<P> Reaction can take place at the solid gas interface, surfaces at very low pressure such as ultra-high vacuum . Via scanning tunneling microscopy, it is possible to observe reactions at the solid gas interface in real space, if the time scale of the reaction is in the correct range . Reactions at the solid gas interface are in some cases related to catalysis . </P> <P> In photochemical reactions, atoms and molecules absorb energy (photons) of the illumination light and convert into an excited state . They can then release this energy by breaking chemical bonds, thereby producing radicals . Photochemical reactions include hydrogen--oxygen reactions, radical polymerization, chain reactions and rearrangement reactions . </P> <P> Many important processes involve photochemistry . The premier example is photosynthesis, in which most plants use solar energy to convert carbon dioxide and water into glucose, disposing of oxygen as a side - product . Humans rely on photochemistry for the formation of vitamin D, and vision is initiated by a photochemical reaction of rhodopsin . In fireflies, an enzyme in the abdomen catalyzes a reaction that results in bioluminescence . Many significant photochemical reactions, such as ozone formation, occur in the Earth atmosphere and constitute atmospheric chemistry . </P> <P> In catalysis, the reaction does not proceed directly, but through reaction with a third substance known as catalyst . Although the catalyst takes part in the reaction, it is returned to its original state by the end of the reaction and so is not consumed . However, it can be inhibited, deactivated or destroyed by secondary processes . Catalysts can be used in a different phase (heterogeneous) or in the same phase (homogeneous) as the reactants . In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products . Additionally, heterogeneous catalysts can dissolve into the solution in a solid--liquid system or evaporate in a solid--gas system . Catalysts can only speed up the reaction--chemicals that slow down the reaction are called inhibitors . Substances that increase the activity of catalysts are called promoters, and substances that deactivate catalysts are called catalytic poisons . With a catalyst, a reaction which is kinetically inhibited by a high activation energy can take place in circumvention of this activation energy . </P>

Where do the atoms in the products of a chemical reaction come from