In the previous post, we started to understand the basic difference between radiation chemistry and photochemistry. We concluded that photochemistry is a branch of radiation chemistry that deals exclusively with radiation in the UV, visible, and IR regions only.

We will try to further understand this difference by going deeper into the atomic structure. We know that an atom is made up of electrons, protons, and neutrons. The protons and neutrons are at the center of the atom- in the nucleus and the electrons revolve around the nucleus in specific orbits.

Structure of an atom.

Though, different electrons have different energy (we know, they occupy different energy levels /orbits), the energy of all these electrons of an atom is typically comparable to the energy of radiation in the UV, visible, or IR region. Thus, when radiation in this range(energy around 1 to 10 eV) is shone upon the atom, specific electrons absorb that energy, get excited, and form products. This is the basis of a photochemical process.

On the other hand, the protons and neutrons sit in the nucleus and are bound by very strong nuclear forces. The energy in the UV, visible, or IR region is far too little to disrupt the nuclear forces and thus, the nucleus remains unaffected in photochemical reactions. However, high-energy radiation (gamma or X-rays) have sufficient energy to overcome these nuclear forces, and thus, they bring about nuclear reactions. These rays have enough energy (e.g.– the energy of a -ray photon is around 1,000,000 ev !!) to ionize the molecules. Thus, they are also called ionizing radiations. These nuclear reactions are very different from the typical reactions we study in chemistry. (We will learn nuclear chemistry in detail later). Now that we have a fair understanding of the difference between nuclear and photochemical reactions, let us begin to study photochemistry basics.


In a photochemical reaction, a molecule absorbs a photon of light and gets energized to an excited state, which leads to the formation of products. Thus, this process consists of two processes-

  1. Primary process – Light is absorbed to produce the excited molecule (light is absorbed only if the energies of the incident radiation and the electrons are same).The absorbed light has energy high enough to dissociate the reactant molecule to form free radicals.
  2. Secondary process – The free radicals undergo further reaction to form products.(To read more about primary and secondary process , click here)

Photochemical reactions are of two types –

  1. Direct light induced or non-sensitized reactions.
    2.Photosensitized reactions.
  1. Direct light induced or non-sensitized reactions – In these kinds of reactions, the reactant molecule actually absorbs quanta of radiation and undergoes various secondary processes to give the final product of the reaction. In photochemistry, this is referred to as Photolysis – which means the breaking of chemical bonds by light energy. The bonds undergo homolytic fission (uniform breaking of the bond) to form free radicals. In very rare cases, photolysis produces ions.

2. Photosensitized reactions – In these kinds of reactions, one of the molecules is a donor(D), which absorbs the incident radiation and becomes excited. It then transfers its excitation energy to another type of molecule called the acceptor(A) and returns to the ground state. This donor molecule thus just acts as a mediator – it absorbs light and transfers it to the acceptor molecule without itself undergoing any chemical change. This donor is called the ‘photosensitizer’ and the process is called photosensitization. We will study these kinds of reactions in upcoming posts.

We are going to learn about different types of reactions in photochemistry. However, we can generally expect the following types of chemical reactions –

1)Unimolecular reactions like dissociation, ionization, and isomerization.

Dissociation is a phenomenon where the bond between atoms is broken. This splitting of the bond generally produces smaller molecules or atoms.

Ionization refers to the separation of an electron from an atom or a molecule. This results in the formation of positively charged ions. As discussed earlier, these types of reactions are a very rare occurrence in the UV, visible, or IR region. Only ionizing radiations (of very short wavelength < 100nm) can bring about such a change in the molecule.

Isomerization is a phenomenon of the formation of isomers. An excited molecule may undergo a rearrangement of its bonds, forming a new molecule. The new molecule is called an isomer of the earlier one and has the same atoms but these atoms are connected differently. This process is called isomerization.

2) Bimolecular reactions where a reaction with a second molecule/atom yields the product. Photosensitization could be a bimolecular process.

3)Reactions that produce an emission of light or luminescence. When an excited species reduces its energy by emitting radiation, the process is termed luminescence. This emission can be immediate (Fluorescence) or delayed (Phosphorescence).

We are going to study all of this in greater detail as we proceed with the discussions n photochemistry in the upcoming posts. Till then,

Be a perpetual student of life and keep learning…

Good day!

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