cesium133transities

Cesium-133 transitions refer to the energy level changes in the cesium-133 atom, with particular emphasis on the hyperfine transitions that underpin the international definition of the second. Cesium-133 has a nuclear spin I = 7/2 and, in its electronic ground state 6S1/2, the interaction between electron and nuclear angular momenta produces two closely spaced hyperfine levels labeled F = 3 and F = 4. The magnetic-dipole transition between these two hyperfine levels within the ground state occurs at a frequency of exactly 9,192,631,770 hertz.

This microwave transition is the basis of the cesium standard used in atomic clocks. The energy difference between the F = 3 and F = 4 states is ΔE = h × f, where h is Planck’s constant and f is the transition frequency. Atomic clocks measure this transition by exposing cesium atoms to microwaves near 9.19 GHz and counting the cycles that yield the maximum transition probability, thereby stabilizing a frequency reference used to keep time.

In addition to the ground-state hyperfine transition, cesium-133 exhibits optical transitions involving excited states, such as the D1 and D2 lines. The D1 line (approximately 894 nm) corresponds to the 6S1/2 → 6P1/2 transition, and the D2 line (approximately 852 nm) corresponds to the 6S1/2 → 6P3/2 transition. These optical transitions are central to laser cooling, trapping, and precision spectroscopy of cesium atoms but are not used to define the SI second.

Since 1967, the second has been defined as the duration of 9,192,631,770 periods of the cesium-133 hyperfine transition, making cesium-133 transitions a foundational element of modern timekeeping.