The cooling of not-fully-ionized ions by means of Doppler laser cooling was introduced by a number of research groups at the same time. Good treatment can be found in the thesis of J.Hangst.
Laser cooling is the result of velocity-selective transfer of photon momentum from a laser beam to a moving ion. In the simplest case cooled particles have two possible energy levels. Particles in resonance with laser beam absorb photons and momentum recoil in direction of the laser beam occures. When particle emittes the photon in relaxsation process, photon is emitted at the random angle and the momentum recoil is in the random drection. Hence, an effective force acts on the particles in the direction of the laser beam. This force depends on the particle velocity by virtue of Doppler shift.
By tuning the frequences of two opposed laser beams so that slow particles are accelerated and fast ones slowed down, velocity spread is reduced and "cooling" achieved.