Raman Spectroscopy

Raman Spectroscopy

A form of vibrational spectroscopy, like IR in some ways

Tyndall Scattering

Light encounters a molecule that is about the same dimension as the wavelength

• Can’t see through the substance

• Examples: fog, smog, milk (molecules aggregate in the liquid and the air

Rayleigh Scattering

originates from collisions within particles that are much smaller than the wavelength.

• small wavelength=more scattering

• Scattering Intensity ∝ 1/λ^4

  • Sky is Rayleigh scattering

  • Rather than view the Rayleigh Scattering as an elastic collision, treat it as a brief excited state
    

Heisenberg Uncertainty Principle

qp=h

uncertainty position times uncertainty in momentum equals Planck’s constant (6.626E-34 J/s).

or

ΔEt=h

uncertainty in NRG multiplied by time equals Plank’s constant

Rayleigh Scatter Virtual State

Due to the extremely short lifetime, the ΔE of the excited state has large uncertainty.

NRG lvl is smeared - a wide range of wavelengths can be used. Creates an elastic process where the NRG of the photon is the same coming in and out of the ES.

Raman Scattering

• Inelastic collisions: A tiny amount of scatter photons with slightly different NRGs

• Will always contain Rayleigh Scattering (photons with the same NRG as the source light)

Stokes Shift

ΔNRG of the scattered photon and the Rayleigh photons was discrete

Stokes Raman Scattering

Photons emit at a lower NRG than the source

Anti-Stokes Raman Scattering

Photons emit at a higher NRG than the source

Shifts

• A function of the sample (chemical entity)

• Independent of the source wavelength

  • Even through different colors of light, the gap is constant, Stokes and Anti are symmetrical about the Rayleigh line

• Correspond to vibrational frequencies

  • Difference of a vibrational gap

Stokes and Anti-Stokes

Both are redundant information. Avoid scanning past the Rayleigh line as the Stokes are weak and the Rayleigh is off scale; would be too intense for detector.

Anti is weaker than Stokes. Anti is before Rayleigh; whereas Stokes is after Rayleigh and Anti.

Perks of Raman

Selection Rules need to be satisfied (photon colliding with a molecule)

Vibrational mode is only IR active if the vibration caused by absorption causes a change in the dipole moment of the molecule

Raman only focuses on vib. modes that cause a change in polarizability of the molecule

IR vs Raman

Almost all vib. modes are IR or Raman active

IR inactive modes will typically be seen by Raman

ΔDipole moment

an expression of how charge is distributed in a molecule

will the vibration make e- orient themselves any differently than the ground state?

Homonuclear vs. Heteronuclear

Homo:

e- sit there and photon will not be absorbed, the molecule stretches symmetrically and will not show in IR.

Hetero

e- is pulled by more electronegative molecule and the molecule is stretched symmetrically. IR active

Polarizability

Size of electron cloud is distorted (ability to distort e- cloud).

symmetrical stretch Homo changes polarizability making it Raman active

symmetrical stretch Hetero changes polarizability makes it Raman active

Badass Approach

Use a double monochromator.

Stray light rejection = 1st mono * 2nd mono

Stray Light Rejection

Monochrmoators reduce stray light by 10^-4 or ^-5

Disadvantages of Raman Spec

1. Working with Weak Signals

   • I ∝ concentration

   • I=kC

   • Intensity of the source must be narrow in bandwidth, it’s difficult to resolve the Raman peaks since they are so close to each other

2. Low Intensity (ppm and less than 0.01M not good)

   • Weak Signals

   • Most photons absorbed return as Rayleigh photons rather than Raman

3. Absorbance and Fluorescence (happens in visible)

   • The presence of a photon processes other than the scattering will interfere

   • Weaken scatter efficiency

     • Sometimes a near IR source is used to collect Raman

Amount of Raman Intesity

Number of raman photons emitted/ number of photons absorbed in virtual state