Infrared spectroscopy measures the infrared (IR) radiation after interacting with a sample, which is directed to the spectrometer’s detector. Thus, the sample absorbs part of the IR radiation, and the difference between the IR signal before and after the interaction with the sample generates the IR spectrum. The sample’s molecules vibrate while the IR radiation interacts with them, reaching a higher energy state. However, how do the molecules stop vibrating and return to their original energy state? I’ve searched for this information in the literature, but I’ve not found a clear answer.
Hey !
When the IR photon hits the molecule, it absorbs the photon and becomes in a vibrational state. In addition, the molecule has always tendency to be back to its original state , so it will be back by doing vibrations. So the vibration modes reflect that the molecule is returning back to its state.
The IR energy that you will give to the molecule let her be back to the original state by doing vibration. Same like a spring that you will give it an energy and it will be back to its original state by oscillation.
Check also Jablonski diagram.
Hope this answer will be good for you
Sincerely.
Thank you Mr. Ali Muhieddine,
Do you know a book about this topic to recommend?
Best wishes,
Murilo
There are many "dark" channels where the molecular vibraional energy can be transfered to. Typically for infrared energy the vibrations are transfered to phonon energy of the bulk material and propagate from the original excitation into the bulk. Think of it as conversion to heat energy in the bulk material. If there are levels between the excited vibrational state and the ground state the molecule can emit a photon and transition to a lower vibrational state - this is not a dark channel. There are many other ways an excited molecule can relax to its ground state. Any book on molecular spectroscopy should go into detail on this topic.
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