Raman基本原理

λscatter> λlaser
What is the Raman effect?
Before Scattering
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O==C==O
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1415.4 1168.3 1264.1 1369.4
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Laser
What does the Raman spectrum look like? EVEN quite subtle differences produce a ‘fingerprint’ which is unique to that particular molecular structure,
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Ethanol CH3CH2OH Methanol CH3OH
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In which spectral range do we measure ?
Note how the range in Å decreases in the UV
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Intensity (a.u.)
• For industrial on-line applications in addition
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785 nm
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economic considerations are of interest: • lifetime • cost
1641.5
What Exactly is Being Measured?
• Raman spectroscopy : scattering phenomenom • MidIR : absorption phenomenom
Mid IR Stokes Rayleigh Anti-Stokes Fluorescence
For pharmaceuticals applications, visible (red) and NIR excitations are the most commonly used ones, mainly to avoid fluorescence emissions
Role of Laser Wavelength
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532 nm
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633 nm
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Which type of information do we access by Raman?
√ Qualitative information : A Raman spectrum provides a fingerprint representing the set of bonds present in the material - Chemical composition, contaminants detection... Vibrational frequencies are sensitive to details of the structure and local environment of a molecule, such as crystal phase, local strain, and degree of crystallinity - Structural information(crystalline/amorphous, polymorphs…) √ Quantitative information: - Macroscopic properties (mechanical resistance, viscosity, constraints, stress...) - Concentrations predictions in mixtures Advantage of the generally sharp and well resolved features for deriving quantitative information - Univariate analysis approach often suffices
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Raman advantages over other analytical techniques
Raman : the “other” vibrational technique
•IR : sensitive to different functional groups (esp. bonds -aromatics, unsaturates) higher spatial resolution (1µm vs. >10µm) sample prep is minimal compatibility with aqueous solutions
What is the Raman effect?
A Light Scattering Process: A photon (Laser source at only one wavelength) bounces off a molecule and loses a small amount of energy equal to the vibrational energy of the molecule.
Real States
Virtual State
Vibrational Spectra
Highly competitive phenomenon
Vibrational States νi Ground State
What is the Raman effect? The change in colour (wavelength) of the Raman scattered light depends upon the molecular bonds under investigation. The change in colour of the scattered light depends on how much energy these individual chemical bonds require to get ‘excited’ to a higher energy level.
O=C=O
O===C===O
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ν 0 -∆ ν
After Scattering
The Raman spectrum stands as the “fingerprint” the ID of the analysed compound
Analysis of liquids, solids and gases
•Optical Microscopy : chemical information •XRD : less material required detect crystalline phase on smaller scale
•ESCA, Auger, TEM : measurements done in ambient conditions information on molecular and/or crystalline phase
The applications of Raman continue to expand…...
Since Raman spectra are derived from molecular vibrations, the technique can be applied to organic, inorganic, solid, liquid and solution samples. The highly specific information contained in Analytical Raman Spectra include molecular identification composition, crystalline phase, and orientation….. And so on and may be applied to :
H H O
Different bonds give rise to the different colours of the scattered light AND HENCE, the spectrum
What does the Raman spectrum look like?
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Rayleigh Scattering
λscatter= λlaser
λlaser
O==C==O
Raman Scattering
Only a very small amount of the light is scattered at a different wavelength ( at different colour(s) - the Raman scatter), most is simply seen as the same wavelength as the starting laser.