Observation of the Hydrogen 1S-2S Two-photon Transition Excited by an Ultraviolet Mode-locked Laser

We report on an observation of the 1S-2S transition in atomic hydrogen using an ultraviolet mode-locked laser. The 243 nm femtosecond laser is produced through second-harmonic generation and sum-frequency generation from a Ti:sapphire mode-locked laser. After cooling the atoms to 7.8 K, a florescence signal with 800 kHz linewidth is observed. Our scheme provides a potential approach to realize the high-precision frequency standard based on hydrogen transition.