Hmm, not sure I understand what's happening there.
Is the attached scene basically what you're trying to do? Just mouse over the box.
It's a little glitchy feeling on/off this way.. Maybe editing the emission of the particles to 0 with a Set Property action on your Enter/Exit trigger would be better. I suppose with one shot you only need to activate it, so that would depend on your FSM and state structure, but it can be done that way like the attached scene.