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Physical Review A

Observing negligible collision trap losses: The case of alkaline-earth metals

Reinaldo L. Cavasso-Filho, Artemio Scalabrin, D. Pereira, Flávio C. Cruz

Published Feb 28, 2003• 10 citations
Publisher

Abstract

We show that when cold collisions account for just a few percent of the total loss in magneto-optical traps, as for alkaline-earth metals, their contribution can be obtained by comparing the load and decay quasiexponential curves. We exemplify it by measuring the collision rate coefficient \ensuremath{\beta} for calcium atoms at small trap laser detuning, which confirms the role of the long-lived ${}^{1}{\ensuremath{\Pi}}_{g}$ state at small internuclear separation. Systematic and simpler measurements of \ensuremath{\beta} for alkaline-earth metals are important for cold collision theory, since these elements have nondegenerate ground state and no hyperfine structure. Our method has general applicability and should considerably reduce the experimental challenges associated with these important measurements.

DOI
10.1103/physreva.67.021402

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We show that when cold collisions account for just a few percent of the total loss in magneto-optical traps, as for alkaline-earth metals, their contribution can be obtained by comparing the load and decay quasiexponential curves. We exemplify it by measuring the collision rate coefficient \ensuremath{\beta} for calcium atoms at small trap laser detuning, which confirms the role of the long-lived 1\ensuremathΠg{}^{1}{\ensuremath{\Pi}}_{g}1\ensuremathΠg​ state at small internuclear separation. Systematic and simpler measurements of \ensuremath{\beta} for alkaline-earth metals are important for cold collision theory, since these elements have nondegenerate ground state and no hyperfine structure. Our method has general applicability and should considerably reduce the experimental challenges associated with these important measurements.
We present measurements of cold collision processes in an alkaline-earth species. Collisions of 88Sr{}^{88}\mathrm{Sr}88Sr pairs in a magneto-optic trap are found to be dominated by very-long-range excitation to the nonradiating ${}^{1}{\ensuremath{\Pi}}_{g}$ state of Sr2∗.{\mathrm{Sr}}_{2}^{*}.Sr2∗​. We find good agreement between measured loss rate coefficients and a modified form of the semiclassical model that includes retardation effects that allow excitation to metastable molecular states.
Trap-loss rates due to hyperfine--structure-changing collisions involving cold Na and K atoms in a mixed species magneto-optical trap have been measured. The investigations were performed in the low intensity regime, where hyperfine--structure-changing collision are believed to be the dominant mechanism for atomic losses. Comparison between homonuclear (Na-Na) and heteronuclear (Na-K) collision rate coefficients reveals a higher loss rate in the case of heteronuclear collisions. Results obtained in the regime of low trap laser intensity are compared with existent loss rates for several alkali metals, showing themselves to be in good agreement with those expected.
We have measured the collisional loss rate for cold strontium atoms held in a magneto-optical trap as a function of light intensity in the regime of low intensity (2-6 mW/cm²). The results confirm our recently proposed model, where we showed that the sudden increase of loss rate at low intensities does not depend on hyperfine structure changing collision only. The model, which is based on radiative escape mechanism and a light intensity dependent escape velocity, is able to reproduce quite well the behavior of the experimental observations. The data here reported may be of importance for a recent proposed application for cold trapped 88Sr atoms, as well as other trappable alkaline earth metal atoms such as Ca.
We calculate the light-induced collisional loss of laser-cooled and trapped magnesium atoms for detunings up to 50 atomic linewidths to the red of the ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ cooling transition. We evaluate loss rate coefficients due to both radiative and nonradiative state-changing mechanisms for temperatures at and below the Doppler-cooling temperature. We solve the Schr\"odinger equation with a complex potential to represent spontaneous decay, but also give analytic models for various limits. Vibrational structure due to molecular photoassociation is present in the trap loss spectrum. Relatively broad structure due to absorption to the ${\mathrm{Mg}}_{2}{ }^{1}{\ensuremath{\Sigma}}_{u}$ state occurs for detunings larger than about ten atomic linewidths. A much sharper structure, especially evident at low temperature, occurs even at smaller detunings due to ${\mathrm{Mg}}_{2}{ }^{1}{\ensuremath{\Pi}}_{g}$ absorption, which is weakly allowed due to relativistic retardation corrections to the forbidden dipole transition strength. We also perform model studies for the other alkaline-earth-metal species Ca, Sr, and Ba, and for Yb, and we find similar qualitative behavior as for Mg.
Collisions between ultra-cold (T << 1K) atoms confined in a magneto- optical trap (MOT) may result in the ejection of atoms from the trap. These collisional loss processes, which involve ground state/excited state collisions, fundamentally limit the density attainable in MOT traps. Although accurate calculations of collision rates between ultra-cold atoms require precise knowledge of the molecular potentials over a long range, the loss rate for Li was recently predicted to be significantly less than for the other alkali metals. Motivated by this result, we are measuring the collisional loss rate of Li in a vapor-cell MOT using a red-detuned laser beam to catalyze the collisions, similar to recent experiments using Rb and Cs. We will report our progress to measure the trap loss rate for {sup 6}Li and {sup 7}Li, which will be compared to investigate an isotopic dependence, as was recently observed for Rb.