Recommendation 2 of the 98th CIPM (2009)

Updates to the list of standard frequencies

The International Committee for Weights and Measures (CIPM),

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 (FSWG) has reviewed several promising candidates for inclusion in the list;

recommends that the following transition frequencies shall be included or updated in the list of recommended standard frequencies:

• the unperturbed optical transition 5s^{2 1}S₀ – 5s 5p ³P₀ of the ⁸⁷Sr neutral atom with a frequency of f = 429 228 004 229 873.7 Hz and a relative standard uncertainty of 1 × 10^–15 (this radiation is already endorsed by the CIPM as a secondary representation of the second);
• the unperturbed optical transition 5s² 1S₀ – 5s 5p 3P₀ of the ⁸⁸Sr neutral atom with a frequency of f = 429 228 066 418 012 Hz and a relative standard uncertainty of 1 × 10^–14;
• the unperturbed optical transition 4s ²S_1/2 – 3d ²D_5/2 of the ⁴⁰Ca⁺ ion with a frequency of f = 411 042 129 776 393 Hz and a relative standard uncertainty of 4 × 10^–14;
• the unperturbed optical transition ²S_1/2 (F = 0) – ²F_7/2 (F = 3, m_F = 0) of the ¹⁷¹Yb⁺ ion with a frequency of f = 642 121 496 772 657 Hz and a relative standard uncertainty of 6 × 10^–14;
• the unperturbed optical transition 6s^{2 1}S₀ (F = 1/2) – 6s 6p ³P₀ (F = 1/2) of the ¹⁷¹Yb neutral atom with a frequency of f = 518 295 836 590 864 Hz and a relative standard uncertainty of 1.6 × 10^–13.

DOI : 10.59161/CIPM2009REC2E