Dynamics in a two-level atom magneto-optical trap

Alkaline-earth-metal atoms present an ideal platform for exploring magneto-optic trap (MOT) dynamics, enabling unique and definitive tests of laser cooling and trapping mechanisms. We have measured the trapping beam intensity, detuning, magnetic-field gradient, trap density, and lifetime dependence of the spring constant κ and damping coefficient α for a ¹S₀-¹P₁ ⁸⁸Sr MOT by fitting the oscillatory response of the atom cloud to a step-function force. We find that the observed behavior of κ and α provide a unified and consistent picture of trap dynamics that agrees with Doppler cooling theory at the level of 10%. Additionally, we demonstrate that the trapped atom temperature can be determined directly from measured value of κ and the trap size, in excellent agreement with free-expansion temperature measurements. However, the experimentally determined temperature is much higher than Doppler cooling theory, implying significant additional heating mechanisms.