Raman spectroscopy can tell us the vibrational modes of molecules or crystals. We can use Raman spectroscopy for chemical and structural characterization.
By interpreting Raman spectra, you can identify chemical substances and obtain structural information. Raman scattering is the result of the interaction between laser and molecular vibrations. This vibration has high sensitivity to changes in chemical composition and structure, so you can identify subtle differences in the molecular environment. Generally speaking, all substances will produce Raman spectra, except for pure metals.
What is Raman spectroscopy?
We display the Raman spectroscopy measurement results in graphical form, namely Raman spectroscopy. The y-axis represents the intensity of scattered light, and the x-axis represents the energy (frequency) of the laser. We focus on the frequency variation of Raman scattering light, so we plotted the x-axis frequency corresponding to the laser frequency. We label the x-axis as Raman frequency shift (in cm-1).
What information can I obtain through Raman spectroscopy?
In short, we utilize the following features of Raman spectroscopy:
Raman frequency shift and relative intensity of all Raman bands of the sample
We can use Raman spectroscopy fingerprinting to identify samples.
Changes in Raman spectra when changing direction or polarization
The intensity and position of Raman bands will vary with the relative direction of the sample. We can prove this by rotating the excitation laser and the polarization of the collected Raman scattering light. By using polarized Raman spectroscopy, you can reveal the symmetry and orientation of anisotropic materials.
Changes in a single spectral band
The spectral band may shift (position), narrow or widen (width), or undergo intensity (height) changes. These changes can respectively reveal information such as compression/tensile stress, changes in crystallinity, and total amount of substance in the sample.
The variation of spectra at different positions on the sample
This will reveal the non-uniformity of matter. You can analyze at any number of sample points or systematically measure a point array (capable of Raman imaging of composition, stress, crystallinity, etc.).
Raman spectroscopy consists of a series of spectral bands, each associated with a vibrational mode. The Raman spectrum of each substance is unique, so you can identify the substance through the spectrum. Some researchers aim to fully understand each Raman band and its relationship with vibrational modes. However, most analysts only use spectral databases to identify samples.
Main characteristics of Raman spectroscopy
Interpretation of Raman Spectroscopy
One way to understand Raman spectroscopy is to consider molecular functional groups as different units. For crystals with a regular lattice (identical atomic arrangement), their Raman spectra are easy to interpret because all atoms are in the same coordination. For example, carbon atoms in diamond are arranged in a regular tetrahedral network. In this case, we usually see a dominant Raman band because the crystal has only one molecular environment.
In contrast, the Raman spectrum of polystyrene is much more complex. Because of its low molecular symmetry and the presence of hydrogen atoms in addition to carbon atoms. In addition, there are various bonds that connect atoms.
Raman spectra of diamond (single band only) and polystyrene (multi band)
Raman spectra of diamond and polystyrene. Due to different bond types, the Raman spectra of polystyrene are much more complex than those of diamond.
The unique vibration frequency of chemical bonds
The vibration frequency depends on the mass of the relevant atoms and the tension of the bonds between them. Heavy atoms and weak bonds have lower Raman frequency shifts. Light atoms and strong bonds have higher Raman frequency shifts.
In the polystyrene spectrum, we observe high-frequency carbon hydrogen (C-H) vibrations at approximately 3000 cm-1. Low frequency carbon carbon (C-C) vibration occurs at approximately 800 cm-1. The higher frequency of C-H vibration compared to C-C vibration is because hydrogen is lighter than carbon.
Similarly, we can see that two carbon atoms connected by a strong double bond (C=C) undergo vibration at approximately 1600 cm-1. This vibration frequency is higher than the two carbon atoms connected by weaker single bonds (C-C, 800 cm-1).
You can use these simple rules to explain many features of Raman spectroscopy.
Raman shift is sensitive to adjacent bonds
If you observe the Raman spectrum carefully, you can see more subtle effects. For example, the C-H vibration of polystyrene appears in two spectral bands at approximately 2900 cm-1 and 3050 cm-1. The carbon atoms in the former band are part of the aliphatic carbon chain, while the carbon atoms in the latter band are part of the aromatic carbon ring.
You can think of the vibrational part of a complex molecule as composed of many simple diatomic vibrations. However, you should also consider the vibrations of larger atomic clusters to fully understand Raman spectroscopy. For example, the Raman spectrum of polystyrene has a band at 1000 cm-1. This is due to the stretching/shrinking "breathing mode" of aromatic carbon rings in polystyrene.
Low frequency Raman band
You can also study the molecular vibrations and rotational modes of low-frequency Raman shifts below 100 cm-1. These Raman bands originate from very heavy atoms or vibrations on a very large scale, such as whole lattice vibrations. Renishaw's Raman instrument can be used to study these modes. You can study various substances and crystals, easily distinguish different crystal forms (polymorphs) and layered structures.
How does Raman spectroscopy identify substances?
In general, we can use software to search spectral databases and identify unknown substances using their unique Raman spectral fingerprints. We use Raman bands in the fingerprint region (from 300 cm-1 to 1900 cm-1) to identify molecules.
Ideally, you can use a high spectral resolution Raman instrument that covers the entire Raman spectrum range. It has higher chemical specificity. You can identify, distinguish, and study more types of substances.
Display Raman spectra of certain substances in suspected counterfeit erectile dysfunction pills. Display Raman spectra of certain chemical substances in suspected counterfeit pills. By searching Renishaw's library of inorganic and mineral spectra, we identified the red spectrum as CaSO4.
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