Photochemical Process
Photochemical Process
To begin a photochemical process, an atom or molecule must absorb a quantum of light energy from a photon. After absorbtion of quanta of light energy, the energy of the atom or molecule increases above its normal level and gets excited state. If a quantum of visible or ultraviolet light is absorbed, then an electron in a relatively low energy state of the atom or molecule is excited into a higher energy state. If infrared radiation is absorbed by a molecule, then the excitation energy affects the motions of the nuclei in the molecule.After the initial absorption of a quantum of energy, the excited molecule can undergo a number of primary photochemical processes. A secondary process may occur after the primary process. The absorption step can be represented by-
M ---Light---> M*
where the molecule M absorbs a quantum of light of appropriate energy to yield the excited M* molecule.
Types of Photochemical Process
Photochemical Process are of two types-Primary Photochemical Process
Secondary Photochemical Process
Primary Photochemical Process
The process of primary photochemical reaction refers to what happens immediately after absorption of light. This can be consider as the first step of the photochemical reaction which obeys the law of photochemical equivalence strictly.Excited molecule M* can undergo various primary photochemical processes as mentioned below-
Primary photochemical process can be of two types–
1. Photophysical Process
2. Photochemical Process
Photophysical Process
These include radioactive transitions, where the excited molecule emits light in the form of fluorescence or phosphorescence and returns back to the ground state. No chemical reaction occurs in a photophysical process.Photochemical Process
In these reactions, the molecule after getting excited by the absorption of light undergoes a chemical change to give chemically different products. The excited molecule can isomerize, rearrange itself, dissociate or react with another molecule as mentioned above.Secondary Photochemical Process
Byproducts (i.e. excited molecules, free radicals) of primary process may be involved in a subsequent thermal reaction referred to as secondary processes and this process may involve in one or more than one steps.For example, the decomposition of HBr occurs as follows-
Evidently, the primary process obeys the law of photochemical equivalence strictly but the secondary process have no concern with the law.
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