  The ac Stark shift in the presence of spontaneous decay is typically
considered to induce an effective dephasing with a scattering rate equal to $
\Gamma_s |\Omega|^2/\Delta^2 $, where $ \Gamma_s $ is the spontaneous decay
rate, $ \Omega $ is the laser transition coupling, and $ \Delta $ is the
detuning. We show that under realistic circumstances this dephasing rate may be
strongly modifed due to non-Markovian behavior. The non-Markovian behavior
arises due to an effective modification of the light-atom coupling in the
presence of the ac Stark shift laser. An analytical formula for the
non-Markovian ac Stark shift induced dephasing is derived. We obtain that for
narrow laser linewidths the effective dephasing rate is suppressed by a factor
of $ Q^2$, where $ Q $ is the quality factor of the laser.
