Energy Acceptance and Energy Delivery method

[dttom]

It has been known that a molecule would absorb or not absorb an energy given to it depending on the method of energy delivery. For example, a molecule may not absorb a photon with an IR-range wavelength compatible to a vibration mode which is not IR-active, hence no IR peak at that wavelength would be observed. Yet, if the method of energy delivery is not through IR but heat, that molecule would absorb the energy given in form of heat and vibrate.

 

My confusion is, both methods confer the same energy, one in form of EM wave, another in form of heat, why the molecule absorb one but not the other? I am not sure if it is about the energy quality of the given energy form (EM probably has a higher quality than heat, so in this sense the molecule absorbs a lower quality form of energy, which personally I think is quite weird...).

[Mr Skeptic]

Heat is transferred in the form of "hot" molecules with higher average kinetic energy than the "cold" molecules, bumping into them and transferring some kinetic energy to them. However, it is an average, and individual molecules can have any of a range of kinetic energies. Also, molecules have more momentum so they can transfer energy via translational motion as well as rotation and vibration.

[dttom]

So you mean there is a range of energy in heat energy transfer so virtually it could take any magnitude of energy so as to excite the molecule to different motion like translation and vibration?

 

I understand that heat could transfer a range of energy but a photon in EM could only take a certain quantized energy. But in the example, the molecule was both given heat and EM (certain frequency). In case of heat given, certainly there were circumstances when the molecule was bumped by another molecule and transfered just enough energy for the molecule to vibrate in a certain manner. In the case of EM, though the frequency matches the vibrational frequency, the molecule just did not absorb the photon energy as it is IR inactive. It is actually the fact that the molecule was treated with the same magnitude of energy but in one case it took it yet in another case it just missed it which confuses me.

[Mr Skeptic]

The vibrational energy is quantized; the translational energy is not. A photon on its own does not have the momentum to change translational motion.

[dttom]

Maybe I put the question in a rather unclear way, let's clarify the matter.

So I think the vibrational energy is quantized in this way, where the separation is constant in most of the cases.

 

If assuming the ground state is at the tip of the well, and vibrational normal mode 1 is IR active, and now an IR-range photon with an energy hv = energy difference between vibrational normal mode 1 and the tip is given to the molecule, the molecule just does not absorb it. This seems to be quite a strange phenomenon to me.

 

Now heat instead is given, there is another molecule colliding with that molecule mentioned, energy equal to the energy difference between mode 1 and tip is transferred, in this case the molecule absorbs the energy and vibrates with normal mode 1. This contrasts with the event when IR is given.