Light Absorption Mechanism: Electrons, IR, UV Heating

[JaWiB]

This is probably a really basic question, but I realized I'm not sure what the answer is. What is the mechanism for light heating an object? Are photons always absorbed by electrons (if they are absorbed at all)?

Maybe this will help explain my question: People often say that infrared light generally heats an object, whereas UV can actually break bonds/ionize atoms. But does infrared do essentially the same thing (excite electrons to higher energies, but below the ionization energy) or does it do something else entirely?

 

[alxm]

Absoprtion of infrared radiation generally corresponds to vibrational/rotational transitions of molecules, rather than electronic ones. Electronic transitions generally occur from the visual range and up, mostly in the UV. Hence the reason for why bond-breaking and ionization occurring there. (e.g. excitation from a bonding to an anti-bonding orbital)

There's coupling between the two, especially from electronic to vibrational. Think about it this way: Each electronic state has a different bunch of associated vibrational states (Why different? Because if the electronic state is different the potential energy surface between the atoms is different). There's no reason an electronic excitation has to be to the vibrational ground state for that exited electronic state. It's typically not, in fact.

So you have http://www.bristol.ac.uk/synaptic/research/techniques/figures/fluorescence.gif" , where the excited molecule first loses its vibrational energy, "non-radiatively" by transferring it to other parts of the molecule, going to the vibrational ground state in the electronic excited state, before the electronic state decays, giving off a photon. (which thus has lower energy since part of it was lost as heat)

So visual light can heat something (but not very efficiently) though this process. Likewise, ultraviolet light can be turned into visible light by a UV-fluorescent compound.

 

[ZapperZ]

Read another thread on a similar topic:

https://www.physicsforums.com/showthread.php?t=414960

I would merge the two threads together, but the merge function messed up several things last time, so I won't.

Zz.

 

[JaWiB]

There's coupling between the two, especially from electronic to vibrational. Think about it this way: Each electronic state has a different bunch of associated vibrational states (Why different? Because if the electronic state is different the potential energy surface between the atoms is different). There's no reason an electronic excitation has to be to the vibrational ground state for that exited electronic state. It's typically not, in fact.

I'm not sure I understand. Electronic state simply refers to the configuration of the electrons around an atom/molecule? Are you saying that the vibrational states are a property of both the nuclei and electrons? (Can you even say that the electrons have their own vibrational energy or do you always treat the molecule as a whole?)

A related question: if you heat a material, do the bond strengths change? In other words, if I had a material at 100K and UV light didn't break the bonds, could I heat to to 200K and then suddenly find that the same wavelength of light would break the bonds?

 

[ZapperZ]

Please note that in most cases, people are asking about solid objects. As I've posted, you can no longer talk about individual atoms and molecules when there is a collective phenomenon involved.

Please read the link. Or go directly to the FAQ thread.

Zz.