Raman Spectroscopy Flashcards
(38 cards)
Summarize the theory of IR absorption.
- IR spectra are results of low energy absorption derived from vibrations and to some extent rotations of molecules
- Absorption is possible when there is a net change in the dipole moment (caused by vibrations or rotations of the molecule)
In an IR spectrum, lower transmittance = […]
more absorbance
What are the two types of instruments in IR instrumentation?
- Dispersive and Fourier Transform
Describe dispersive IR instrumentation.
- Components are similar to UV-VIS spectrometers
- i.e., radiation source
- monochromator or filter
- sample holder
- detector connected to amplifier system
Largely now replaced by FT-IR
Describe FT IR spectroscopy instrumentation.
- Uses an interferometer rather than monochromator, and polychromatic IR light to generate an interferogram.
Describe the interferogram before and after the Fourier Transformation.
- Before: The interferogram is intensity of signal vs. time (or mirror position)
- Mirror position changes at a fixed velocity
- After: Intensity (absorbance/transmittance) vs. frequency
- An IR spectrum
The FT takes the signal, and determines the amount of each wavelength/wavenumber that went into the signal.
What does Fourier Transformation accomplish?
- Takes the signal and determines the amount of each wavelength/wavenumber that went into the signal.
What is Raman spectroscopy?
How does it work?
- Used to study the molecular vibrations and rotations of a sample
Uses monochromatic laser light, usually at VIS or near-IR wavelengths.
Works by shining a light source, usually a laser, onto a sample and measuring the light that is scattered by the sample.
What information does Raman spectroscopy provide? [4]
- Chemical structure
- Phase and polymorphy
- Crystallinity
- Molecular interactions
Non-destructive chemical analysis
Describe Raman spectra. [3]
- Molecule specific
- Contain information about the vibrational modes of the molecule
- Have sharp features, allowing identification of the molecule by its spectrum
Describe the Raman Effect.
- When a Raman event is observed, energy from incident laser photons is transferred to the analyte, resulting in emitted photons that are shifted to different frequencies.
- This inelastic form of scattering only affects approximately 1 in 10 million photons an is known as Raman scattering, or the Raman effect.
Inelastic scattering
What is the difference between elastic and inelastic scattering?
i.e., Raleigh vs. Raman scattering
- Elastic: most of the light is scattered without a change in energy - this is termed Rayleigh scattering
- Inelastic: a small proportion of photons is scattered with a loss or gain of energy to molecular vibrations - this phenomenon is termed the Raman effect, or Raman scattering
What is the difference between Stokes and Anti-stokes shift?
- Both are forms of Raman scattering
- Stokes: when emitted photons are shifted to a lower energy
- Anti-stokes: when emitted photons are shifted to a higher energy
Are stokes or anti-stokes shift more commonly used in Raman applications?
- Stokes (emitted photons are shifted to a lower energy)
Stokes shift is when scattered photons are shifted to a lower energy.
True or False?
True.
Stokes shift is when emitted photons are shifted to a higher energy.
True or False?
False.
Stokes shift is when emitted photons are shifted to a lower energy.
Anti-stokes shift is when emitted photons are shifted to a higher energy.
True or False?
True.
Anti-stokes shift is when emitted photons are shifted to a lower energy.
True or False?
False.
Anti-stokes shift is when emitted photons are shifted to a higher energy.
Describe the frequency shift of the scattered light in Raman scattering.
Compare Stokes to Anti-stokes.
- Proportional to the vibrational energy of the molecules in the sample
- By analyzing the frequency shifts of the scattered light, we can determine the molecular structure of the sample and identify its chemical composition.
What are the ‘virtual levels’ of photon molecular interactions?
Compare the energy shifts between UV-VIS, IR, and Raman spectroscopy.
- Virtual levels refers to Rayleigh and Raman scattering effects.
- Not as high as the first excited state
What are the characteristics of Raman scattering? [3]
- Very weak effect: only 1 in 10^7 photons is Raman scattered
- True scattering process: virtual state is a short-lived distortion of the electron cloud which creates molecular vibrations
- Strong Raman scatterers: C=C and pi-bonds
What works well with Raman spectroscopy? [4]
- Organic materials including active pharmaceutical ingredients, organic solvents, polymers, harmful narcotics, and explosives
- Polyatomic inorganics such as magnesium sulfate, sodium bicarbonate, titanium dioxide, and calcium phosphate
- Molecules containing only single bonds: C-C; C-H; or C-O (e.g., aliphatic; sugar; starch; cellulose)
- Highly polar small molecules such as ethanol
What does not work well with Raman spectroscopy? [4]
- Materials with no covalent bonds: purely ionic species (e.g., NaCl)
- Highly fluorescent samples including plant-based materials
- Black or dark-coloured samples as the materials may completely absorb the laser light
- Any substance with weak Raman signal within the region being examined (e.g., water in the region 200 to 2000 cm^-1)
- Most metals and elemental substances
Compare between Raman and IR spectroscopy.
Raman
- Observes the shift in vibration from an incident source
- Change in polarizability of a bond is required for a vibrational mode to be Raman active
- Occurs at all wavelengths
- Weak signal
- Sharp spectral features for molecular fingerprinting
IR
- Examines the wavenumber at which a functional group has a vibrational mode
- A change in dipole moment is required for a vibrational mode to be IR active
- Only observed in IR spectral regions
- High water absorption
- Broad spectral features
Complementary techniques
