I'm doing a small paper on Magneto-Optical Traps, trying to explain how they work and I just encountered another paper which talked about ground states and excited states (J = 0, J = 1) and magnetic substates (m = 0 for J = 0 and m = -1, 0 or 1 for J = 1). I understand what ground states and excited states are, but can someone explain magnetic substates? Also, I'm confused by this: It's from near the bottom of page 2 from http://physweb.bgu.ac.il/COURSES/LAB_C/Laser%20Cooling%20and%20trapping/colorado%20mot.pdf [Broken]. Could anyone tell me in laymans terms what exactly the bolded sentence means? As far as I understand, the atoms absorb photons, which excites them, then they emit them. What does this have to do with the magnetic substate? Edit: I see now that I probably should have put this in the Atomic, Solid State, Comp. Physics subforum.