Absolute frequency measurement of the In-115(+) 5s(2 1)S(0)-5s5p P-3(0) transition

We have measured the absolute frequency of the In-115(+) 5s(2) S-1(0)-5s5p P-3(0) clock transition at 236.5 nm with an accuracy of 3.3 parts in 10(11) . For this measurement, a frequency synthesis chain was used which links th e indium clock transition to a methane-stabilized He-Ne laser at 3.39 mu m and a Nd:YAG laser at 1064 nm whose second harmonic was locked to a hyperfi ne 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 In-115(+) clock transition was determined to 1 267 40 2 452 914(41) kHz, where the accuracy is limited by the uncertainty of the iodine reference. This measurement represents an improvement of more than t hree orders of magnitude in accuracy compared to previous measurements of t he line. (C) 1999 Elsevier Science B.V. All rights reserved.