13-06-2012, 04:47 PM
Raman Spectroscopy
Raman Spectroscopy.ppt (Size: 866.5 KB / Downloads: 54)
Raman Principle
When a sample is irradiated with monochromatic light
Different physical phenomena happen:
1. Most of the incident light is transmitted without interaction
2. Roughly 10-4 part of the light is elastically scattered
with no energy absorption --->Rayleigh line
3. A part of the incident light may be absorbed by electronic transitions
and is emitted as ---> Fluorescence
4. Only 10-8 of the incident light is inelastically scattered and interacts
with the sample .The energy is partly absorbed ---> Raman spectrum
Fluorescence Vs Raman
Scattering processes are instantaneous whereas fluorescence involves an intermediate step.
Fluorescence requires actual levels whereas Raman can occur through virtual states.
Scattering process is intrinsically weak as compared to Fluorescence. Hence scattering is masked in the presence of fluorescence.
Laser with excitation
wavelength of 785nm
is used to eliminate
Fluorescence.
The Raman Effect
The Raman effect occurs when light falls on a molecule and interacts with the electron cloud.
A photon excites the molecule from the ground state to a virtual energy state.
When the molecule relaxes it emits a photon and it returns to a different vibrational state.
Raman Shift’s
Stokes shift:-If the final vibrational state of the molecule is more energetic than the initial state, then the emitted photon will be shifted to a lower frequency. This shift in frequency is designated as a Stokes shift.
Anti-Stokes shift:-If the final vibrational state is less energetic than the initial state, then the emitted photon will be shifted to a higher frequency, and this is designated as an Anti-Stokes shift.
Quantum Mechanics: Normal modes
Only certain vibrational frequencies and atomic
displacements allowed
- Linear molecules: 3N-5
- Non-linear molecules: 3N-6
(e.g. bending, stretching, out of plane deformation)
Vibrational spectrum of a molecules depends upon
- Masses of the atoms in molecules
- The strength of chemical bonds
- Atomic arrangements .
Raman Spectroscopy.ppt (Size: 866.5 KB / Downloads: 54)
Raman Principle
When a sample is irradiated with monochromatic light
Different physical phenomena happen:
1. Most of the incident light is transmitted without interaction
2. Roughly 10-4 part of the light is elastically scattered
with no energy absorption --->Rayleigh line
3. A part of the incident light may be absorbed by electronic transitions
and is emitted as ---> Fluorescence
4. Only 10-8 of the incident light is inelastically scattered and interacts
with the sample .The energy is partly absorbed ---> Raman spectrum
Fluorescence Vs Raman
Scattering processes are instantaneous whereas fluorescence involves an intermediate step.
Fluorescence requires actual levels whereas Raman can occur through virtual states.
Scattering process is intrinsically weak as compared to Fluorescence. Hence scattering is masked in the presence of fluorescence.
Laser with excitation
wavelength of 785nm
is used to eliminate
Fluorescence.
The Raman Effect
The Raman effect occurs when light falls on a molecule and interacts with the electron cloud.
A photon excites the molecule from the ground state to a virtual energy state.
When the molecule relaxes it emits a photon and it returns to a different vibrational state.
Raman Shift’s
Stokes shift:-If the final vibrational state of the molecule is more energetic than the initial state, then the emitted photon will be shifted to a lower frequency. This shift in frequency is designated as a Stokes shift.
Anti-Stokes shift:-If the final vibrational state is less energetic than the initial state, then the emitted photon will be shifted to a higher frequency, and this is designated as an Anti-Stokes shift.
Quantum Mechanics: Normal modes
Only certain vibrational frequencies and atomic
displacements allowed
- Linear molecules: 3N-5
- Non-linear molecules: 3N-6
(e.g. bending, stretching, out of plane deformation)
Vibrational spectrum of a molecules depends upon
- Masses of the atoms in molecules
- The strength of chemical bonds
- Atomic arrangements .