Rb85

Rubidium-85 (85Rb) is one of the two naturally occurring isotopes of the chemical element rubidium (Rb). It has 37 protons and 48 neutrons, giving a nominal atomic mass of about 84.9118 atomic mass units. In nature, 85Rb accounts for roughly 72 percent of terrestrial rubidium. The nucleus possesses nonzero spin, with a nuclear spin I = 5/2, which leads to multiple hyperfine levels in atomic states.

85Rb is stable on experimental timescales, in contrast to the other natural rubidium isotope, 87Rb, which is radioactive and decays to strontium-87 with a very long half-life (about 4.88 × 10^10 years). Because of its stability, 85Rb is not subject to radioactive decay and is commonly encountered in laboratories and products that use rubidium.

In research and practical applications, 85Rb is widely used in atomic physics, particularly in laser cooling and ultracold-atom experiments. It is frequently employed alongside 87Rb to study isotopic effects and to calibrate instruments. The two isotopes share chemical properties, but their different nuclear spins and isotope shifts produce distinct hyperfine structures; the common 780 nm D2 optical transition is used for cooling and trapping, with isotope-specific tuning and trapping parameters.

Overall, 85Rb is a stable, naturally abundant isotope of rubidium that plays a central role in experimental atomic physics and related disciplines.