Stephanie Gadbois

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Stephanie Gadbois
LambDip Spectroscopy
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Lamb Dip Spectroscopy
Follow me, the Cephalopod with the laser, for Q
A

• First proposed by W.E. Lamb in 1964 as a method
  to overcome Doppler broadening and improve
  resolution power.
• In 1969 C.C. Costain first observed through
  experiment using an effusive beam to eliminate
  Doppler broadening.
• Applied in microwave and millimeter wave
  spectroscopy and later to laser spectroscopy

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Recall Doppler Broadening

• If an atom or molecule moving towards a detector
  with a velocity (va) has an observed transition
  frequency (fobs) it is related to the actual
  frequency by
• fobs f /(1-va/c)

• Since the molecules follow Maxwell Boltzman
  velocity distributions..
• This resembles a Gaussian statistical shape.
• Doppler broadening width

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What W.E. Lamb predicted

• Molecules which have the zero velocity component
  away from the source absorb radiation of the same
  frequency whether or not the radiation is
  traveling towards or away from the Reflector,
  resulting in saturation.
• If saturation occurs then no further absorption
  takes place when it travels back to the Reflector
  and a dip in the power output results in the
  center of the line.
• The width of the dip is called the natural line
  width (the inverse of the lifetime).

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Basic Principle of Lamb Dip Spectroscopy

• Figure A shows the shape of an absorption line
  broadened by the Doppler effect and the usual
  source-cell-detector approach.
• Figure B shows the modification by a Reflector
  and the characteristic Lamb Dip that results from
  light beams traveling in both directions within
  the laser cavity.

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Applications of Lamb Dip Spectroscopy to Lasers

• Doppler broadening can be reduced by introducing
  a laser beam perpendicular to the velocity vector
  of a collimated molecular beam.
• The absorption of laser light is a function of
  the laser frequency and can be measured with high
  resolution.
• The position of the Lamb Dip gives the location
  of the transition frequency having no doppler
  shift (i.e. doppler-free absorption).

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Doppler-free Absorption

• A molecule moving with a velocity along a beam
  feels the beam frequency and the frequency of the
  reflected beam, so when a molecule absorbs the 2
  photons of different (counter-propagating) light
  beams, the transition energy is doppler-free.

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Results high resolution doppler-free electronic
spectra

• From the high resolution spectra, energy spectral
  lines can be assigned and this gives accurate,
  detailed information on electronic states,
  molecular structures, potential energy curves,
  electron spin and nuclear spin states.
• This method is very useful to study the spectra
  of polyatomic molecules.

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Assumptions of Lamb Dip Spectroscopy

• That all other broadening factors must be small
  in comparison to doppler broadening
• The radiation introduced must pass linearly
  through the cell and be monochromatic when
  compared to the natural line width.
• The power source must be sufficiently high in
  order for saturation to occur.

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Lamb Dip Spectroscopy is often called Saturation
Spectroscopy

• It is based on the velocity selective saturation
  of Doppler-broadened molecular transitions, now
  the resolution is no longer limited by the
  Doppler width but rather the more narrower Lamb
  dip width.
• Even if the Doppler profiles of 2 transitions
  overlap their narrow Lamb dips can be clearly
  separated.

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Useful Applications of Lamb Dip Spectroscopy

• High resolution of polyatomic molecules
• Enhancing computer memory and storage
• A tightly focused laser beam is used to burn a
  hole in a thin film at a location that
  corresponds to a bit. Then the second focused
  laser can detect that hole and read the bit.

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What is the theory behind Lamb Dip /Saturation
Spectroscopy?

• The narrow resonances, i.e. Lamb dips, that
  appear at the center of non-homogenous broadened
  lines interacting with counter-propagating laser
  beams essentially resulted from holes burned in
  the Maxwell-Boltzman velocity distribution.
• This provided a way to find the center frequency
  of the line and remove the inhomogeneous line
  width.

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References

• Hollas, J. Micheal. Modern Spectroscopy. 4th
  Edition. John Wiley Sons Inc. (2004).
• Hollas, J. Michael. High Resolution Spectroscopy.
  2nd Edition. John Wiley Sons Inc. (1998).
• Demtroder, Wolfgang. Laser Spectroscopy Basic
  Concepts and Instrumentation. 2nd Edition.
  Springer-Verlag Berlin Heildelberg. (1998).
• Stenholm, Stig. Foundations of Laser
  Spectroscopy. John Wiley Sons Inc. (1984).
• Levenson, Mark. Introduction to Nonlinear Laser
  Spectroscopy. Academic Press. (1982).
• Meijer,Gerard, Ubachs,Wim, Ter Meulen,J.J., and
  Dymanus,A. Molecular High-Resolution Lamb Dip
  Spectroscopy on OD and SiCl in a Beam. Chemical
  Physics Letters. Vol 139, (6). 603-611. 1987.
• Remillard,D, and Weber,W.H. Sub-Doppler
  Resolution Limited Lamb-Dip Spectroscopy of NO
  with a Quantum Cascade Distributed Feedback
  Laser. Optics Express. Vol 7, (7). 243-249. 2000.