Last week I started reading about Raman spectroscopy and also reviewed a bit of IR. The main idea that I took away from the information is that the Raman effect is all about virtual excited states. These states are the result of light hitting a molecule with an amount of energy that isn’t quite enough to promote it to a true excited electronic state but is still enough to change its vibrational state. This virtual excitation happens very quickly, leaving most of the light to re-emit at the same energy level as it went in at (good ole Rayleigh scattering). The amount that doesn’t re-emit at the same energy level, our friendly non-Rayleigh scattering, is what we care about instead.
The re-emitted light that differs from the incident radiation emerges in a way that can be used to identify the sample. This is because the energy gaps correspond to the molecule’s vibrational modes, which are as unique as a fingerprint. Most of the light re-emits at a lower energy level than the incident radiation due to vibrational promotion of the molecule, with these losses being visualized as Stokes lines. The rest of the re-emitted light emerges at a higher energy level, but this happens at a lower frequency because it is much less likely that the molecule will be demoted to the ground state. These gains are visualized as anti-Stokes lines, and are less intense.
All of this Raman business is different from IR spectroscopy because you don’t rely on permanent dipoles. Instead, Raman-active molecules experience changes in polarizability during vibrations. This basically means that Raman-active molecules contain bonds between elements whose electron densities can be altered based on the distortion that vibrational extremes cause. Therefore, Raman and IR can be used together to help figure out what’s going on.
Alright, so this has all been pretty new to me and I have so much more to learn. The rest of the term is going to be spent on Raman and on figuring out the confocal, but in the meantime Allison, Bobby and I picked out the cutest of the baby cats in the world:
Say hi to Carnot!!! (orange and white)