We have measured the absolute optical frequency of the cesium line at 335 THz (895 nm). This frequency provides an important link for a new determination of the fine structure constant . The line has been compared with the fourth harmonic of a methane stabilized He-Ne laser at 88.4 THz ( ). To measure the frequency mismatch of 18.39 THz between and the line a frequency comb spanning around 244 000 modes of a Kerr-lens mode-locked laser was used. We find 1 167 688 (81) kHz for the hyperfine splitting of the state and 335 116 048 807 (41) kHz for the hyperfine centroid from which we derive .
- Received 17 December 1998
©1999 American Physical Society