Atomic clock - optical pumping

Moth

I am doing some research on atomic clocks. From what I have read, I understand that in Rb atomic clocks you:

-optically pump a cell of Rb, so that the electrons are all in a single hyperfine state
-subject the cell to microwaves of appropriate frequency, so that the electrons will be stimulated into the other hyperfine groundstate
-see how well the microwave frequency worked
-modify the frequency accordingly and try again

However, I am unclear whether the optical pumping puts the electrons in the higher F=1 state or the lower F=2 state (and the microwaves then attempt to do the opposite). Can you tell me which is used?