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It is stated that water is extremely sensitive to THz radiation, absorbing big amounts of this radiation, this being a reason why skin measurements are safe to do using this range of frequencies (1) (2) (3)

But I don't find a good explanation as to why this is so.

The last link states that the reason behind the absorption of THz by water is "due to rotational transition of polar water molecules" but I don't understand this statement.

My understanding:

Water molecules vibrations comprise a range of frequencies in the THz range, so when radiation of a specific frequency hits a water molecule vibrating in that same frequency it causes them to "absorb" the radiation.

This still does not explain why they absorb the radiation and I don't understand the reason as to why.

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  • $\begingroup$ You'd probably need to clarify the level of "why" you wish for. A reasonable but useless answer may be: "We can model an H2O molecule's interaction with THz photons using a quantum electrodynamical model, and it turns out it's likely to absorb those photons. So we'd expect a real water molecule to act that way as well". Paraphrasing Pratchett: "Because of quantum". What would you consider a satisfying answer - what level of detail do you need? In the simplest terms, there are electron state transitions that overlap with THz photon energies. $\endgroup$
    – Kuba hasn't forgotten Monica
    Commented Mar 30, 2021 at 18:13
  • $\begingroup$ Thank you for helping. A graduate-level of understanding would be great, I am just starting a PhD soon in this area and I know that skin imaging is used because of this, but I don't understand how the water molecules can simply absorb the radiation and not be ionised, and what makes the water molecules such good absorbers. $\endgroup$
    – user7077252
    Commented Mar 30, 2021 at 18:17
  • $\begingroup$ THz radiation is not special in that respect, neither is a water molecule. Why can some light be absorbed by some molecule without ionising it? We know it happens and it's very common: sunlight doesn't ionise wood, for example :) Should give you a good starting point. $\endgroup$
    – Kuba hasn't forgotten Monica
    Commented Mar 30, 2021 at 18:54

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It sounds like you are trying to get a picture of why it happens, not so much to go through the math.

Imagine a water molecule. It is shape like a shallow V. It vibrates every way it can -- the arms lengthen and shorten, the angle varies, the whole thing spins around three axes and all their combinations, etc. Each kind of vibration has a resonant frequency where it is particularly good at absorbing energy, and then radiating it away again.

Now imagine a second molecule. One of the hydrogens is close to the base of the V of the first one, where the oxygen is. They are attracted some because they have opposite charges and they are closer to each other than they are to anything else except the other parts of their own molecules. This is called hydrogen bonding. It's a weaker force than the covalent bonds, but it's there. Every now and then that hydrogen will get close enough that it isn't clear which of the three hydrogens are covalently bonded to that oxygen. And then one of them will get carried away elsewhere, and it's a 2/3 chance it will be one of the others. The water molecule has changed. Water molecules at room temperature are continually breaking up and reforming like millenials at a singles bar.

So each of the other bonds has its resonant frequency. But these hydrogen bonds vary randomly in their distance and strength. They don't have a single resonant frequency, they are smeared out all over the place.

There's more, but that's a start.

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