Recommendation 1 of the 20th CCTF (2015)

Updates to the list of standard frequencies

The Consultative Committee for Time and Frequency (CCTF), together with 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,

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

(this radiation is already endorsed by the CIPM as a secondary representation of the second);

• the unperturbed optical transition 6s^{2 1}S₀ – 6s6p ³P₀ of the ¹⁹⁹Hg neutral atom with a frequency of f_199Hg = 1 128 575 290 808 154.8 Hz and an estimated relative standard uncertainty of 6 × 10⁻¹⁶;
• the unperturbed optical transition 6s ²S_1/2 – 4f 13 6s^{2 2}F_7/2 of the ¹⁷¹Yb⁺ ion with a frequency of f_{171Yb+ (octupole)} = 642 121 496 772 645.0 Hz and an estimated relative standard uncertainty of 6 × 10⁻¹⁶ (this radiation is already endorsed by the CIPM as a secondary representation of the second);
• the unperturbed optical transition 6s ²S_1/2 (F = 0, m_F = 0) – 5d ²D_3/2 (F = 2, m_F = 0) of the ¹⁷¹Yb⁺ ion with a frequency of f_{171Yb+ (quadrupole)} = 688 358 979 309 308.3 Hz and an estimated relative standard uncertainty of 6 × 10⁻¹⁶ (this radiation is already endorsed by the CIPM as a secondary representation of the second);
• the unperturbed optical transition 5s ²S_1/2 – 4d ²D_5/2 of the ⁸⁸Sr⁺ ion with a frequency of f_88Sr+ = 444 779 044 095 486.6 Hz and an estimated relative standard uncertainty of 1.6 × 10⁻¹⁵ (this radiation is already endorsed by the CIPM as a secondary representation of the second);
• the unperturbed optical transition 4s ²S_1/2 – 3d ²D_5/2 of the ⁴⁰Ca⁺ ion with a frequency of f_40Ca+ = 411 042 129 776 398.4 Hz and an estimated relative standard uncertainty of 1.2 × 10⁻¹⁴;
• the unperturbed optical transition 1S – 2S of the ¹H neutral atom with a frequency of f_1H = 1 233 030 706 593 514 Hz and an estimated relative standard uncertainty of 9.0 × 10⁻¹⁵.
  Note: This frequency corresponds to half of the energy difference between the 1S and 2S states;
• the unperturbed optical transition 5s^{2 1}S₀ – 5s5p ³P₀ of the ⁸⁷Sr neutral atom with a frequency of f_87Sr = 429 228 004 229 873.2 Hz and an estimated relative standard uncertainty of 5 × 10⁻¹⁶ (this radiation is already endorsed by the CIPM as a secondary representation of the second);
• the unperturbed optical transition 6s^{2 1}S₀ – 6s6p ³P₀ of the ¹⁷¹Yb neutral atom with a frequency of f_171Yb = 518 295 836 590 864.0 Hz and an estimated relative standard uncertainty of 2 × 10⁻¹⁵ (this radiation is already endorsed by the CIPM as a secondary representation of the second);
• the unperturbed ground-state hyperfine transition of ⁸⁷Rb with a frequency of f_87Rb = 6 834 682 610.904 310 Hz and an estimated relative standard uncertainty of 7 × 10⁻¹⁶
  (this radiation is already endorsed by the CIPM as a secondary representation of the second).

and also recommend 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
  f_a1 = 564 074 632.42 MHz
  λ_a1 = 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 5S_1/2 - 5P_3/2 crossover between the d and f hyperfine components of the saturated absorption at 780 nm (D2 transition); The values
  f_{d/f crossover} = 384 227 981.9 MHz
  λ_{d/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 to the d/f crossover in a rubidium cell external to the laser;

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

and ask the CIPM for adoption.

DOI : 10.59161/CCTF2015REC1E

Recommended values of standard frequencies