Complex sentence: There are only a few points on our frequency control dial that have any effect on that atom--at those frequencies, the photon slams into the atom, the atom starts drifting away from the laser, and later the atom releases the photon.
Split into simpler sentences: There are only a few points on our frequency control dial that have any effect on that atom. At those frequencies, the photon slams into the atom, the atom starts drifting away from the laser, and later the atom releases the photon.