Unlike most Doppler cooling techniques, light in the gray molasses must be blue -detuned from its resonant transition; the resulting Doppler heating is offset by polarization-gradient cooling. Qualitatively, this is because the choice of | F = 2 ⟩ ⟩ → → | F ′ = 2 ⟩ ⟩ means that the AC Stark shifts of the three levels are the same sign at any given position. Selecting the potential energy maxima as the sites of optical pumping to the dark state requires the overall light to be blue-detuned; in doing so, the atoms in the bright state traverse the maximum potential energy difference and thus dissipate the most kinetic energy. A full quantitative explanation of the molasses force with respect to detuning can be found in Hänsch's paper.