Infrared Laser Excitation to Study Transitions in Fullerene (C60) Molecule

Raman spectroscopy is the study of inelastic scattering of light. A Raman active sample should show a change in polarizability. It provides amongst other things, important information about the vibrational state of matter. Raman studies in scattering entails a sum of both creation and annihilation of photons and law of energy conservation applies to only the overall process and not to individual events. Infrared laser beam is a good choice for studying absorption and emission characteristics of fullerene molecule. The photo physical, photochemical and optical properties have already been studied in detail by many scientists. We intend to study the infrared laser induced characteristics of fullerene molecule. It is expected that laser interaction with C60 molecule can break the carbon bond, creating more free electrons resulting in formation of new compounds. Citation: Subramaniam TY, Premanand P (2018) Infrared Laser Excitation to Study Transitions in Fullerene (C60) Molecule. J Laser Opt Photonics 5: 189. doi: 10.4172/2469-410X.1000189


Introduction
The fullerene molecule C 60 was discovered by Kroto, et al. [1]. C 60 has much weaker inter-molecular forces than diamond, another allotrope of carbon. It has a much lower melting point and boiling point than diamond. Therefore, less energy is required to break the forces of attraction. Raman spectroscopy was named in the honour of its discoverer Sir. Raman CV, who along with Krishnan KS published the first paper on this technique. Raman spectroscopy (RS) is a versatile method for analysis of a wide range of samples. It resolves most of the limitations of other spectroscopic techniques. Qualitative analysis can be performed by measuring the frequency of scattered radiation while quantitative analysis can be performed by measuring the intensity of the scattered radiation. Raman spectroscopy is basically a light scattering technique. The frequency of a small fraction of the scattered radiation is found to be different from the frequency of a monochromatic incident radiation [2]. It probes molecular vibrations. After the excitation, for the total energy of the system to remain constant even after the molecule moves to a new rovibronic state, the scattered photon shifts to a different energy, and therefore a different frequency can be seen [3] (Figure 1).

Experiment
In our Raman spectroscopy experiment, the sample C 60 is illuminated with a monochromatic laser, (Nd: YAG laser beam with a wavelength of λ=1.064 µm) which interacts with the C 60 molecules and generated a scattered radiation. Thus, the scattered light, having a wavelength different from that of the incident wavelength is used to construct a Raman spectrum. This type of scattering (inelastic) arises due to the inelastic collisions between the incident monochromatic radiation and the molecules of the C 60 .We has given the sample containing C 60 , pure (99.99%), research grade, nano particles obtained from M/s. Merck and Co. (formerly Sigma-Aldrich) for analysis with an instrument named Bruker RFS27, at the Sophisticated Analysis Instrumentation Facility (SAIF), Indian Institute of Technology (IIT-M), Chennai. The sample analysis was performed from the wave number range of 4000 cm -1 to 400 cm -1 . Raman spectrophotometers can be either a dispersive or a non-dispersive unit. In a dispersive unit, a prism or a grating will be used for analysis. In a non-dispersive unit, a Michelson interferometer is used. In FT-Raman, an interferometer is used for analysis.

Analysis
The spectrum recorded is shown here below, in Figure 2 below. In the above spectrum, the peaks seen at 1467.30 cm -1 ,493.96 cm -1 ,269.36 cm -1 , are due to resonance Raman lines (Q branch), which goes to highest values recorded in any spectrum. These could not be attributed to Rayleigh scattering, since the recorded spectrum itself is about Raman intensity only and not recorded in any arbitrary unit (a.u).The peaks recorded at 1573.59 cm -1 , 2841.02 cm -1 , 1420.51 cm -1 , 1264.94 cm -1 , 1099.23 cm -1 ,771.25 cm -1 ,566.95 cm -1 , and at 431.15 cm -1 are all due to either Stokes shift or due to anti-Stokes shift lines. The resonance Raman lines (RR) is a name given to a laser frequency which is close in energy to an electronic transition of a compound or a crystal under examination. This frequency can lead to a greatly enhanced intensity of the Raman scattering, which facilitates the study of compounds present at low concentrations [4]. In a Raman spectrum, Q branch will be present and the same will be absent in an infrared spectrum. The O and the S branches are similar to the R and P branches in an infrared spectrum, as shown in Figure 3  It is not necessary that all the Raman lines will have their corresponding infrared bands and similarly, all infrared bands may not have their corresponding Raman lines. Generally, Raman lines are symmetrically displaced about the parent lines. When the temperature increases, their individual separations from the parent lines decrease. In order to find more anti-Stokes lines that are present in the spectrum Herzberg has contributed enormously to the study of molecules using different types of radiations present in the electromagnetic spectrum. He is regarded as the Father of Molecular Spectroscopy. The anti-Stokes line is much less intense than the Stokes line. This occurs because only molecules that are vibrationally excited prior to irradiation can give rise to the anti-Stokes line. Hence, in Raman spectroscopy, only the more intense Stokes line is normally measured. Hence, from the recorded spectrum above, we find that the lines having the energies at 1573.59 cm -1 , 771.25 cm -1 , 431.15 cm -1 are attributed to the Stokes shift and the    that we have recorded, we took an expanded plot between 3000 cm -1 to 2500 cm -1 region. The recorded spectrum is shown here below (Figure 4).
Whereas only one line could be seen between 3000 cm -1 to 2500 cm -1 in the spectrum as shown in Figure 4 above, we could see that from the Figure 5 above, there is more anti-Stokes lines present due to rovibronic transitions of the C 60 molecule.
The intensities shown in the Y-axis is evident that these lines could not surface in the earlier spectrum (Figure 3), because they were weak in intensity. This leads us to conclude that, on a laser excitation, the C 60 molecule forms many smaller compounds from C 60 itself, with a lower concentration.
The rovibronic transitions are mainly due to the different stretching and bending modes that are present due to laser excitation of the C 60  molecule. The number of normal modes of vibration is 3N-6, where N is the number of atoms in a molecule. Therefore, the total number of modes present is 3 X 60 -6=180-6=174 modes. The normal modes are due to symmetric stretching, asymmetric stretching and bending modes. The equation for an harmonic oscillator is used to describe the energy of various levels in a molecule. The levels are unequally placed with the spacing between the energy levels decreasing, as it goes towards higher energies.
Here, x e and y e are the an harmonic constants. The negative sign is placed in front of the second term for convenience so that all values of x e and y e are positive [5].
From the Figure 3, the lines due to 2841.02 cm -1 , 1573.59 cm -1 , 1467.30 cm -1 are attributed to the C-H, C=O, C=N, C=C stretching bonds.
From the Figure 5, we note that, the lines due to 2934 cm -1 , 2878.67 cm -1 , 2816.38 cm -1 , are due to C-H stretching bond.
We also recorded a Raman spectrum in the famous "finger print" region (1500 cm -1 to 100 cm -1 ), to see the presence of Stokes and anti-Stokes shifts. The spectrum recorded is in the region between 1000 cm -1 to 100 cm -1 , shown below ( Figure 5).

Conclusion
FT-Raman scattering result using an Nd: YAG laser excitation for the fullerene (C 60 ) molecule were studied and new vibrational transitions and formation of new compounds established from the recorded spectrum. Several lines are newly reported for the first time here. The new intense, or the not-so-intense lines are attributed to Stokes and anti-Stokes shifts respectively.