Raman Spectroscopy

Raman spectroscopy is an inelastic scattering technique that uses monochromatic light to study rotational, vibrational and other low frequency modes in a system.124 _{The technique} was discovered in 1928 by C.V. Raman who used a telescope and a specific arrangement of lenses to converge and focus the sunlight to a liquid sample.125 A system of optical filters was used to show the presence of scattered radiation with altered frequency.

In Raman scattering a sample is illuminated by laser light which distorts electron clouds to form short-lived states known as virtual states. Now if only the deformation of electron clouds occurs then owing to the small mass of electrons photons will be scattered with very small change of frequency (or a no change at all) – Rayleigh scattering.126 However, if a change in molecular polarization potential takes place then energy will be either transferred from photon to the molecule or vice versa. This leads to inelastic scattering of light – Raman scattering.124-126 Raman scattering is a very weak process and the intensity of a strong Raman signal is only about one thousandth of the intensity of Rayleigh scattering.124 The basic process is shown in Fig. 2.10.

Figure 2.10 Different light scattering techniques. m n Vibrational states Virtual states Stokes scattering Rayleigh scattering Anti-stokes scattering

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The Raman effect can be explained by a simple classical treatment as follows:126 Consider a photon beam with frequency ߥ଴ accompanied by an oscillating electric field E:

ܧ ൌ ܧ଴ܿ݋ݏʹߨߥ଴ݐ (2.3)

which interacts with an electron cloud to induce a dipole

ܲ ൌ ߙܧ (2.4)

where α is the polarizability of the molecule. An interaction between polarizability and normal vibrational modes (Q) will lead to the Raman effect. For a non-linear molecule with N atoms there will be 3N-6 vibrational modes. Normal modes are defined as:

ܳ ൌ ܳ௝଴ܿ݋ݏʹߨߥ௝ݐ (2.5)

where νj is the harmonic frequency of the jth normal mode.

For a vibrating molecule, the polarizability term is given by the following Taylor expansion

ߙ ൌ ߙ଴൅ଵ_ଶ൤_డொడఈ

ೕ൨ ൅ ܳ௝൅ ڮ (2.6)

Substituting (2.3), (2.5) and (2.6) in (2.4) will get to

ܲ ൌ ߙ଴ܧ଴ܿ݋ݏʹߨߥ଴ݐ ൅ଵ_ଶ൤_డொడఈ

ೕ൨ ܧ଴ܳ௝

଴_{ൣܿ݋ݏ൛ʹߨ൫ߥ}

଴൅ ߥ௝൯ൟ ൅ ܿ݋ݏ൛ʹߨ൫ߥ଴െ ߥ௝൯ݐൟ൧ (2.7)

The first term in the above equation represents Rayleigh scattering and it has same frequency as the incident radiation. The second ൫ߥ଴൅ ߥ௝൯ and third ൫ߥ଴െ ߥ௝൯ terms

represents anti-stokes and stokes Raman scattering respectively.126 Stokes scattering occurs when the energy absorbed by a molecule from its ground vibrational state m lifts it to the higher energy state n. The photon emitted by the molecule on its descent from the excited state will therefore have less energy than the absorbed one. Due to thermal agitation, some molecules already reside in the excited state n and scattering from this state to the lower energy state m is known as anti-stokes scattering. In this case the emitted photon will have a higher energy than the absorbed photon.127 _{The intensities of stokes and anti-stokes scattering}

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differ but depend on the initial state of the molecule. In case of thermal equilibrium where most of the molecules are at low energy levels stokes scattering will be dominant.

Whenever there is an interaction between matter and energy there are certain transitions between atomic or molecular level that take place. The possibility of occurring any transition depends upon physical structure of the molecule and many other properties. These transitions are governed by selection rules and each spectrum has its own selection rule. A specific transition will be Raman active only if there is a change in polarizability of molecule upon interaction with light while a transition will be Infrared active if there is a change in dipole moment of the molecule upon interaction with light.128

Figure 2.11 The LabRam instrument at Victoria University, Wellington was used for solid- state Raman measurements.

Raman spectroscopy can be applied to investigate the bulk structure of hybrid materials and can provide very useful information about the chemical and physical properties of the reaction components and products. In the past, Raman spectroscopy has been used to identify the purity of transition metal oxides as well as to determine the influence of incorporating ligands, with different lengths and bonding preferances, on overall structure of the hybrid organic-inorganic frameworks.74 The effect of doping hybrid materials with metal atoms can also be investigated by Raman measurements where a difference in the Raman peak shape and/or new phonon-modes in the spectra of intercalated materials can appear as a result of doping.91,129 _{A literature review suggests} that Raman spectroscopy should provide information about the following three aspects of ligand-metal interaction within the hybrid framework:74,126,127,130-136 (1) the presence of organic ligands in the structure, (2) the extent of structural modifications that occur when different ligands are incorporated in similar frameworks, (3) the existence of different

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coordination enviornments offered by inorganic molecules for ligand attachment.