Photodissociation

Photodissociation, also called photolysis, photodecomposition, or photofragmentation, is a chemical reaction. In this reaction, molecules of a chemical compound break apart after they take in light, or photons. This happens when one or more photons touch a molecule, making it split into two pieces.

Here, “light” means many kinds of radiation. This includes vacuum ultraviolet (VUV), ultraviolet (UV), visible, and infrared (IR) parts of the electromagnetic spectrum. To break strong bonds in molecules, photons with energy from visible, UV, or VUV light are usually needed. Sometimes, IR photons can be strong enough to pull parts away from certain kinds of molecules or to break apart some special molecule groups.

Photoinduced proton transfer

Photoacids are special molecules. When they absorb light, they give away a particle called a proton. This changes them into a photobase. We can write this as AH → h ν A⁻ + H⁺. After this change, the proton and the acid can return to their normal state.

Photoacids are useful in science. They help scientists make 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. These radicals 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 a 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 learn what interstellar clouds are made of. This 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 examples 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 are very rare for our galaxy, the Milky Way.

If a gamma-ray burst happened close 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 through a process called photodissociation. This might create chemicals that could harm the ozone layer, which protects Earth from harmful sunlight. Scientists think such an event might have happened long 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.