[Compaq]

Every time someone talks about IR vs Raman, I get a little confused. Because, what is really the difference between the two techniques?. The transitions follow different selection rules, so clearly there is something fundamentally different between the transitions. However, both involve absorbing electromagnetic radiation to excite a molecule into an excited vibrational level.

My understanding right now is this: The fundamental difference between IR and Raman spectroscopy is that the molecules are excited into higher vibrational modes in different ways - even though both ways involve electromagnetic radiation. For a vibrational mode to absorb radiation, either **a)** the molecule's dipole moment needs to change during the vibration, or **b)** the molecule's polarizability needs to change during the vibration. We call the first situation IR spectroscopy, while we call the second situation Raman spectroscopy.

But here's another thing. For IR measurements we use infrared radiation, while we usually use visible light for Raman measurements. During (normal) Raman measurements, we excite to some virtual vibrational state, while during IR absorption measurements, we do not excite to virtual states. Considering that we use higher energy light during Raman measurements, this makes sense, but is this a necessary condition for it to be called Raman? Could we do Raman with IR lasers? In the Rayleigh equation, we see that the intensity of the Raman signal is proportional to the inverse of the wavelength to the 4th power, and think a Professor once said that this is the reason why we use lower wavelengths: to obtain much more Raman signal. So the fact that we excite to virtual states is just a consequence of us using lower wavelengths to get a higher Raman signal?

I'm sorry for a long and perhaps confusing question, but I have been thinking about this for some time now, and I thought I's just ask.

[mjc123]

Basically, you've got the key difference - in IR the radiation interacts with the molecule via the dipole moment, in Raman via the polarisability. It's not quite right to say that "both involve absorbing electromagnetic radiation to excite a molecule into an excited vibrational level". IR involves absorption; Raman involves scattering, with a change in energy of the scattered photon. This can be regarded as absortion of one photon (to the "virtual" state) and emission of another of different energy. Raman is thus a 2-photon process, while IR is 1-photon. This affects e.g. the change in angular momentum, which is why the rotational selection rules are different. Because the dipole moment and the polarisability tensor have different symmetries, IR and Raman often excite different vibrational modes of a molecule, which makes them useful complementary techniques for structural characterisation.

Because the scattered light loses energy equal to the molecular transition energy, it makes no sense to use (mid)IR radiation to look for vibrational transitions. (You can use near IR lasers, like CO₂.) The λ^-4 law is why we use visible lasers - along, of course, with the ready availability of such lasers. Another reason is that (surprise surprise) IR radiation is absorbed by vibrationally active species; using visible light can avoid the problem of absorption. Thus you can do Raman on aqueous solutions in glass vials, where IR is difficult or impossible due to the strong absorption of IR by glass and water. (Strongly coloured samples can give problems with absorption of visible laser light, this is where near-IR may be useful.)

Edit: I meant near-IR, not far-IR! Wavelengths ca. 1000 nm.

[Compaq]

Okay, I see now that my confusion is largely because I am not confident in the fundamental differences between absorption, scattering (or reflections, for that matter). I am just a mere chemist, and sometimes I feel like we should have been taught more physics during our education. Whenever you want to go into real detail in some "chemical" area, I and up lacking in my physics (or maths) background.

[P]

He's right - absorption vs scattering  -  the number of scatters per incident light is much lower (orders of magnitude) than the number of incidences of absorption in the IR spectroscopy too...  i.e. the signal is much weaker.

The Raman signal can be enhanced using silver, gold or platinum molecular islands, which use surface phonon resonances to enhance the Raman spectrum or to observe Raman spectra that are usually too weak.