair of ultrafast radiation pulses, each of whose temporal widths is less than the duration ${\mathrm{\ensuremath{\tau}}}_{\mathrm{c}}$ of a collision between the A and A\ensuremath{'} atoms. The excitation profile of the laser-assisted collision is calculated as a function of detuning of the laser field frequency from the initial- to final-state transition frequency. For radiation pulses separated by time T, Ramsey fringes appear separated in frequency by ${\mathrm{T}}^{\mathrm{\ensuremath{-}}1}$. The modulation depth and shift of the central Ramsey fringe as a function of (T/${\mathrm{\ensuremath{\tau}}}_{\mathrm{c}}$) provide information about the collisional interaction. Both the weak- and strong-field regions are considered and comparison with laser-assisted collisions using excitation pulses having temporal widths greater than ${\mathrm{\ensuremath{\tau}}}_{\mathrm{c}}$ is made.
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.
We describe the experimental observation of the resonant collisional energy transfer Na(ns)+Na(ns)+qh\ensuremath{\nu}\ensuremath{\rightarrow}Na(np)+Na[(n-1)p]+(q○)h\ensuremath{\nu} which occurs when the Na atoms in a strong microwave field are tuned in energy with a static field so that the energy resonance Ens{E}_{\mathrm{ns}}Ens​+Ens{E}_{\mathrm{ns}}Ens​=Enp{E}_{\mathrm{np}}Enp​+${E}_{n\mathrm{\ensuremath{-}}1}$,p\ifmmode\pm\else\textpm\fi{}mh\ensuremath{\nu} occurs. We describe our experimental observations and provide a detailed description of a nonperturbative theory which we have developed to explain our observations. The theory is essentially developed in terms of the resonant dipole-dipole collisions between ``microwave dressed'' atoms. Using an extension of the approach we used previously to describe resonant dipole-dipole collisions in the absence of microwaves, we are able to take into account all the coherences in the problem in a completely straightforward way.