SiO

SiO, or silicon monoxide, is a diatomic molecule composed of silicon and oxygen. It is a high-temperature, reactive species that is not stable at room temperature and is typically generated in flames, plasmas, or astrophysical environments where strong heating occurs. In the gas phase, SiO has a polar covalent bond and a well-defined electronic ground state. Its infrared fundamental vibration lies near 8 micrometres, a spectral feature that enables detection by infrared spectroscopy. The molecule also exhibits rotational–vibrational transitions that are used to study high-temperature gas mixtures and shocked regions.

SiO is notable in astronomy and astrochemistry. It is detected in the outflows around oxygen-rich evolved stars, including Mira variables and red supergiants, where SiO maser emission arises in the circumstellar envelopes. SiO lines are used to probe shock- and flame-like conditions in interstellar and circumstellar environments. In the laboratory, SiO is produced by high-temperature oxidation of silicon or by laser ablation in an oxygen-containing atmosphere.

Chemically, SiO tends to react with oxygen to form silicon dioxide (SiO2), and in silicate melts or grains it can act as an intermediate species. It is not a stable industrial compound but serves as a model molecule for studying gas-phase silicon chemistry and silicon–oxygen bonding.