Why does water easily absorb THz radiation? 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.
 A: 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.
