a Clock Transition for a Future Optical Frequency Standard with Trapped Atoms

We report the observation of the 5s^{2 1}S₀-5s5p ³P₀ transition in ⁸⁷Sr. Its frequency is 429 228 004 235 (20) kHz. A resonant laser creates a leak in a magneto-optical trap (MOT): atoms build up in the metastable ³P₀ state and escape the trapping process, leading to a detectable decrease in the MOT fluorescence. This line has a natural width of 1 mHz and can be used as an optical frequency standard using atoms confined in a light shift free dipole trap.