A magneto-optical trap (MOT) is commonly used for cooling and trapping a substance by Doppler cooling. In the process of Doppler cooling, the red detuned light would be absorbed by atoms from one certain direction and re-emitted in a random direction. The electrons of the atoms would decay to an alternative ground states if the atoms have more than one hyperfine ground level. There is the case of all the atoms in the other ground states rather than the ground states of Doppler cooling, then system cannot cool the atoms further.