RAMAN SPECTROSCOPY

HISTORY

Light scattered by molecules is shifted in frequency by diffrences in the vibrational energy levels -Stokes line -Anti Stokes line RAMAN EFFECT

What is Raman spectroscopy? Raman Spectroscopy is a non-destructive chemical analysis technique which provides detailed information about chemical structure, phase and polymorphy, crystallinity and molecular interactions. It is based upon the interaction of light with the chemical bonds within a material. Raman is a light scattering technique, whereby a molecule scatters incident light from a high intensity laser light source. Most of the scattered light is at the same wavelength (or colour) as the laser source and does not provide useful information – this is called Rayleigh Scatter. However a small amount of light (typically %) is scattered at different wavelengths (or colours), which depend on the chemical structure of the analyte – this is called Raman Scatter.

RAYLEIGH AND RAMAN  Rayleigh scattering: occurs when incident EM radiation induces an oscillating dipole in a molecule, which is re-radiated at the same frequency  Raman scattering: occurs when monochromatic light is scattered by a molecule, and the scattered light has been weakly modulated by the characteristic frequencies of the molecule –STOKES LINE –ANTI-STOKES LINE

Stokes and anti-Stokes Raman spectral lines of CCl 4

Incident radiation excites “virtual states” (distorted or polarized states) that persists for a shorter time (10^- 14 secs) Inelastic scattering of a photon when it is incident on the electrons in a molecule STOKES LINE When inelastically-scattered, the photon loses some of its energy to the molecule (Stokes process). It can then be experimentally detected as a lower-energy scattered photon ANTI STOKES LINE The photon can also gain energy from the molecule (anti-Stokes process)

POLARISABILITY the “deformability” of a bond or a molecule in response to an applied electric field. It is the most important selection rule for Raman spectrum Intensity of the Raman peak depends on Polarisabilty of each molecules of a substance

INDUCED ELECTRIC DIPOLE MOMENT An electric field can distort the electron cloud of a molecule, thereby creating an “induced” electric dipole moment The oscillating electric field associated with EM radiation will therefore create an oscillating induced electric dipole moment which in turn will emit, i.e. scatter, EM radiation

A linear molecule of N atoms has (3 translational deg. freedom+ 2 rotational ) 3N-5 normal modes of vibration Non-linear molecule: 3N-6 modes\ Normal modes: Stretching motion between two bonded atoms Bending motion between three atom connected by two bonds Out-of-plane deformation modes Vibration of molecules

VIBRATIONAL MODES SYMMETRICAL ASYMMETRICAL WAGGING TWISTING SCISSORING ROCKING

 Vibrational modes that are more polarizable are more Raman-active  Examples: –N 2 (dinitrogen) symmetric stretch  cause no change in dipole (IR-inactive)  cause a change in the polarizability of the bond – as the bond gets longer it is more easily deformed (Raman-active) –CO 2 asymmetric stretch  cause a change in dipole (IR-active)  Polarizability change of one C=O bond lengthening is cancelled by the shortening of the other – no net polarizability (Raman- inactive) RAMAN ACTIVE VIBRATIONAL MODES

SOURCE SELECTORS AND FILTERS DETECTORS RAMAN INSTRUMENTATION

Lasers Laser wavelengths ranging from ultra-violet through visible to near infra-red can be used. Ultra-violet: 244 nm, 257 nm, 325 nm, 364 nm Visible: 457 nm, 473 nm, 488 nm, 514 nm, 532 nm, 633 nm, 660 nm Near infra-red: 785 nm, 830 nm, 980 nm, 1064 nm

LASERS Gas lasers Solid-state lasers Semiconductor lasers Other: Dye laser (tunable) Metal-vapor laser (deep UV) Ti: Sapphire (solid-state tunable)

SOLID STATE LASER

SAMPLING SYSTEMS

RAYLEIGH LINE REJECTION Optical filters these optical components are placed in the Raman beam path, and are used to selectively block the laser line (Rayleigh scatter) whilst allowing the Raman scattered light through to the spectrometer and detector. Each laser wavelength requires an individual filter. Edge. An edge filter is a long pass optical filter which absorbs all wavelengths up to a certain point, and then transmits with high efficiency all wavelengths above this point. Holographic notch. A notch filter has a sharp, discrete absorption which for Raman is chosen to coincide with a specific laser wavelength.

DETECTORS Multichannel - Charge Coupled Device (CCD) -Diode Arrays Single Channel - Avalanche Photodiode (APD) - Photomultiplier (PM) - Single Diodes

A CCD (Charge Coupled Device) is a silicon based multichannel array detector of UV, visible and near-infra light. They are used for Raman spectroscopy because they are extremely sensitive to light (and thus suitable for analysis of the inherently weak Raman signal), and allow multichannel operation (which means that the entire Raman spectrum can be detected in a single acquisition). CCD DETECTOR

PHOTO MULTIPLIER

Other forms of Raman spectroscopy

Hand held Raman spectroscopes  Handheld Raman instruments are useful for the identification of chemicals  Designed for safe for use in manufacturing plant environment, for military and chemical weapons applications, etc…

RAMAN RESONANCE SPECTROMETER  UV lasers allow for better Raman performance, because of the 1/ 4 dependence of scattering, but fluorescence is a problem  With lasers in the nm region, the Raman spectrum can be “fit” in the region above the laser but below the normal Stokes- shifted fluorescence spectrum

SURFACE ENHANCED RAMAN SPECTROSCOPY  SERS is a form of Raman spectroscopy that involves a molecule adsorbed to the surface of a nanostructured metal surface which can support local surface plasmon resonance (LSPR) excitations  The Raman scattering intensity depends on the product of the polarizability of the molecule and the intensity of the incident beam; the LSPR amplifies the beam intensity when the beam is in resonance with plasmon energy levels – leads to signal enhancements of >10 6

Selection rules A mode will be Raman active if it induces a change in the polarizability (I) of the molecule. Dipole moment induced by the electric field E of a laser photon is P = I E Change in polarizability Change in the volume of electron cloud Symmetric stretching modes will be (intensely) Raman active, IR inactive

At various conditions  Heating/Cooling – typically suitable for temperatures in the range -196 o C to 600 o C, or ambient to 1500 o C, these stages can be used for solids, powders and liquids.  Catalysis – a variant of the heating/cooling stages above, but designed to have preheated gases forced through a catalyst matrix. Suitable for temperatures up to 1000 o C, and gas pressures up to 5bar.  Tensile Stress – allows structural changes in a sample to be monitored under tensile stress. Forces up to 200N can be used with these stages.

 Pressure – Diamond Anvil Cells (DAC) allow analysis at pressures up to 50GPa, with elevated temperatures.  Humidity – control of sample temperature and humidity allows analysis of solvent-adsorbate interactions, and the effect of humidity on a sample’s structure.

 Semi-conductors Stress, contamination, super lattices structure and defect investigations, hetero structures, doping effects.  Polymers Polymorphs identification, blend morphology, monomers and isomers analysis, crystallinity, orientation, polymerisation.  Geology / Mineralogy/Gemmology Fluid inclusions, gemstones, phase transitions, mineral behaviour under extreme conditions, mineral structures FIELDS OF APPLICATIONS

 Carbon compounds DLC (Diamond Like Carbon), nanotubes, Fullerenes characterisation, diamond, graphite, intercalation compounds, film quality analysis, hard-disk coatings analysis  Life Science Bio-compatibility, DNA analysis, drug/cell interaction, immuno globulins, nucleic acids, chromosomes, oligosaccharides, cholesterol, lipids, cancer, metabolic accretions, inclusion of foreign materials and pathology. CONTINUED

 Forensics Illicit drugs and narcotics, paints, pigments, varnishes, fibres, explosives, inks, gems and other geological specimens, gunshot residues.  Chemistry Phase transitions, catalysts, corrosion, oxides, electrochemistry, solid lubricants, silicon compounds, surfactants, emulsions, aqueous chemistry, solvents analysis CONTINUED

Fluorescence can often contaminate Raman spectra. The use of a Near-IR 785 nm laser radically reduces the possibility of fluorescent contamination, but there can still be some fluorescent problems. More expensive Sample heating through the intense laser radiation can destroy the sample or cover the Raman spectrum It is not suitable for metal alloys. LIMITATIONS

 Can be used with solids and liquids  It is highly non-destructive  No sample preparation needed  Not interfered by water  Non-destructive  Highly specific like a chemical fingerprint of a material  Raman spectra are acquired quickly within seconds  Samples can be analyzed through glass or a polymer packaging  Laser light and Raman scattered light can be transmitted by optical fibers over long distances for remote analysis ADVANTAGES