Photodissociation

Photodissociation, also known as photolysis, photodecomposition, or photofragmentation, is a chemical reaction where molecules of a chemical compound break down after absorbing light, or photons. This process happens when one or more photons interact with a target molecule, causing it to split into two fragments.

In this context, "light" includes a wide range of radiation, such as vacuum ultraviolet (VUV), ultraviolet (UV), visible, and infrared (IR) regions of the electromagnetic spectrum. To break covalent bonds, photons with energies from visible, UV, or VUV light are usually needed. Meanwhile, IR photons can sometimes be strong enough to detach ligands from coordination complexes or to break apart supramolecular complexes.

Photoinduced proton transfer

Photoacids are special molecules that, when they absorb light, give away a particle called a proton to become a photobase. This process can be written as AH → h ν A⁻ + H⁺. After this change and a quick return to their normal state, the proton and the acid can come back together again.

Photoacids are useful in science for creating quick changes in pH during experiments with ultrafast lasers.

Photolysis in the atmosphere

Photolysis is an important process in Earth's atmosphere. It helps change harmful substances like hydrocarbons and nitrogen oxides into other pollutants, such as peroxyacyl nitrates. This process is part of what creates photochemical smog.

One key reaction happens when ozone (O₃) breaks down into oxygen (O₂) and a single oxygen atom. This oxygen atom can then help create hydroxyl radicals, which are important for cleaning the atmosphere by breaking down hydrocarbons. Another important reaction involves nitrogen dioxide (NO₂) breaking down into nitric oxide (NO) and oxygen. This reaction helps form tropospheric ozone.

Photolysis also plays a role in creating the ozone layer in the stratosphere. Here, ultraviolet light breaks apart oxygen molecules (O₂) into single oxygen atoms. These atoms then combine with other oxygen molecules to form ozone (O₃). However, photolysis can also break down CFCs and release chlorine atoms, which can damage the ozone layer.

Astrophysics

In astrophysics, photodissociation is an important process that breaks down molecules. Because space is a vacuum, molecules and tiny particles called free radicals can stay around for a long time. Photodissociation helps scientists understand what interstellar clouds are made of, which is where stars are born.

One example is water breaking into hydrogen and hydroxyl when it absorbs light. Another example is methane breaking into methyl and hydrogen. These processes show how light can change molecules in space.

Atmospheric gamma-ray bursts

Orbiting satellites detect about one gamma-ray burst each day. These bursts come from very far away, and they are very rare events for any single galaxy like the Milky Way.

If a gamma-ray burst happened close enough to Earth in the Milky Way, it could affect our planet's atmosphere. The burst's energy could break down molecules like nitrogen and oxygen using a process called photodissociation. This could create chemicals that damage the ozone layer, which protects Earth from harmful sunlight. Scientists think such an event might have happened a very long time ago, possibly affecting life on Earth.

Multiple-photon dissociation

Single photons in the infrared range usually don't have enough energy to break molecules apart. But, if a molecule absorbs many infrared photons, it can gain enough energy to split into smaller pieces. This process, called multiple-photon dissociation, can happen using strong lasers like a carbon dioxide laser or a free-electron laser. It can also occur through long exposure to regular heat radiation, known as black-body radiation, in a method called blackbody infrared radiative dissociation.