These varying thermodynamically stable contact angles are known as metastable states. Rydberg atoms are an example of metastable excited atomic states. Consequently, the transformation of the metastable TiO polymorphs to rutile is irreversible. Chromium also has two metastable nuclear isomers. There are three mechanisms with which supersaturation occurs&colon; precipitation, nucleation, and metastable. At least 21 metastable nuclear isomers exist. Many other polyoxometalate anions exist as metastable species. At least four isotopes have metastable excited states. The supersaturated solution can then return to the metastable region. As an example, consolidation occurs so rapidly that metastable microstructures may be retained. The superscript "m" indicates a metastable isotope. Few dispute the large number of metastable vacua. The origin of the stabilization mechanism is different for the stable and metastable quasicrystals. The Doppler limit has been verified with a gas of metastable helium. Transitions from metastable excited levels are typically those forbidden by electric dipole selection rules.