Raman spectroscopy uses monochromatic light, usually from a laser, to illuminate a sample. Most of the light is elastically scattered (Rayleigh scattering) at the same wavelength as the incident light. A small amount is inelastically scattered (Raman scattering) at optical frequencies different from the incident light. This difference in scattering wavelengths corresponds to vibrational and rotational energy levels of molecules, providing a unique spectral fingerprint to identify chemical bonds and molecular structure.

8. Chemistry of Raman Spectroscopy
• Monochromatic light applied to sample
• Incident light is scattered
– Rayleigh (elastic) and Raman (inelastic)
• Rayleigh scatter is filtered out
• The returned scattered light is a different
wavelength
• This difference corresponds to an energy
shift which provides a unique chemical
fingerprint

10. Careful observation, however, reveals that:
A very small fraction of the radiation is transmitted at all angles from the
original path and that the intensity of this scattered radiation increases
with particle size.
Types of Scattering:
• Rayleigh scattering
Scattering by molecules or aggregates of molecules with dimensions
significantly smaller than the wavelength of the radiation.
Its intensity is proportional to:
- The inverse fourth power of the wavelength (.
- The dimensions of the scattering particles.
• The square of the polarizability of the particles.
• An everyday manifestation of Rayleigh scattering is the blue color of
the sky, which results from greater scattering of the shorter
wavelengths of the visible spectrum.

11. The blue color of the sky is caused by the scattering of sunlight off the
molecules of the atmosphere.
This scattering, called, is more effective at short wavelengths (the blue end of
the visible spectrum).
Therefore the light scattered down to the earth at a large angle with respect to
the direction of the sun's light is predominantly in the blue end of the spectrum.

12. Raman Scattering
• The Raman scattering effect differs from ordinary
scattering in that part of the scattered radiation suffers
quantized frequency changes.
• These changes are the result of vibrational energy level
transitions that occur in the molecules as a consequence
of the polarization process.

13. • Basic Physical Realization
– Illuminate a specimen with laser light (e.g. 532nm)
– Scattered (no absorbed) Light in two forms
• Elastic (Rayleigh) → scattered = incident
• InElastic (Raman) → scattered  incident
– Light Experiences a “Raman Shift” in Wave Length.

14. Raman vs. Rayleigh
• Raman Intensity is About 0.1 ppm of the Incoming
Laser Intensity

15. • Inelastic Light Scattering Mechanisms
• Raman Shift Can be:
– To Longer WaveLengths (Stokes Scattering)
• Loses Energy – Predominant Raman Shift
– To Shorter WaveLengths (AntiStokes Scattering)
• Gains Energy – Subordinate Raman Shift
Raman Stokes Scattering Raman AntiStokes Scattering

24. 14-6 The Doppler Effect
The Doppler effect is the change in pitch of a sound when the source and observer
are moving with respect to each other.
When an observer moves toward a source, the wave speed appears to be higher,
and the frequency appears to be higher as well.

25. Application of Doppler Effect
Nexrad: Next Generation Weather
Radar

26. Ultrasonic Scanner

27. The cavitron ultrasonic surgical
aspirator (CUSA)
Neurosurgeons use a cavitron ultrasonic surgical aspirator (CUSA) to “cut out” brain
tumors without adversely affecting the surrounding healthy tissue.

36. Bohr Model of the Atom
Electrons in Atoms
nucleus (+)
electron (-)
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

37. Atomic Spectrum
How color tells us about atoms

38. Prism
• White light is made up
of all the colors of the
visible spectrum.
• Passing it through a
prism separates it.
Author: Thomas V. Green Jr.

39. If the light is not white
• By heating a gas or
with electricity we can
get it to give off
colors.
• Passing this light
through a prism does
something different.
Author: Thomas V. Green Jr.

40. Atomic Spectrum
• Each element gives off
its own characteristic
colors.
• Can be used to
identify the atom.
• How we know what
stars are made of.
Author: Thomas V. Green Jr.

41. • These are called
line spectra
• unique to each
element.
• These are emission
spectra
• The light is emitted
given off.
Author: Thomas V. Green Jr.

42. Line-Emission Spectrum
ground state
excited state
ENERGY IN PHOTON OUT
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
656 nm486 nm410 nm
434 nm
Wavelength (nm)
PrismSlits

43. Bohr Model
• electrons exist only in orbits with specific
amounts of energy called energy levels
• Therefore…
• electrons can only gain or lose certain
amounts of energy
• only certain photons are produced
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

44. Bohr Model
1
2
3
4
5
6
• Energy of photon
depends on the
difference in energy
levels
• Bohr’s calculated
energies matched the
IR, visible, and UV
lines for the H atom
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
nucleus

45. Other Elements
• Each element has a unique bright-line
emission spectrum.
i.e. “Atomic Fingerprint”
Helium
Bohr’s calculations only worked for hydrogen!

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

46. Bohr’s Experiment
Kelter, Carr, Scott, Chemistry A Wolrd of Choices 1999, page 76 Animation by Raymond Chang – All rights reserved.

47. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
(a) Electronic absorption transition
(b) H2 emission spectrum (top), H2
absorption spectrum (bottom)

48. continuous spectrum
absorption spectrum
emission spectrum
hot source
gas
absorption spectrum
emission spectrum

49. Hydrogen Spectral Lines
Lyman series
(ultraviolet)
Balmer series
(visible)
Paschen series
(infrared)
Frequency
(hertz)
1016 1015 1014
7 6 5 4 3 2 1n =

50. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
(ultraviolet)
(visible)
(infrared)
HYDROGEN SPECTRAL LINES

51. Hydrogen Spectral Lines
A B C D E F
Lyman series (UV)
A B C D E
Balmer (Visible)
A B C D
Paschen (IR)
E1
E2
E3
E4
E5
E6
Energy
Bohr’s model of the atom accounted mathematically for the energy of each of the transitions shown.
IR
region
UV
region
656 nm
486 nm
434 nm
410 nm
Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 97
ionization