Absolute Frequency Measurement
The 486 nm radiation of the dye laser is transfered through an optical fiber to the frequency comb [17] to compare it phase-coherently with a Cs atomic clock. In this way the absolute frequency of the hydrogen 1S-2S transition is determined with one of the most precise measurement tools. Subtracting the Lamb shift from the measured frequency a value for Rydberg may be derived from the remainder. On the other hand if one uses a value for the for Rydberg constant determined in a different experiment and a measured for the 2S Lamb shift [8] a value of the 1S Lamb shift may be derived. This is an important result since the Lamb shift of the 1S level is the largest of all hydrogen energy levels and it is not accessible to radio frequency methods. The most general way to determine the constants given above is to use not only the 1S-2S transition frequency but all available hydrogen data [16].