Yeah, I remember some of that remotely from my IR spectroscopy segments of orgo- and, later, biochem; but mostly we were just told to think of water like three little balls hooked to springs, Hook’s Law was flashed before our eyes, and then we all memorized the peaks like good little biomonkeys.
Resonance isn’t a requirement for absorption. An electrical resistor absorbs lots of energy without being resonant at the applied frequency. In fact, a resonant circuit doesn’t absorb energy in the sense that it is turned into heat. The energy is stored and transferred back and forth between kinetic energy and potential energy with just a little input kick now and then to replace the losses, usually small. Think of a playground swing.
I don’t know anything about electronics, just spectroscopy. I still fail to see how a water molecule is going to absorb a microwave photon if that photon’s energy is not equal to the difference between two energy levels in an allowed transition. Water has allowed rotational transitions at the microwave level, and I understand that its microwave absorption spectrum is going to get quite smeared since it’s in food. I always thought that the water was (rotationally) excited by the photons, and it then proceeded to nonradiatively transfer this energy to translational energy. I’m certainly not an expert on the topic, hence my confusion. Could one of you folks who seems to know what’s going on here clear this up for me?
So far as I understand, this is correct, except that there are so many allowed energy levels so close together that they all get smeared out, and you don’t really have “levels” any more. You have a whole band of allowed or almost-allowed energies, and any photon anywhere in that band (not just at some precisely tuned energy) can be absorbed. I don’t know exactly what sort of mode is excited, but you’re correct that once any mode is excited, it’ll tend to equipartition that energy with other modes and with translation.
I’m a little out of my field, and depth here, but if energy is merely transferred from rotational to translational it is conserved and doesn’t appear as heat, does it?
It seems to me that what is required is that the rotational or translational energy
has to be non-conservatively dissipated in the surrounding molecules. On the macro scale it would be by friction but I’m not sure about what happens on the atomic or molecular level.
That’s not quite true. Microwave ovens have an output of about 800 watts, while flashlights have between 5 to 30 watts, maybe a little more for some special purpose flashlights. My guess is that a 800 watt flashlight would be quite capable to heat up a plate of soup. 
A 800 watt flashlight with an incandescent bulb would heat up a plate a soup, but only because it emits in the IR range as well as the visible. If it was a flashlight with a flourscent bulb, my guess is that it would do very little heating.
There’s a common misconception that infrared is synonymous with heat. It’s not. A hot object can emit any sort of light, if it’s hot enough, and light of any sort will heat up an object, if it’s absorbed. The only reason that we think of infrared in particular as being heat is that most hot objects we encounter are not hot enough to produce visible light or above. But if your soup is reasonably dark in color and opaque, then 800 watts of pure visible light would heat it up just as well as 800 watts of pure infrared.
Translational energy pretty much is heat, if not a very technical definition of it.
Yeah. I got to thinking about that and realized that temperature is a measure of the kinetic energy of the molecules so if they are vibrating they have a temperature. However, I don’t think the incoming energy has to be at the resonant frequency of the molecule in order to make it vibrate.
That’s cool. Too bad they didn’t include some emission spectra.
I note that, in the visible range, water absorbs very poorly. But it’s interesting to see in the far UV, it does an amazing job of absorbing. What is this UV energy emitted as? I shine UV light through electrolite solutions all the time, to view nucleic acids stained with fluorescing dyes suspended in agarose gels. Maybe this light is near UV, and hence the water is relatively transparent. Or maybe some of the far UV is being re-emitted by the water as more UV, which in turn lights up the dye? The solution itself (very mostly water) does not appear to fluoresce at all…would it heat up? I wish I knew what the emission spectra of the UV transilluminaters I use are. I do know they can give me a hell of a burn even with only a few minutes’ exposure if I don’t use protective clothing and a plastic face shield.
I think you’re only going to get a blackbody spectrum from liquid water.
Photoelectrons would be my guess.
Well, not that many of you are likely to care, but this is the transilluminator I use most frequently, and it has a “312 nm” bulb. I assume that’s a peak wavelength of emission; I can definitely see a purplish light that it also emits, which is maybe some violet light visible light. I assume there is a shoulder on the deep UV end, too. Water is quite transparent at this wavelength.
Water in the air must be almost opaque to shorter UV rays. Does all that absorbed energy heat it up?