[jkmunk]

Dear Forum,

I am wondering what happens to the energy absorbed in spectroscopy. I do know something about this, so please allow me to begin by describing what I know, which is mainly fluorescence.

A fluorescent molecule absorbs light at some wavelength in its absorption spectrum. The energy in the light excites the molecule to a higher state. Some of the energy dissipates as heat in a process known as relaxation, while the rest gets emitted as fluorescence at a wavelength longer than the wavelength absorbed, which was used to excite the molecule. The energy represented by relaxation and fluorescence emission (and possibly other intramolecular processes) exactly match the energy of the exciting light. So the energy budget balances perfectly.

So, the relaxational energy - does it get emitted as heat, i.e. infrared light?

To continue:

In an IR spectroscopy experiment, IR is absorbed, causing the molecule's bonds to vibrate. How does this energy dissipate? Does it also leave the molecule as heat, i.e. infrared light? If so I think, intuitively, that the IR emitted should have the same wavelength as that absorbed, in order to keep the energy budget balanced. If that's correct, the molecule really "just" absorbs the IR and then emits identical IR, albeit probably in different directions. So the molecule causes IR scattering!

Is this a correct understanding? Or does the energy leave the molecule in other ways? If so, how?

I am looking forward to any replies, and I will be happy to clarify if necessary.

Thank you in advance.

/Jens

[mjc123]

Most of the energy is dissipated by collisions between molecules, resulting in the energy being equilibrated among all the energy levels of the sample - translational, vibrational, rotational etc. - in accordance with Boltzmann's distribution. So if the sample absorbs IR radiation (at specific frequencies) it will heat up (maybe only a little), and it will emit broadband IR radiation (which it is doing at room temperature anyway; the increase will usually be relatively small). So no, it's not IR scattering (though that happens too, cf. Raman spectroscopy).

It is better to say that the energy absorbed leaves the sample as heat (which may be transferred to the surroundings by conduction and convection, as well as radiation), not that it leaves the molecule as heat. Heat is essentially a bulk concept; it doesn't have much meaning applied to individual molecules.

[Irlanur]

heat, i.e. infrared light

I have the feeling that you think heat and IR light are the same. That's not really true.