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arXiv:physics/0311039 · Physical Review A

Radiative collisional heating at the Doppler limit for laser-cooled magnesium atoms

Jyrki Piilo, Emil Lundh, Kalle‐Antti Suominen

Published Jul 26, 2004• 6 citations
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Abstract

We report Monte Carlo wave function simulation results on cold collisions between magnesium atoms in a strong red-detuned laser field. This is the normal situation, e.g., in magneto-optical traps (MOT). The Doppler limit heating rate due to radiative collisions is calculated for $^{24}\mathrm{Mg}$ atoms in an MOT based on the $^{1}S_{0}\text{\ensuremath{-}}^{1}P_{1}$ atomic laser cooling transition. We find that radiative heating does not seem to affect the Doppler limit in this case. We also describe a channeling mechanism due to the missing $Q$ branch in the excitation scheme, which could lead to a suppression of inelastic collisions, and find that this mechanism is not present in our simulation results due to the multistate character of the excitation process.

DOI
10.1103/physreva.70.013410

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We report Monte Carlo wave function simulation results on cold collisions between magnesium atoms in a strong red-detuned laser field. This is the normal situation, e.g., in magneto-optical traps (MOT). The Doppler limit heating rate due to radiative collisions is calculated for 24Mg^{24}\mathrm{Mg}24Mg atoms in an MOT based on the 1S0\ensuremath-1P1^{1}S_{0}\text{\ensuremath{-}}^{1}P_{1}1S0​\ensuremath-1P1​ atomic laser cooling transition. We find that radiative heating does not seem to affect the Doppler limit in this case. We also describe a channeling mechanism due to the missing QQQ branch in the excitation scheme, which could lead to a suppression of inelastic collisions, and find that this mechanism is not present in our simulation results due to the multistate character of the excitation process.
We use quantum scattering methods to calculate the light-induced collisional loss of laser-cooled and trapped magnesium atoms for detunings up to 30 atomic linewidths to the red of the ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ cooling transition. Magnesium has no hyperfine structure to complicate the theoretical studies. We evaluate both the radiative and nonradiative mechanisms of trap loss. The radiative escape mechanism via allowed ${}^{1}{\ensuremath{\Sigma}}_{u}$ excitation is dominant for more than about one atomic linewidth detuning. Molecular vibrational structure due to photoassociative transitions to bound states begins to appear beyond about ten linewidths detuning.
Magnesium atoms are cooled in a magneto-optical trap (MOT) using the 3s2 S0 → 3s3p P1 resonance transition. Magnesium is a simple atom which offers a unique possibility for comparison to the Doppler theory of laser cooling. We measure trap parameters as a function of laser-intensity, -detuning and magnetic field gradient. We find the main features well accounted for by Doppler theory, but temperature measurements gave significant higher values than predicted by the Doppler theory. We also observe radiation pressure effects within the MOT that limit the maximum achievable density. Comparing our results to predictions by models in the literature shows a good agreement. Recently we have improved our set-up and now trap about 150 × 106 atoms.
In the past, light-assisted cold collisions between laser cooled atoms have been widely studied in magneto-optical atom traps (MOTs). We describe here theoretical studies of dynamical interactions, specifically cold collisions, between atoms trapped in near-resonant, dissipative optical lattices. The developed quantum-mechanical model is based on Monte Carlo wave-function simulations and combines atomic cooling and collision dynamics in a single framework. It turns out, that the radiative heating mechanism affects the dynamics of atomic cloud in a red-detuned lattice in a way that is not directly expected from the MOT studies. The optical lattice and position dependent light-matter coupling introduces selectivity of collision partners. Atoms, which are most mobile and energetic, are strongly favored to participate in collisions, and are more often ejected from the lattice, than the slow ones in the laser parameter region selected for study. For blue-detuned lattices, we study how optical shielding emerges as a natural part of the lattice and look for ways to optimize the effect. The simulations are computationally very demanding and would obviously benefit from the simplification schemes. We present some steps to this direction by showing how it is possible to calculate collision rates in near-resonant lattices in a fairly simple way.
Magnesium atoms are cooled in a magneto-optical trap (MOT) using the 3s(2 1)S(0) --> 3s3p P-1(1) resonance transition. Magnesium is a simple atom which offers a unique possibility for comparison to the Doppler theory of laser cooling. We measure trap parameters as a function of laser-intensity, -detuning and magnetic field gradient. We find the main features well accounted for by Doppler theory, but temperature measurements gave significant higher values than predicted by the Doppler theory. We also observe radiation pressure effects within the MOT that limit the maximum achievable density. Comparing our results to predictions by models in the literature shows a good agreement. Recently we have improved our set-up and now trap about 150 x 10(6) atoms.
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.