What happens in chemicals that makes them photosensitive? Like I know that chemicals get affected by temperature, and their molecules start moving faster, so is photosensitivity basically similar that when light particles come in, something happens? If so, what exactly happens?
In: Chemistry
Electrons orbit atoms and molecules. Quantum mechanics tells us these orbits can only be at certain energy levels, like rungs on a ladder. The differences between energy levels determine what types of light particles (photons) can be absorbed by the electrons. When a photon with the right amount of energy hits an electron it “kicks” it up to a higher step on the energy ladder. This energy is now stored in the electron’s orbit and can be used to do work, like converting carbon dioxide and water into sugar.
Photosensitive pigments have electron arrangements that accept photons down into the visible light range, while most atoms and molecules do not. In general if you get to a high enough energy band, like X-rays, the photons will be strong enough to affect any electron arrangement.
It depends on the chemical, but basically the atom “catches” the photon and uses the energy to put an electron in a higher orbital or knock it off completely. This changes the molecules bonding characteristics, triggering chemical reactions.
When a molecule absorbs light an electron is moved from the ground state to a higher energy “excited state”. From here, the electron can do one of three basic things. First is nonradiative decay, where the electron makes its way back to the ground state and the energy difference between the ground and excited state is released as heat. The next thing that can happen is radiative decay. Here the electron makes its way back to where it came from, but the excess energy is given off as light. The third thing the molecule can do is react. Depending on the molecule, while the electron is in the excited state it is unpaired, so it can do various things from there, such as creating a leaving group (how diazos create carbenes), it can simply cleave off to form a radical (Norrish), or it can react in the excited state where the molecule will have some diradical character. There are many other things it can do as well. Photochemistry is complicated.
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