We report observation of Bose-Einstein condensation (BEC) of a trapped
, dilute gas of atomic hydrogen. The condensate and normal gas are stu
died by two-photon spectroscopy of the 1S-2S transition. Interactions
among the atoms produce a shift of the resonance frequency proportiona
l to density. The condensate is clearly distinguished by its large fre
quency shift. The peak condensate density is 4.8 +/- 1.1 x 10(15) cm(-
3), corresponding to a condensate population of 10(9) atoms. The BEC t
ransition occurs at about T = 50 mu K and n = 1.8 x 10(14) cm(-3). [S0
031-9007(98)07617-0].