Absolute frequency measurement of the In+ 5s S0–5s5p P0 transition

We have measured the absolute frequency of the 115 In+ 5s 2 1 S0–5s5p 3 P0 clock transition at 236.5 nm with an accuracy of 3.3 parts in 1011. For this measurement, a frequency synthesis chain was used which links the indium clock transition to a methane-stabilized He–Ne laser at 3.39 μm and a Nd:YAG laser at 1064 nm whose second harmonic was locked to a hyperfine component in molecular iodine. A frequency gap in the chain of 1.43 THz at 850 nm was bridged with the help of an optical frequency comb generator. The frequency of the 115 In+ clock transition was determined to 1 267 402 452 914 (41) kHz, where the accuracy is limited by the uncertainty of the iodine reference. This measurement represents an improvement of more than three orders of magnitude in accuracy compared to previous measurements of the line.