oxygen. Because of the differences in their electron shells, singlet oxygen has different chemical and physical properties than triplet oxygen, including absorbing and emitting light at different wavelengths. It can be generated in a photosensitized process by energy transfer from dye molecules such as rose bengal, methylene blue or porphyrins, or by chemical processes such as spontaneous decomposition of hydrogen trioxide in water or the reaction of hydrogen peroxide with hypochlorite. Ozone Triatomic oxygen (ozone, ) is a very reactive allotrope of oxygen that is a pale blue gas at standard temperature and pressure. Liquid and solid have a deeper blue color than ordinary , and they are unstable and explosive. In its gas phase, ozone is destructive to materials like rubber and fabric and is damaging to lung tissue. Traces of it can be detected as a pungent, chlorine-like smell, coming from electric motors, laser printers, and photocopiers, as it is formed whenever air is subjected to an electrical discharge. It was named "ozon" in 1840 by Christian Friedrich Schönbein, from ancient Greek ὄζειν (ozein: "to smell") plus the suffix -on, commonly used at the time to designate a derived compound and anglicized as -one. Ozone is thermodynamically unstable and tends to react toward the more common dioxygen form. It is formed by reaction of intact with atomic oxygen produced when UV radiation in the upper atmosphere splits . Ozone absorbs strongly in the ultraviolet and in the stratosphere functions as a shield for the biosphere against mutagenic and other damaging effects of solar UV radiation (see ozone layer). Tropospheric ozone is formed near the Earth's surface by the photochemical disintegration of nitrogen dioxide in the exhaust of automobiles. Ground-level ozone is an air pollutant that is especially harmful for senior citizens, children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma. The immune system produces ozone as an antimicrobial (see below). Cyclic ozone Cyclic ozone is a theoretically predicted molecule in which its three atoms of oxygen bond in an equilateral triangle instead of an open angle. Tetraoxygen Tetraoxygen had been suspected to exist since the early 1900s, when it was known as oxozone. It was identified in 2001 by a team led by Fulvio Cacace at the University of Rome. The molecule was thought to be in one of the phases of solid oxygen later identified as . Cacace's team suggested that probably