The Hydrogen Spectrometer
In the 1S-2S experiment hydrogen atoms are excited by longitudinal Doppler-free two photon excitation at 243 nm. This radiation is generated by a frequency doubled ultrastable dye laser at 486 nm. The UV radiation is then resonantly enhanced in a linear cavity inside a vacuum chamber . Atomic hydrogen from a gas discharge is directed through a nozzle cooled with liquid helium. The cold atomic beam is oriented along the axis of the enhancement resonator. After the atoms have passed a distance L, usually around 10 cm, a small electric field that mixes the metastable 2S state (lifetime 1/7 sec) with the fast decaying 2P state is applied. To reduce the residual second order Doppler-effect we use a delayed detection scheme where the fluorescence at 121 nm is detected only after the exciting light has been switched off for a period T, typically between 0.5 and 1.8 ms. In this way we set an upper limit on the second order Doppler-shift below 1 kHz.