Geos 306, Lecture 12
- Spectroscopy is the study of the interaction between matter and radiated energy, especially light.
In particular, the electric component of the electromagnetic light interacts with electrons in a crystal,
and depending upon the structure of the crystal, responds with a new signal that we can measure and interpret.
X-ray diffraction is one of those interactions where the crystal produces a new signal
that has the same wavelength as the incident light; known as elastic scattering.
In contrast, Raman spectroscopy occurs during an interaction where the incident light has a different wavelength than the dispersed light;
known as inelastic scattering.
As part of your class project, you will be given the opportunity to conduct a Raman spectroscopic experiment
that might help you identify your unknown sample.
- The electromagnetic spectrum:
- The spectrum/sensitivity that our eyes can see:
- Electromagnetic radiation is a transverse wave, composed
of two parts, an electric and a magnetic component, oriented 90° from each other.
Each wave might have its polarization
in a different direction.
The velocity of all electromagnetic waves, c, is the same in a vacuum; its most recent measurement is 299,792.458 km/sec
or ~186,000 mi./sec .
Its speed always decreases when travelling through matter. The speed of light in a transparent material is c/n, where n = the refractive index of the material.
The speed of light in diamond where n = 2.43, for instance, is = 77,000 mi/sec.
- When light travels into a crystal, it causes the electrons in the crystal to oscillate, like the water molecules in ocean waves.
This oscillation couples with the oscillations of the atoms themselves in the crystal, already vibrating to and fro primarily due to heat
or zero-point quantum energy. Like the oscillations of a cork on the ocean, or maybe immersed seaweed.
The amplitudes of the atomic vibrations are illustrated in the figure below for quartz,
showing its atomic displacement ellipsoids, or, in statistics terms, probability distribution functions.
The ellipsoids in the figure below were drawn to enclose 99% of the volume of space occupied by the vibrating atom.
The coupling of the two sets of motion comes with an energy shift, similar to the Doppler effect,
where the wavelength of the incident light gets shifted by the energy of the atomic oscillations.
- The Raman spectrum is an emission of electromagnetic waves shifted in energy from the incident radiation.
As long as the incident light is more energetic than the energy required to induce vibrations,
there is an observable Raman spectrum.
- Here is an example of a typical Raman spectrum collected on a topaz crystal.
The x-axis is a measure of the frequency of the light emitted during the inelastic scattering, and the y-axis is the intensity recorded by the detectors.
The zero point on the x-axis is towards the left and is not shown.
Remember that the position of the Raman peaks represent the change in energy of the incident light,
so at zero we would see almost the full intensity of the incident laser light.
This has been removed from the signal by a filter in the spectrometer. Otherwise, this signal would drown the Raman signals
because the Raman signals are at least one million times weaker in intensity.
There are a certain number of expected peaks for a given crystal structure, generally correlated to the number of unique bond pairs.
The intensity of each peak depends on the distortion of the electron density in the crystal due to that vibration.
- You can consider each peak in the spectrum to represent the energy shift of the incident light due to the coupled motion of pairs or groups of atoms.
The energy shift is greater with increasing x, on the right of the graph. The stronger bonds are also towards the right.
This link shows an example of the sort of
coupled motions that lead to distinct Raman peaks in calcite.
- The figure below shows a plot of the frequencies of the asymmetric modes of a number of different
anionic groups in minerals versus the bond lengths of the atoms that are involved. The larger frequency shifts are associated with the shortest bonds,
again illustrating the principle that the stronger bonds are short and longer bonds are weak.
The Raman Experiment
- The Raman spectrometer that you will use in class is illustrated below.
- However, it is not very useful for illustrating the Raman setup,
so the image below represents a more open system that shows what is going on in the experiment.
- In your class project, you will place your unknown material in the Raman instrument, focus the microscope
on the mineral and then collect a spectrum. Your TA will help with the details.
After a few seconds of collecting, the spectrum will become sharp and you can search the database for a match.
Our library can only be used to identify a mineral that we have already examined.
We cannot identify a mineral whose spectrum is not already in the database.
The database that we use is being built by the
It is anticipated that someday there will be handheld Raman instruments, like the tricorder from StarTrek, or the one in the image below.
Wenk and Bulakh, chapter 12 Spectroscopic techniques