[KayVL]

I am in the last year of high school and I have to do a project about FT-IR and FT-NIR.
I've already looked at several sites and videos but it's not clearly mentioned and I don't really understand.

It may be a stupid question but will molecules BEGIN to vibrate (stretch and bend) because the bonds of functional groups absorb the energy from infrared radiation with a specific frequency. Or do the bonds of the functional groups aready vibrate by themselves before they make contact with IR radiation?

Thank you in advance.

[Corribus]

Molecules always vibrate, even (counterintuitively) at absolute zero temperature - a result of the uncertainty principle. When a molecule receives the appropriate amount of infrared energy, some of that energy is absorbed, and the absorbed energy is converted into kinetic energy. Which is to say, the the molecule vibrates faster. For molecules, only certain amounts of energy are allowed to be absorbed, and that allowed amount of energy depends on the structure of the molecules. Which is why infrared spectroscopy (analyzing which energies of light a certain molecule absorbs) can be used to understand molecular structure, or identify unknown molecules. The higher energy vibration after absorption of light does not persist indefinitely. Eventually the vibrating molecule sheds the excess energy and "slows down" back to a lower energy vibration, in a process liberating heat. Which is why infrared lamps can heat up, for example, food.

[KayVL]

Thank you very much for your explanation.

Kind regards
Kay