Re: how were the lamb shift experiments done?

Well, here's a general description of how I understand these experiments were done...

Since the Lamb shift is so small (~1057 MHz), it is necessary to use very long wavelengths to observe this energy. Willis Lamb performed his measurements using microwaves. To measure the shift, he first had to excite atoms in the 2s(1/2) state hydrogen up to 2p(1/2) state because the atoms cannot directly transition from 2s(1/2) to 1s(1/2) due to selection rules. From the 2p(1/2) level, Lamb exposed the atoms to microwave radiation at 2395 MHz, then varied the magnetic field until it produced transitions from the 2p(1/2) to 2p(3/2) levels. Once in the 2p(3/2) state, he could measure the *allowed* transition from the 2p(3/2) to the 1s(1/2) state. The measurements showed that the zero-magnetic field splitting of these levels corresponded to 1057 MHz.(1)

It is possible to get more information on this by reading Lamb's Nobel Prize lecture from 1955.

I hope this helps!
Amber