Index

DEFININGCONSTANT: hyperfine transition frequency of Cs

  • PID: si:definingconstant:caesium_transition_frequency:2019
  • Definition: The unperturbed ground state hyperfine transition frequency of the caesium 133 atom, ΔνCs, is exactly 9 192 631 770 Hz.
  • Source: SI Brochure 9th Ed. 2019, p 127
  • Formatted: 9 192 631 770 Hz
  • Numerical Value: 9192631770
  • Unit: Hz
  • Unit in Base SI: s-1
  • Notes
    1. This value was fixed by the CGPM in 1967 (Resolution 1) and reiterated by the CGPM in 2018 (Resolution 1).
    2. The caesium transition frequency is an atomic parameter which may be affected by the environment, such as electromagnetic fields. However, the underlying transition is well understood, stable and a good choice as a reference transition under practical considerations.
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