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Raman spectroscopy

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principle instrumentation working of the raman spectroscopy is explained
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RAMAN SPECTROSCOPY
Transcript
Page 1: Raman spectroscopy

RAMANSPECTROSCOPY

Page 3: Raman spectroscopy

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

RAMAN EFFECT

Page 4: Raman spectroscopy

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 0.0000001%) is scattered at different wavelengths (or colours), which depend on the chemical structure of the analyte – this is called Raman Scatter.

Page 5: Raman spectroscopy

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

Page 8: Raman spectroscopy

Stokes and anti-Stokes Raman spectral lines of CCl4

Page 9: Raman spectroscopy

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)

Page 11: Raman spectroscopy

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 spectrumIntensity of the Raman peak depends on Polarisabilty of each molecules of a substance

[Title picture]

Page 13: Raman spectroscopy

INDUCED ELECTRIC DIPOLE MOMENT

An electric field can distort the electron cloudof a molecule, thereby creating an “induced”electric dipole moment

The oscillating electric field associated withEM radiation will therefore create anoscillating induced electric dipole momentwhich in turn will emit, i.e. scatter, EMradiation

Page 14: Raman spectroscopy

•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 bondsOut-of-plane deformation modes

Vibration of molecules

Page 15: Raman spectroscopy

VIBRATIONAL MODES

SYMMETRICAL ASYMMETRICAL WAGGING

TWISTING SCISSORING ROCKING

Page 16: Raman spectroscopy

· Vibrational modes that are more polarizable are more Raman-active

· Examples: – N2 (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)

– CO2 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

Page 17: Raman spectroscopy

SOURCESELECTORS AND FILTERS

DETECTORS

RAMAN INSTRUMENTATION

Page 20: Raman spectroscopy

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 nmVisible:  457 nm, 473 nm, 488 nm, 514 nm, 532 nm, 633 nm, 660 nmNear infra-red:  785 nm, 830 nm, 980 nm, 1064 nm

Page 21: Raman spectroscopy

LASERS•Gas lasers

•Solid-state lasers

•Semiconductor lasers

•Other: Dye laser (tunable)

•Metal-vapor laser (deep UV)

•Ti: Sapphire (solid-state tunable)

Page 23: Raman spectroscopy

SOLID STATE LASER

Page 24: Raman spectroscopy

SAMPLING SYSTEMS

Page 25: Raman spectroscopy

SAMPLING SYSTEMS

Page 26: Raman spectroscopy

RAYLEIGH LINE REJECTIONOptical 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.

Page 27: Raman spectroscopy

DETECTORS

Multichannel- Charge Coupled Device (CCD)-Diode Arrays

Single Channel- Avalanche Photodiode (APD)- Photomultiplier (PM)- Single Diodes

Page 28: Raman spectroscopy

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

Page 30: Raman spectroscopy

PHOTO MULTIPLIER

Page 31: Raman spectroscopy

[Divider picture]

Other forms of Raman

spectroscopy

Page 32: 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…

Page 33: Raman spectroscopy

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 245-266 nm region, the Raman spectrum can be “fit” in the region above the laser but below the normal Stokes-shifted fluorescence spectrum

Page 34: Raman spectroscopy

SURFACE ENHANCED

RAMAN SPECTROSCO

PY

· 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 >106

Page 35: Raman spectroscopy

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

Page 36: Raman spectroscopy

At various conditionsHeating/Cooling – typically suitable for temperatures in the range -196oC to 600oC, or ambient to 1500oC, 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 1000oC, 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.  

Page 37: Raman spectroscopy

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.

Page 38: Raman spectroscopy

Semi-conductorsStress, contamination, super lattices structure and defect investigations, hetero structures, doping effects.PolymersPolymorphs identification, blend morphology, monomers and isomers analysis, crystallinity, orientation, polymerisation .Geology / Mineralogy/GemmologyFluid inclusions, gemstones, phase transitions, mineral behaviour under extreme conditions, mineral structures

FIELDS OF APPLICATIONS

Page 39: Raman spectroscopy

Carbon compoundsDLC (Diamond Like Carbon), nanotubes, Fullerenes characterisation, diamond, graphite, intercalation compounds, film quality analysis,hard-disk coatings analysisLife ScienceBio-compatibility, DNA analysis, drug/cell interaction, immuno globulins, nucleic acids, chromosomes, oligosaccharides, cholesterol, lipids, cancer, metabolic accretions, inclusion offoreign materials and pathology.

CONTINUED

Page 40: Raman spectroscopy

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

CONTINUED

Page 41: Raman spectroscopy

•Fluorescence can often contaminate Raman spectra.The use of a Near-IR 785 nm laser radically reduces thepossibility of fluorescent contamination, but there canstill 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

Page 42: Raman spectroscopy

Can be used with solids and liquidsIt is highly non-destructiveNo sample preparation neededNot interfered by waterNon-destructiveHighly specific like a chemical fingerprint of a materialRaman spectra are acquired quickly within secondsSamples can be analyzed through glass or a polymer packagingLaser light and Raman scattered light can be transmitted by optical fibers over long distances for remote analysis

ADVANTAGES


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