2-PHOTON SPECTROSCOPY OF TRAPPED ATOMIC-HYDROGEN

We report observation of the 1S-2S two-photon transition in hydrogen c onfined in a magnetic trap at submillikelvin temperatures. The excitat ion spectrum can display a sharp central feature arising from periodic motion of the trapped atoms. The metastable 2S atoms remain trapped, and have a lifetime close to the natural lifetime of 122 ms. These dev elopments open the way to achieving the transition's natural linewidth . We have also demonstrated that the temperature of the gas can be det ermined from the line shape, providing an important tool for the study of cold trapped hydrogen. The resolution in these experiments appears to be limited by laser stability, currently 3 kHz.