Caesium monoxide or caesium oxide is an chemical compound with the chemical formula . It is the simplest and most common oxide of the caesium. It forms yellow-orange hexagonal crystals.
Caesium oxide is used in photocathodes to detect infrared signals in devices such as image intensifiers, vacuum photodiodes, photomultipliers, and TV camera tubesL. R. Koller described the first modern photoemissive surface in 1929–1930 as a layer of caesium on a layer of caesium oxide on a layer of silver. It is a good electron emitter; however, its high vapor pressure limits its usefulness.
Elemental magnesium reduces caesium oxide to elemental caesium, forming magnesium oxide as a side-product:
is hygroscopic, forming the corrosive CsOH on contact with water.