Clock transition for a future optical frequency standard with trapped atoms

We report direct excitation of the strongly forbidden 5s{sup 2} {sup 1}S{sub 0}-5s5p {sup 3}P{sub 0} transition in {sup 87}Sr. 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 {sup 3}P{sub 0} state and escape the trapping process, leading to a detectable decrease in the MOT fluorescence. This line has a natural width of 10{sup -3} Hz and can be used for a new generation of optical frequency standards using atoms trapped in a light-shift-free dipole trap.