In a Raman spectroscopy experiment, a sample is irradiated with a given wavelength of light and the intensity and energy of the inelastically scattered radiation are measured. The energy difference between the incident radiation and scattered radiation gives information about the energy levels in the system.
Image Credit: lucadp/Shutterstock.com
Most Raman spectroscopy is performed with visible or UV radiation sources, and it is the vibrational energy levels of the system that are probed. Most vibrational energy levels have spacing between 50 to 4000 cm-1 and absorb light in the infrared region.
When using Raman spectroscopy to probe vibrational transitions, the energy shift between the incident and scattered is in the typical infrared energy range.
However, while probing the vibrational structure of a molecule is the most common application of Raman spectroscopy, it can also be used to probe the rotational or electronic structure of a molecule.1,2
Raman spectroscopy is usually used to investigate the vibrational structure of a molecule or material, as the energies and intensities of the vibrational transitions provide a unique ‘fingerprint’ of the molecular species. This fingerprint can then be used to identify and quantify the species of interest.
Probing electronic or rotational states provides different information on the sample than the vibrational fingerprint. The electronic structure or material governs many of its optical properties and how the sample will interact with light.
In many research areas, such as the development of new materials for optoelectronics, there has been a great deal of interest in probing the relationship between modifications in the electronic structure on how this influences the optical properties with a view to designing more efficient materials for a given application.3
Investigating these properties means using spectroscopies that can probe different electronic transitions in the sample of interest.
Absorption-based spectroscopies, such as UV-vis, are commonly used to probe the electronic structure of molecules and materials. The intensity of the transition is governed by the overlap integrals of the initial and final states, meaning some states have a powerful and intense absorption and are described as ‘optically bright’, whereas others are optically dark.
As Raman spectroscopy uses different selection rules that depend on the polarizability of a given transition, electronic Raman spectroscopy can be used to probe states that would be dark in absorption-based methods.4 Therefore, electronic Raman can provide a highly complementary technique to be used in conjunction with absorption spectroscopies.
For materials science, where the electronic transitions are often more closely spaced than for molecular species, Raman spectroscopy is often used to probe the density of the electronic states.5 The energetic shifts of the phonon modes can be used to work out the electron concentrations, which can be a way of working out the degree of doping in a material.
One of the advantages of using Raman spectroscopy for profiling materials is its non-destructive technique. Raman spectroscopy does not require any additional sample preparation. Variations of the method, such as surface-enhanced Raman spectroscopy (SERS) can also be used to increase the signal levels in the Raman experiment that can be problematically low for many types of samples.5
X-ray Raman Scattering
One development that has a significant impact on the types of electronic Raman experiments that are possible is the advances in X-ray free-electron laser technologies.
While there are still a limited number of X-ray free-electron lasers worldwide, the current sources have made it possible to generate ultra-intense X-ray pulses that have enabled several new non-linear spectroscopies, including Raman techniques for probing electronic states.6
One method is X-ray Raman scattering, which, while still an experimentally challenging measurement to perform, is capable of performing element-selective probing of both occupied and unoccupied electronic energy levels.7 The ability to probe both occupied and unoccupied states simultaneously makes stimulated X-ray Raman spectroscopy a very information rich-method that can provide information on valence electronic energy levels without suffering from many of the issues that valence spectroscopic methods, such as UV-vis absorption do.
The inherent element selectivity of X-ray spectroscopies also means that X-ray Raman methods are very well-suited to probing complex multi-element species. For example, many metalloproteins have thousands of atoms, and it becomes nearly impossible to resolve the individual electronic transitions in their valence spectra as the resulting bands are broad and featureless.
However, in an X-ray experiment that can selectively probe the metal site, it is possible to recover much more selective, detailed information on the electronic structure of the system with signals that only arise from a single element – in many cases, the metal center of the protein would be the target of choice.
While a challenging experiment, with free-electron laser sources moving towards higher peak brightness and repetition rates, X-ray Raman may soon become a more widely used method for probing the electronic structure of a sample with Raman spectroscopy.
References and Further Reading
- Rouzée, A., Boudon, V., Lavorel, B., Faucher, O., & Raballand, W. (2005). Rotational Raman spectroscopy of ethylene using a femtosecond time-resolved pump-probe technique. Journal of Chemical Physics, 123(15). https://doi.org/10.1063/1.2069866
- Tanaka, S., & Mukamel, S. (2002). Coherent X-Ray Raman Spectroscopy: A Non-linear Local Probe for Electronic Excitations. Physical Review Letters, 89(4), 4–7. https://doi.org/10.1103/PhysRevLett.89.043001
- Moliton, A., & Hiorns, R. C. (2004). Review of electronic and optical properties of semiconducting π-conjugated polymers: Applications in optoelectronics. Polymer International, 53(10), 1397–1412. https://doi.org/10.1002/pi.1587
- Koningstein, J. A., & Grunberg, P. (1971). Electronic Raman Spectra. VII. Raman Spectra of the Lanthanides. Canadian Journal of Chemistry, 49(13), 2336–2344. https://doi.org/10.1139/v71-376
- Cantarero, A. (2015). Raman Scattering Applied to Materials Science. Procedia Materials Science, 9, 113–122. https://doi.org/10.1016/j.mspro.2015.04.014
- Neyman, P. J., Colson, W. B., Gottshalk, S. C., Todd, A. M. M., Blau, J., & Cohn, K. (2017). Free electron lasers in 2017. Proceedings of the 38th International Free-Electron Laser Conference, FEL 2017, 204–209. https://doi.org/10.18429/JACoW-FEL2017-MOP066
- Rohringer, N. (2019). X-ray Raman scattering: a building block for non-linear spectroscopy. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 377, 20170471.