Two-Photon Spectroscopy of Trapped Atomic Hydrogen.

We have performed two-photon laser spectroscopy on the 1S-2S transition in trapped atomic hydrogen. The measured linewidth allows for in situ determination of the sample temperature. We predicted and observed transitions between trap states due to momentum transfer between the laser intensity profile and the quantized atomic motion perpendicular to the laser beam. In this regime, with the appearance of sidebands, the resolution is no longer limited by time-of-flight and we have achieved fractional spectral resolution of 1.7 times 10^{-12}, limited by laser jitter. We have also measured 2S decay lifetimes around 110ms, close to the natural radiative decay time of 122ms. The estimated sensitivity should allow us to detect as few as 10^7 Bose-Einstein condensate atoms with the present apparatus. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.).