1s2s

1S2S is a compact designation used in atomic physics and spectroscopy to describe configurations and transitions involving the first and second shells of light atoms, most often seen in two contexts: the electronic configuration 1s2s in helium-like systems and the 1S–2S transition in hydrogen and hydrogenic ions.

Electronic configuration: In helium and other two-electron or helium-like ions, the arrangement of one electron in the 1s orbital and the other in the 2s orbital is written as 1s2s. The coupling of electron spins yields a singlet state, 1S0, and a triplet state, 3S1. The 2^3S1 triplet state is metastable, with a comparatively long lifetime, and is exploited in laser cooling, precision spectroscopy, and atomic clocks. In astrophysics, the 1s2s configuration of neutral helium gives rise to metastable levels such as 2^3S1, which produces infrared and optical transitions (for example, the He I 10830 Å line) used to diagnose stellar atmospheres and winds.

Hydrogen and other hydrogenic systems: The notation 1S–2S most often refers to the electric dipole-forbidden single-photon transition between the ground 1S state and the excited 2S state. Because it cannot occur via one-photon emission or absorption, it is typically accessed by two-photon processes and studied with high-precision frequency metrology. The 1S–2S transition in hydrogen has played a central role in tests of quantum electrodynamics and in determinations of fundamental constants, with measurements reaching fractional uncertainties well below 10^-15.

Overall, 1S2S is a shorthand that appears in atomic-level notation across spectroscopy, quantum optics, and astrophysics to denote either the 1s2s electronic configuration or the 1S–2S transition, depending on context.