Recommendation 1 of the 16th CCL (2015)

Updates to the list of standard frequencies

The Consultative Committee for Length (CCL),

considering that

a common list of “Recommended values of standard frequencies for applications including the practical realization of the metre and secondary representations of the second” has been established;

the CCL-CCTF Frequency Standards Working Group (WGFS) has reviewed several candidates for updating the list;


that the following transition frequencies shall be included in the list of recommended values of standard frequencies:

  • Absorbing molecule 127I2, saturated absorption a1 component, R(36)32-0 transition.

    The values
    fa1 = 564 074 632.42 MHz

    λα1 = 531 476 582.65 fm

    with an estimated relative standard uncertainty of 1 × 10−10 apply to the radiation of a frequency-doubled diode DFB laser, stabilized with an iodine cell external to the laser.

  • Absorbing atom 87Rb crossover between the d and f hyperfine components of the saturated absorption at 780 nm (D2 transition)

    The values
    fd/f crossover = 384 227 981.9 MHz

    λd/f crossover = 780 246 291.6 fm

    with an estimated relative standard uncertainty of 5 × 10−10 apply to the radiation of a tunable External Cavity Diode Laser, stabilized using the 3rd derivative technique.

Note: The value of the standard uncertainty is assumed to correspond to a confidence level of 68 %. However, given the very limited number of available data there is a possibility that in hindsight this might not prove to be exact.

and asks the CIPM for approval.

reference The reader should note that the official version of this Resolution is the French text

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