Hello. In this video,
we're going to talk about Raman scattering. Raman scattering is an
inelastic process in contrast to Rayleigh
scattering which is elastic, and that's because it
results in a change in the quantum state of the
molecule being probed. It occurs because of modulation of the scattered radiation, the dipole radiation by
state to state transitions, and the polarizability is modulated as shown in the equation at the
bottom of the slide. If the process results
in an increase in molecular energy it's
called Stokes scattering, if it decreases the
molecular energy it's called anti-Stokes
scattering, and like Rayleigh scattering
its non-resonance but it does cause a
change in quantum states. These two diagrams
illustrate these processes. Start with the diagram on the right where you have two
different energy levels, say vibrational energy
levels in there, the spacing is shown there, and so Stokes scattering
is light that is scattered from the lower vibrational state, and then results in
light being absorbed as it were such that
you'll end up back in excited state and anti-Stokes scattering
is the opposite. I struggled with this a bit when I was first
learning about it, people were talking about it
in terms of use in my field. They showed these
dotted lines and they called them virtual states. That doesn't really
make sense to me, it is really just about
the modulation of the polarizability by the
presence of the molecule. So a better way of
looking at it is the diagram on the left
which is a spectrum, and the center of spectrum, this peaked line is
Rayleigh scattering, and then the two humps around the Rayleigh scattering
line are rotational Raman. In other words, the scattering is between rotational
levels and then the two humps further
out to the right and left are vibrational Raman, and that is exactly what you would see if
you took a spectrum of scattered light from a
gaseous medium of some kind. Again, the measurement setup is actually identical to
Rayleigh scattering. Laser goes in through some
medium, it's scattered, we collect the
scattered light with some optics onto a detector and we get an expression for the intensity somewhere to
that given in the slide. Here is an example of
vibrational Raman. Most people do Raman
scattering on nitrogen because the concentration of
nitrogen and combustion flows for example is
usually quite larger, it's usually the most
prevalent species in a combustion flow where you're burning air so that
makes the signal larger, and it doesn't change very
much and a lot is known about it and they want to use
it to measure temperature. The reason they can measure the temperatures is because you get the various
vibrational bands. You pick up the various
vibrational bands and by fitting the spectrum you
can recover a temperature. This is an example
of rotational Raman and I think both of these images, I'm not sure if they are
from the same paper but this rotational Raman images from the paper cited
that Mike Drake and Rosenblatt worked at General Motors research
laboratories in Michigan and were interested in
making measurements and engines which they subsequently did I believe and certainly
other people have done that. So with Raman scattering you can get a measure of
population distributions, you can recover
temperature, and so on. Here are some Raman
cross sections. The Raman cross sections for the various molecules
listed are relative to the nitrogen cross-section and the nitrogen cross section
is given at the bottom. It's a pretty big number
compared to Rayleigh scattering. The difference is that
you're scattering off the concentration
of some molecule rather than the entire gas, and so a Rayleigh signal will always dominate by quite
a bit the Raman signal. But nonetheless, Raman scattering
is a useful technique. So bad is it essentially
for this video I will just say that there are other spectroscopic
techniques. They get more complicated. These are the "simple
ones" four wave mixing, coherent, anti-Stokes, Raman
scattering, and so on. If you get into this field, you'll undoubtedly
learn about them. But this set of videos
has given you an idea about what spectroscopy
is all about. So thanks very much for
listening and have a great day.