‘准分子’ (zhǔn fēn zǐ) is a scientific term borrowed from English ‘excimer’, short for ‘excited dimer’. It refers to a short-lived, unstable molecule formed when two atoms (often noble gases like argon or krypton, or a noble gas and a halogen) bond only in an excited electronic state — not in their ground state. The character 准 means ‘quasi-’ or ‘pseudo-’, indicating something that resembles but isn’t a true molecule; 分 means ‘part’ or ‘component’, as in ‘molecule’ (分子); and 子 is a common suffix for small particles or units, reinforcing the atomic/molecular sense.

This term appears primarily in physics, chemistry, and medical engineering — especially in contexts involving lasers. Excimer lasers, for example, are widely used in eye surgery (LASIK) and semiconductor manufacturing due to their precise, high-energy ultraviolet light. While highly technical, the word may appear in science textbooks, medical device documentation, or news reports about advanced medical treatments.