Absolute frequency measurement of the R(12) 26-0 and R(106) 28-0 transitions in /sup 127/I/sub 2/ at /spl lambda/=543 nm

A direct phase coherent determination of the absolute frequency of the a
₁₅
component in the R(12) 26-0 and b
₁₀
component in the R(106) 28-0 transitions in
¹²⁷
I
₂
at 543 nm has been made at the International Bureau of Weights and Measures. Lasers from the Czech Metrology Institute (Czech Republic), the National Institute of Metrology (China), the Centro Nacional de Metrologia (Mexico), the Standards, Productivity and Innovation Board (Singapore), the Centre for Metrology and Accreditation (Finland), the Danish Institute of Fundamental Metrology (Denmark), the National Physical Laboratory (U.K.), and the International Bureau of Weights and Measures took part. The mean frequency values found are f(a
₁₅
)=551579856480.4 kHz, u
_c
=4.8 kHz and f(b
₁₀
)=551580162397.1 kHz, u
_c
=4.6 kHz for the current group of lasers, where u
_c
is the combined standard uncertainty. This constitutes a 30-fold improvement in accuracy for this radiation, which is recommended for the realization of the definition of the meter. If only lasers equipped with an iodine cell temperature control are considered, the group of lasers shows a dispersion of 7 kHz. This value is indicative of the level of reproducibility that can be expected at present for this type of standard