Physical mechanism how matter absorbs radiation.

[avicenna]

I'll like to know the actual physical mechanism how matter absorbs EM radiation. Take the specific example how copper metal absorbs IR radiation and the copper having its temperature raised.
 

[exchemist]

1 hour ago, avicenna said:

I'll like to know the actual physical mechanism how matter absorbs EM radiation. Take the specific example how copper metal absorbs IR radiation and the copper having its temperature raised.
 

I think it is harder to explain for a metal than for a molecular substance. In molecules, there needs to be a dipole for the electric vector of the radiation to couple to, which excites vibrations in the molecule. For example, nitrogen gas does not absorb in the IR as the molecule has no dipole, whereas water vapour does, because the hydrogen atoms carry a partial +ve charge while the oxygen atoms carried a partial -ve charge.

In a metals there is a lattice of +ve cores and a sea of surrounding delocalised electrons. Lattice vibrations (phonons) can presumably be excited by the +ve cores coupling to the electric vector of the radiation, but I'm guessing a bit as I never covered the relevant solid state physics at university. Perhaps someone else can comment further. 

[swansont]

I think a classical model works better for a metal. The varying EM field causes the electrons in the conduction band to oscillate; the field transfers its energy and momentum to the metal (mostly to the electrons) The increased KE of the electrons manifests as an increase in temperature 

[avicenna]

Maybe no easy answer because we know too little yet about light.

The wiki says it is the bound electrons that absorbs radiation; then how the electrons energy get transferred  to the nucleus kinetic energy. I think temperature not dependent on the KE of electrons, but only in the KE of the nucleus or center of mass.  

[swansont]

4 minutes ago, avicenna said:

Maybe no easy answer because we know too little yet about light.

The issue is that we can’t investigate what’s happening at the level of detail you seem to be asking about. We have classical and quantum models about how radiation interacts, but it doesn’t tell us what’s happening at very small scales. We can’t probe what the photon is doing, because the photon is the probe.

 

4 minutes ago, avicenna said:

The wiki says it is the bound electrons that absorbs radiation; then how the electrons energy get transferred  to the nucleus kinetic energy. I think temperature not dependent on the KE of electrons, but only in the KE of the nucleus or center of mass.  

The temperature manifests itself both in terms of the CoM kinetic energy and the excitation of the electrons; for the latter the number of excited electrons varies as e^-(E/kT) where E is the excitation energy

i.e. more electrons will be excited at higher temperature, or if the system has a smaller excitation energy

[exchemist]

1 hour ago, avicenna said:

Maybe no easy answer because we know too little yet about light.

The wiki says it is the bound electrons that absorbs radiation; then how the electrons energy get transferred  to the nucleus kinetic energy. I think temperature not dependent on the KE of electrons, but only in the KE of the nucleus or center of mass.  

It is not only bound electrons that absorb radiation. That is true for electronic transitions, absorbing light in the visible and UV range. However IR and microwaves stimulate vibrations and rotation, respectively, of polar molecules. So long as you have a degree of charge separation, you have in principle something that can couple to radiation, if the scale and frequency are right.  

But I think you must be right that to explain a temperature rise we need to account for how the nuclei are set in motion, in the form of lattice vibrations. A mere sympathetic oscillation of the conduction band electrons setup by the radiation will generate reflection rather than absorption, it seems to me. And of course metals are good at reflecting radiation.

So I think we need to look at the interaction of electron motion with lattice vibration. It feels to me as if this is connected with the mechanism behind electrical resistance in metals. There has to be a process whereby coordinated induced motion of electrons is converted into randomised motion of electrons and vibrations of the lattice. But I don't know the relevant solid state physics.