The Nobel Prize in Physics 1930 "for his work on the scattering of light and for the discovery of the effect named after him" Sir Chandrasekhara Venkata.

The Nobel Prize in Physics 1930"for his work on the scattering of light and for the discovery of the effect named after him"

Sir Chandrasekhara Venkata Raman India Calcutta University Calcutta, India

b. 1888d. 1970

+ + +

- -

Electron cloud oscillates at same freq. as EM field

4λ

1 α Intensity

((

((

+

-

+

-

-

-+

+

Frequency of scattered light same as frequency of incoming lightFrequency of scattered light same as frequency of incoming light

A. Rayleigh Scattering (Elastic collision)

“virtual” states

in = out

So

B. Infrared Absorption

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+

--

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IR Absorption if the vibration results in a change in dipole moment

Frequency of absorbed light frequency of vibrationFrequency of absorbed light frequency of vibration

So

S1

C. Raman Scattering (Inelastic collision)

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Frequency of scattered light different from frequency of incoming lightFrequency of scattered light different from frequency of incoming light

Raman Selection Rule: Raman scattering is “allowed” when the vibration results in a change in electron cloud Polarizability

4λ

1 α Intensity

h”

h’h”

h’

“scattered” photons(’ and ”)

h

(a) (b) (c)

a) Stokes Shift – photon energy transferred TO the vibration

b) Rayleigh Band

c) Anti-Stokes – scattered photon absorbs energy FROM the vibration

The Stokes and Anti-Stokes bands always occur at the same frequencies (shifts) irregardless of the incoming excitation frequency (o).

“virtual” states

So

S1

Stokes Anti-Stokes

= result of direct IR absorption/emission

or

If band is also IR active, then the Raman shift shows up at the same wavenumber as in the IR spectrum,

e.g. the C=O band at 1600 cm-1 in the IR spectrum is at a Raman shift of 1600 cm-1

Raman can be “complementary” if the band is not IR active but does show up in the Raman spectrum

Basic components are a laser source, a sample illumination system, and a spectrometer.Basic components are a laser source, a sample illumination system, and a spectrometer.

Source (Sec 18B-1) - Raman signals are inherently weak so need a high-power laser source

514.5 nm Ar+ laser

1064 nm Nd:YAG

laser

4λ

1 α Intensity

Sample Illumination System for Liquids (Sec 18B-2)

•laser wavelengths from 488-1064 nm can pass through ordinary glass optics, so no need for special halide optics

•lasers can be tightly focused so small sample containers can be used such as capillary tubes

•a major advantage of Raman is the signal produced by water is very small, unlike in IR spectroscopy which produces an enormous water background

Usually only collect the more intense Stokes linesUsually only collect the more intense Stokes lines

A. Dispersive w/CCD detector

BP = interference bandpass filter

BR = Rayleigh Band rejection filter

B. Fourier Transform Raman (FT-Raman)

Fiber-optic excitation and collection optics - the Vis and near IR easily transmitted up to 100's of meters.

Fiber-optic excitation and collection optics - the Vis and near IR easily transmitted up to 100's of meters.

•laser light collected by a microscope objective which tightly focuses it into the fiber optic (diameter < mm)

•collection fiber optics return the Raman scatter to the monochromator, geometrically arranged to fit into the entrance slit

•the fiber optic probe can be inserted directly into hostile chemical environments