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Atomic collisions in atom traps

Alan C. Gallagher

Published May 21, 1990• 0 citations
OpenAlex

Abstract

Atomic velocities in atom traps are so slow that radiative lifetimes are often shorter than the duration of a collision. In current traps this effect is greatly enhanced by the very long range resonant interaction between identical atoms. Thus collisional motion on a particular molecular potential is often interrupted by radiative transfer between molecular states of A–A, A*–A, or A*–A* parentage.

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Associative ionization in collisions of slowed and trapped sodium

Alan C. Gallagher

Collisions of trapped atoms occur over time intervals that are often longer than radiative lifetimes. This leads to radiative interruption of excited-state collision dynamics and constrains collisional interactions to atom pairs that are in close proximity at the time of excitation. Using classical dynamics I show that this causes Na associative ionization to be dominated by a four-step process in which the longer-range ${\mathrm{Na}}^{\mathrm{*}}$-Na interaction initiates inward motion that is completed as ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$\ensuremath{\rightarrow}${\mathrm{Na}}_{2}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$+e. Thus low-temperature associative ionization does not result from ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$ collisions. I predict strong single- and two-wavelength dependences, which have a molecular character due to the close proximity at the time of photoabsorption. These are amenable to experimental observation. I also calculate saturation that results from the depletion of interacting pairs. This produces very-low-power saturation and explains the associative ionization measurements of Gould et al. [Phys. Rev. Lett. 60, 788 (1988)].

Physical Review A•Oct 1, 1991•30 citations

Effects of laser light on ultracold atom collisions

V. Sánchez-Villicaña

Measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms are presented. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below $\\sim$70 $\\mu$K. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.^ We report on measurements of trap loss collisions between ultracold Rb atoms which are confined in a magneto-optical trap. Both isotopes $\\sp{85}$Rb and $\\sp{87}$Rb have been studied over wide range of trap laser intensities and detunings. The trap loss collision rates exhibit variations over 3 orders of magnitude. We observe significant differences between the isotopes. At large detunings the ability of the trap to recapture products of the hyperfine-changing collisions is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room temperature background gas have also been measured.^ Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine changing collisions between ultracold $\\sp{87}$Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\\sim$50% and are in reasonable agreement with a simple Landau-Zener model.^ Finally, we have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of $\\sim$3 have been observed. ^

•Jan 1, 1996•0 citations

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Inelastic atomic collisions constitute loss mechanisms for neutral atom traps, and the study of trap-loss rates may provide detailed information on the long-range interaction between atoms and on the dynamics of very slow collisions. Gallagher and Pritchard (1989) have given a simple two-state model for such collisions which includes a radiative transition or a single curve crossing at short range representing, for example, fine-structure-changing collisions. In order to better describe the more complicated case found in alkali metals, we have extended this model to include several excited hyperfine structure levels with curve crossings at long range. We find that collisions in a laser field that are capable of expelling atoms from the trap can be suppressed in some laser frequency regions and enhanced in others, in comparison with the predictions of the simple two-state model. Using this model to represent radiative escape via the 2 u potential of alkali dimer molecules we are able to greatly improve agreement between theory and experiment, including new measurements on 85 Rb reported here.

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We have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of \ensuremath{\sim}3 have been observed. This indicates that the atomic pair-distribution function can be significantly distorted in a typical laser trap environment.

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We have confined up to 10{sup 4} {sup 85}Rb atoms in an optical dipole force trap operating up to 65 nm from atomic resonance. For detunings greater than 16 nm, we obtained long confinement times, limited only by background gas collisions, without additional cooling. However, for trapping laser tunings between 4 nm and 16 nm to the red of the lowest Rb (D1) resonance, we observed a novel trap loss mechanism which results in much shorter-trap lifetimes. The trap loss rate is very sensitive to the laser wavelength, and exhibits a complex spectrum. This trap loss may arise from a photo-associative collisional process in which an initially free pair of colliding ground state atoms is promoted to a bound molecular excited state. The bound excited molecule could then be lost from the trap by photoionization, or by spontaneous decay to an untrapped ground state. The results of our preliminary experiments and of further experiments to clarify this novel trap loss process will be presented.

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Resonant-collision spectroscopy of Rydberg atoms

R. C. Stoneman, M. D. Adams, Thomas F. Gallagher

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.

Physical Review Letters•Mar 30, 1987•25 citations
Atomic velocities in atom traps are so slow that radiative lifetimes are often shorter than the duration of a collision. In current traps this effect is greatly enhanced by the very long range resonant interaction between identical atoms. Thus collisional motion on a particular molecular potential is often interrupted by radiative transfer between molecular states of A–A, A*–A, or A*–A* parentage.
Collisions of trapped atoms occur over time intervals that are often longer than radiative lifetimes. This leads to radiative interruption of excited-state collision dynamics and constrains collisional interactions to atom pairs that are in close proximity at the time of excitation. Using classical dynamics I show that this causes Na associative ionization to be dominated by a four-step process in which the longer-range Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗-Na interaction initiates inward motion that is completed as Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗+Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗\ensuremath{\rightarrow}${\mathrm{Na}}_{2}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$+e. Thus low-temperature associative ionization does not result from Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗+Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗ collisions. I predict strong single- and two-wavelength dependences, which have a molecular character due to the close proximity at the time of photoabsorption. These are amenable to experimental observation. I also calculate saturation that results from the depletion of interacting pairs. This produces very-low-power saturation and explains the associative ionization measurements of Gould et al. [Phys. Rev. Lett. 60, 788 (1988)].
Measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms are presented. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below $\\sim$70 mu\\mumuK. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.^ We report on measurements of trap loss collisions between ultracold Rb atoms which are confined in a magneto-optical trap. Both isotopes sp85\\sp{85}sp85Rb and sp87\\sp{87}sp87Rb have been studied over wide range of trap laser intensities and detunings. The trap loss collision rates exhibit variations over 3 orders of magnitude. We observe significant differences between the isotopes. At large detunings the ability of the trap to recapture products of the hyperfine-changing collisions is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room temperature background gas have also been measured.^ Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine changing collisions between ultracold sp87\\sp{87}sp87Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\\sim$50% and are in reasonable agreement with a simple Landau-Zener model.^ Finally, we have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of $\\sim$3 have been observed. ^
Inelastic atomic collisions constitute loss mechanisms for neutral atom traps, and the study of trap-loss rates may provide detailed information on the long-range interaction between atoms and on the dynamics of very slow collisions. Gallagher and Pritchard (1989) have given a simple two-state model for such collisions which includes a radiative transition or a single curve crossing at short range representing, for example, fine-structure-changing collisions. In order to better describe the more complicated case found in alkali metals, we have extended this model to include several excited hyperfine structure levels with curve crossings at long range. We find that collisions in a laser field that are capable of expelling atoms from the trap can be suppressed in some laser frequency regions and enhanced in others, in comparison with the predictions of the simple two-state model. Using this model to represent radiative escape via the 2 u potential of alkali dimer molecules we are able to greatly improve agreement between theory and experiment, including new measurements on 85 Rb reported here.
We have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of \ensuremath{\sim}3 have been observed. This indicates that the atomic pair-distribution function can be significantly distorted in a typical laser trap environment.
We have confined up to 10{sup 4} {sup 85}Rb atoms in an optical dipole force trap operating up to 65 nm from atomic resonance. For detunings greater than 16 nm, we obtained long confinement times, limited only by background gas collisions, without additional cooling. However, for trapping laser tunings between 4 nm and 16 nm to the red of the lowest Rb (D1) resonance, we observed a novel trap loss mechanism which results in much shorter-trap lifetimes. The trap loss rate is very sensitive to the laser wavelength, and exhibits a complex spectrum. This trap loss may arise from a photo-associative collisional process in which an initially free pair of colliding ground state atoms is promoted to a bound molecular excited state. The bound excited molecule could then be lost from the trap by photoionization, or by spontaneous decay to an untrapped ground state. The results of our preliminary experiments and of further experiments to clarify this novel trap loss process will be presented.
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.