Clock transition for a future optical frequency standard with trapped atoms

We report direct excitation of the strongly forbidden 5s²¹S₀-5s5p³P₀ transition in ⁸⁷Sr. Its frequency is 429 228 004 235(20) kHz. A resonant laser creates a small leak in a magneto-optical trap (MOT): atoms build up to 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 10^-3Hz and can be used for a new generation of optical frequency standards using atoms trapped in a light-shift-free dipole trap.