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Physical Review Letters

Laser-Induced Ionizing Collisions of Ultracold Krypton Gas in the 1s5 Metastable State

Hidetoshi Katori, Fujio Shimizu

Published Nov 7, 1994• 58 citations
Publisher

Abstract

The line shape of laser-induced ionizing collisions of ultracold Kr gas is investigated near the ${1s}_{5}[4p^{5}5s:^{3}P_{2}]$ and ${2p}_{9}[4p^{5}5p:^{3}D_{3}]$ transition. The rate increases several times when the laser is red tuned by a few times of the natural width, while it sharply decreases for a slight blue detuning. This frequency dependence of the collision rate is due to the collision dynamics characteristic of ultracold atoms in a near-resonant light. Our experimental results on metastable Kr atoms show a good qualitative agreement with semiclassical Monte Carlo simulation of the cold collision dynamics.

DOI
10.1103/physrevlett.73.2555

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The line shape of laser-induced ionizing collisions of ultracold Kr gas is investigated near the 1s5[4p55s:3P2]{1s}_{5}[4p^{5}5s:^{3}P_{2}]1s5​[4p55s:3P2​] and 2p9[4p55p:3D3]{2p}_{9}[4p^{5}5p:^{3}D_{3}]2p9​[4p55p:3D3​] transition. The rate increases several times when the laser is red tuned by a few times of the natural width, while it sharply decreases for a slight blue detuning. This frequency dependence of the collision rate is due to the collision dynamics characteristic of ultracold atoms in a near-resonant light. Our experimental results on metastable Kr atoms show a good qualitative agreement with semiclassical Monte Carlo simulation of the cold collision dynamics.
Ultracold ionizing collisions of laser-cooled Kr isotopes in the 1s5{\mathit{s}}_{5}s5​[4${\mathit{p}}^{5}5s5s5s{:}^{3}$P2{\mathit{P}}_{2}P2​] metastable state are investigated. When the collision energy is reduced below the p-wave centrifugal barrier of 80 \ensuremath{\mu}K, the ratio of the ionizing collision rate between polarized and unpolarized 83Kr^{83}\mathrm{Kr}83Kr atoms is found to decrease. This is a direct evidence of the quantum interference of two identical Fermi particles. The experimental results are compared with the calculation based on the simplified inelastic collision theory.
We present results for controlled optical collisions of cold, metastable Ne atoms in a magneto-optical trap. The modification of the ionizing collision rate is demonstrated using a control laser tuned close to the (3s)3P2 to (3p)3D3 cooling transition. The measured ionization spectrum does not contain resonances due to the formation of photoassociated molecules connected to the omega = 5 excited state potential as predicted by Doery et al (1998 Phys. Rev. A 57 3603-20). Instead, we observe a broad unresolvable ionization spectrum that is well described by established theory (Gallagher and Pritchard 1989 Phys. Rev. Lett. 63 957). In order to induce collisions between pairs of trapped metastables, a control laser is tuned to the red of the transition where we measure a factor of 4 increase in the rate for ionizing collisions. We also investigate the effect of optical shielding by tuning the laser to the blue of the transition and observe a factor of 5 suppression in the ionization rate.
We report on the dynamics of ultracold collisions induced by near-resonant frequency-chirped light. A series of identical chirped pulses, separated by a variable delay, is applied to an ultracold sample of 85Rb^{85}\mathrm{Rb}85Rb, and the rate of inelastic trap-loss collisions is measured. For small detunings of the chirped light below the atomic resonance, we observe that the rate of collisions induced by a given pulse can be increased by the presence of an earlier pulse. We attribute this to the enhancement of short-range collisional flux by the long-range excitation of atom pairs to an attractive molecular potential. For larger detunings and short delays, we find that a leading pulse can suppress the rate of collisions caused by a following pulse. This is due to a depletion of short-range atom pairs by the earlier pulse. Comparison of our data to classical Monte Carlo simulations of the collisions yields reasonable agreement.
We have studied the optical collisions of cold, metastable helium atoms in a magneto-optical trap. We have detected the rate of Penning and associative ionization of two metastable helium atoms at a temperature of 1 mK with and without nearly resonant light. We find that the associative ionization rate is increased with more than a factor 20 due to the presence of the light field. We present a simple, semiclassical model, which accounts for all the observed features, and which agrees, also on an absolute scale, with our experimental results.
We have observed optically induced Feshbach resonances in a cold ( <1 mK) sodium vapor. The optical coupling of the ground and excited-state potentials changes the scattering properties of an ultracold gas in much the same way as recently observed magnetically induced Feshbach resonances, but allows for some experimental conveniences associated with using lasers. The scattering properties can be varied by changing either the intensity or the detuning of a laser tuned near a photoassociation transition to a molecular state in the dimer. In principle this method allows the scattering length of any atomic species to be altered. A simple model is used to fit the dispersive resonance line shapes.
In laser-induced collisions between ultracold atoms, the combination of low velocities and long-range interactions results in collision times which can exceed the excited-state lifetime. We use a cooperative effect between two lasers to explicitly observe this time dependence. The first laser, tuned near resonance, excites the atom pair at long range and enhances the collisional flux available for short-range excitation by a second, far-detuned laser. Using pulsed excitation, we find this collisional process to take place on a ${10}^{\ensuremath{-}6}\mathrm{s}$ time scale, in reasonable agreement with trajectory simulations.