Cäsium133Atoms

Cäsium-133 atoms are neutral atoms of the element cesium with mass number 133. Cesium, atomic number 55, is an alkali metal with a single valence electron. Natural cesium is almost entirely 133Cs, which is the only stable isotope; other cesium isotopes are radioactive. In the ground state, the electronic configuration is [Xe] 6s1. The metal is soft, highly reactive, and readily oxidizes in air; it is typically stored under mineral oil to prevent contact with moisture.

Nuclear properties: The nucleus has spin I = 7/2, giving hyperfine structure in the ground state 6S1/2. The hyperfine transition between F=4 and F=3 is used as the basis for many cesium frequency standards. The radiation frequency of this transition is 9,192,631,770 hertz.

The SI second is defined by this transition since 1967; cesium clocks provide the primary standard for timekeeping and frequency metrology. Cesium-133 atoms are central to microwave clocks and are used in laboratory spectroscopy and calibration of timekeeping systems such as GPS. In experiments, cold-atom techniques, optical pumping, and magneto-optical traps are used to study coherence and quantum properties.

Safety and handling: Cesium metal is highly reactive; it reacts violently with water and oxygen and is stored under mineral oil or in an inert atmosphere to prevent contact.