[GrapheneBlanket]

Hi friends. I had a question about infrared spectroscopy. I am aware that IR spectroscopy works because when a continuum of IR radiation is blasted on a sample, the frequencies corresponding to the different vibrational modes of the sample are absorbed and a smaller amount of that particular frequency reaches the detector, allowing us to probe the vibration states of our compound.

What I am confused about is what happens to the energy after it is absorbed. In ultraviolet absorption, after the photon is absorbed, it is re-emitted. Why does this not occur with IR spectroscopy? Is the energy released thermally in the case of IR? The energy of the photon cannot remain in the sample forever, correct? Thanks in advance for the help,

-GB

[Arkcon]

Briefly, yes the IR radiation absorbed is converted into molecular vibration, and that is one definition of heat.  To be completely though, UV energy absorbed may not be re-emitted, it may also be lost to heat, or a molecule may decompose and react.

[fledarmus]

This does occur in IR spectroscopy, and you can have both emission and absorption measurements of both UV and IR spectroscopy. It depends on the instrument and how your detector is related to your source of radiation.

If you want to measure the absorbance of a sample, you put the detector in a direct line with the source, and your sample in between. If there is no sample, any radiation emitted by the source will be detected by the detecter. When you interpose the sample, any frequencies which can affect your sample get absorbed. Your detector is detecting any radiation NOT absorbed by the sample. Absorbed radiation is reemitted in random directions, so that energy will be missing from the energy seen by the detector.

If you want to measure the emission of the sample, you put the detector at 90 degrees to the sample. If there is no sample, the detector will see none of the radiation emitted by the source. When you interpose your sample, the sample will absorb certain frequencies and re-emit them. Since the absorbed radiation is reemitted in random directions, some of it will be emitted at 90 degrees to the source beam, and the detector will detect it. Whatever radiation is detected comes from radiation emitted by the sample.

These types of absorption-emission choices are common to any radiation source. Mechanical considerations and technical aspects based on noise and detection limits determine whether instruments use absorption or emission detection, and both types have instruments have been used on all types of radiation that I am aware of.

[GrapheneBlanket]

That really clears things up. I realize I took no consideration into  the fact that once the energy is absorbed by the molecule, it can be released in many different forms and many different directions and not necessarily all at once. And to assume that it would all be re-emitted at the same frequency in the same direction as the incident radiation is simply short-sighted.
Thanks for the *delete me*