Fig. 9.6 The force as a function of the velocity in the optical molasses technique (solid lines) for (a) $\delta = -\Gamma/2$, and (b) $\delta = -\Gamma$. The damping is proportional to the slope of the force curve at $v = 0$. Note that the force is negative for $v > 0$ and positive for $v < 0$, so the force decelerates atoms. The forces produced by each of the laser beams separately are shown as dotted lines—these curves have a Lorentzian line shape and they are drawn with an FWHM of $\Gamma$ appropriate for low intensities. For $\delta = 0$ (not shown in the figure) the forces from the two laser beams cancel each other for all velocities. The velocity capture range is approximately $\pm \Gamma/k$.
