In this paper we experimentally and theoretically investigate laser coolingof Strontium 88 atoms in one dimensional optical molasses. In our case, sincethe optical cooling dipole transition involves a $J_g=0$ groundstate, noSisyphus-type mechanisms can occur. We are thus able to test quantitatively thepredictions of the Doppler-cooling theory. We have found, in agreement withother similar experiments, that the measured temperatures are systematicallylarger than the theoretical predictions. We quantitatively interpret thisdiscrepancy by taking into consideration the extra-heating mechanism induced bytransverse spatial intensity fluctuations of the optical molasses. Experimentaldata are in good agreement with Monte-Carlo simulations of our theoreticalmodel. We thus confirm the important role played by intensity fluctuations inthe dynamics of cooling and for the steady-state regime.
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