Photodissociation,photolysis,photodecomposition, orphotofragmentation is achemical reaction in whichmolecules of achemical compound are broken down by absorption of light (photons). It is defined as the interaction of one or more photons with one target molecule that dissociates into two fragments.[1]
Here, “light” is broadly defined as radiation spanning thevacuum ultraviolet (VUV),ultraviolet (UV),visible, andinfrared (IR) regions of theelectromagnetic spectrum. To breakcovalent bonds,photon energies corresponding to visible, UV, or VUV light are typically required, whereas IR photons may be sufficiently energetic to detachligands fromcoordination complexes or to fragment supramolecular complexes.[2][3]
Photoacids are molecules that upon light absorption undergo aproton transfer to form the photobase.
In these reactions, the dissociation occurs in the electronically excited state. After proton transfer and relaxation to the electronic ground state, the proton and acid recombine to form thephotoacid again.
Photoacids are a convenient source to induce pH jumps inultrafast laser spectroscopy experiments.
Photolysis occurs in the atmosphere as part of a series of reactions by which primarypollutants such ashydrocarbons andnitrogen oxides react to form secondary pollutants such asperoxyacyl nitrates. SeePhotochemical smog.
The two most important photodissociation reactions in thetroposphere are firstly:
which generates an excited oxygen atom which can react with water to give thehydroxyl radical:
The hydroxyl radical is central toatmospheric chemistry as it initiates theoxidation of hydrocarbons in the atmosphere and so acts as adetergent.
Secondly the reaction:
is a key reaction in the formation oftropospheric ozone.[4]
The formation of theozone layer is also caused by photodissociation. Ozone in the Earth'sstratosphere is created by ultraviolet light striking oxygen molecules containing two oxygenatoms (O2), splitting them into individual oxygen atoms (atomic oxygen). The atomic oxygen then combines with unbrokenO2 to createozone,O3.[5] In addition, photolysis is the process by whichCFCs are broken down in the upper atmosphere to form ozone-destroying chlorinefree radicals.[6]
Inastrophysics, photodissociation is one of the major processes through which molecules are broken down (but new molecules are being formed). Because of thevacuum of theinterstellar medium, molecules andfree radicals can exist for a long time. Photodissociation is the main path by which molecules are broken down. Photodissociation rates are important in the study of the composition ofinterstellar clouds in whichstars are formed.
Examples of photodissociation in the interstellar medium are (hν is the energy of a singlephoton of frequencyν):
Currently, orbiting satellites detect an average of about onegamma-ray burst (GRB) per day.[7] Because gamma-ray bursts are visible to distances encompassing most of theobservable universe, a volume encompassing many billions of galaxies, this suggests that gamma-ray bursts must be exceedingly rare events per galaxy.[8]
Measuring the exact rate of gamma-ray bursts is difficult, but for a galaxy of approximately the same size as theMilky Way, the expected rate (for long GRBs) is about one burst every 100,000 to 1,000,000 years.[8] Only a few percent of these would be beamed toward Earth. Estimates of rates of short GRBs are even more uncertain because of the unknown beaming fraction, but are probably comparable.[9]
A gamma-ray burst in the Milky Way, if close enough to Earth and beamed toward it, could have significant effects on thebiosphere. The absorption of radiation in the atmosphere would cause photodissociation ofnitrogen, generatingnitric oxide that would act as a catalyst to destroyozone.[10]
The atmospheric photodissociation
would yield
(incomplete)
According to a 2004 study, a GRB at a distance of about akiloparsec could destroy up to half of Earth'sozone layer; the direct UV irradiation from the burst combined with additional solar UV radiation passing through the diminished ozone layer could then have potentially significant impacts on thefood chain and potentially trigger a mass extinction.[11][12] The authors estimate that one such burst is expected per billion years, and hypothesize that theOrdovician-Silurian extinction event could have been the result of such a burst.
There are strong indications that long gamma-ray bursts preferentially or exclusively occur in regions of low metallicity. Because the Milky Way has been metal-rich since before the Earth formed, this effect may diminish or even eliminate the possibility that a long gamma-ray burst has occurred within the Milky Way within the past billion years.[13] No such metallicity biases are known for short gamma-ray bursts. Thus, depending on their local rate and beaming properties, the possibility for a nearby event to have had a large impact on Earth at some point in geological time may still be significant.[14]
Single photons in theinfrared spectral range usually are not energetic enough for direct photodissociation of molecules. However, after absorption of multiple infrared photons a molecule may gain internal energy to overcome its barrier for dissociation. Multiple-photon dissociation (MPD;IRMPD with infrared radiation) can be achieved by applying high-power lasers, e.g. acarbon dioxide laser, or afree-electron laser, or by long interaction times of the molecule with the radiation field without the possibility for rapid cooling, e.g. by collisions. The latter method allows even for MPD induced byblack-body radiation, a technique calledblackbody infrared radiative dissociation (BIRD).
