Helistate

Helistate is a term used in atomic physics to denote a metastable electronic state of a helium atom or helium-based system. The most common reference is to the 2^3S1 metastable state of neutral helium-4, and the analogous state in helium-3. These states are long-lived and can be populated and controlled in laboratory conditions, making them useful as a resource state in experiments.

Physically, the helium 2^3S1 metastable state lies about 19.8 eV above the ground state and possesses a lifetime on the order of thousands of seconds in vacuum. It is produced by discharge sources or electron impact and can be accessed by laser excitation via allowed transitions to higher triplet states. In many experiments the state is trapped and cooled using standard laser cooling schemes, often in a magneto-optical trap (MOT).

Applications and research use of helistates include high-precision spectroscopy, studies of quantum degeneracy in helium, and atom interferometry. They serve as convenient two-electron systems for testing interactions, collisional dynamics, and fundamental constants. Because of their relatively simple electronic structure combined with long lifetimes, they are a focal point in ultracold-atom physics.

Terminology regarding helistate is not a widely formal term in standard reference works; researchers more commonly refer to metastable helium as He* and specify the 2^3S1 state. The usage of helistate may appear informally in some texts or discussions.