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Physical Review A

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

P. Pillet, Ravinder Kachru, Nam Huu Tran, W. W. Smith, Thomas F. Gallagher

Published Aug 1, 1987• 37 citations
Publisher

Abstract

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 ${E}_{\mathrm{ns}}$+${E}_{\mathrm{ns}}$=${E}_{\mathrm{np}}$+${E}_{n\mathrm{\ensuremath{-}}1}$,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.

DOI
10.1103/physreva.36.1132

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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 $\mathrm{ns}$ state lies midway between $(n\ensuremath{-}1)p$ and an $\mathrm{nl}$ state or between $\mathrm{np}$ 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} ${10}^{9}$ ${\mathrm{\AA{}}}^{2}$ 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} ${10}^{8}$ ${\mathrm{\AA{}}}^{2}$) is observed at a relatively low electric field \ensuremath{\sim} 17 V/cm.

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Physical Review A•Feb 1, 1994•7 citations
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​+En\ensuremath−1{E}_{n\mathrm{\ensuremath{-}}1}En\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.
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.
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.
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.
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.
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.
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.