Laser Isotope Separation LIS

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Laser Isotope Separation(LIS)

• Stanley A. Knott
• CHM 466
• 12/4/07

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Outline

• Background
• Basic Concepts of LIS
• General Methods
• Closer look at three commonly used methods
• Potential Advantages/Disadvantages
• A look at a Silicon Isotope Separation Study

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Background

• Early Laser Isotope Separation (LIS) techniques
  were developed in the early 1970s.
• Made possible due to the development of tunable
  lasers covering a spectral range from 2000 Å to
  20 ?m.

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Basics

• LIS methods rely on isotopic shifts.
• Shifts are based on differences in masses,
  volumes, and nuclear spins.
• 415 millikaiser shift for Uranium is shown.

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Basics

• Experiments are typically run at pressures of no
  more than a few torr to reduce pressure
  broadening of peaks.
• Experiments are also typically run at lowered
  temperatures to reduce doppler broadening.
• Techniques for narrowing in special situations
• Cooling of gas in a supersonic jet
• Atomic beams

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Basics

• All LIS techniques begin with the selective
  photo-excitation of an atom or molecule with the
  correct isotopic composition.

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General LIS Methods
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Photoionization

• The most commonly used photophysical method for
  the separation of substances at the atomic level.

• Photoionization is a two step process.
• Isotopically selective excitation.
• Ionization of only the excited atoms.

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Photoionization

• Various pathways used for the ionization of a
  sample.

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Photodissociation

• This method is commonly used for the separation
  of substances at the molecular level.

• Photodissociation is also a two step
  process.
• Isotopically selective excitation.
• Dissociation of only the excited molecular
  vibrational states.

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Photodissociation

• Various pathways for the dissociation of a
  molecular species.

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Photochemical

• Excited atoms or molecules usually have a
  higher chemical reactivity than an unexcited
  one.
• The probability of overcoming an energy
  barrier is increased due to internal energy.

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Advantages

• High selectivity in separation
• Extraction of desired isotope
• Low energy consumption
• Short starting time
• Possibility of remote separations
• Universality

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Disadvantages

• Most separations require kW or MW power levels
  for reasonable productivity.
• High equipment costs

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Silicone Isotope Separation

• Joint Swiss and Russian project.
• Goal of research is to overcome high cost of
  isotope separation required to improve the
  operation of semiconducting devices.
• Natural Silicone Isotope Abundances
• 28Si 92.1
• 29Si 4.7
• 30Si 3.2
• Utilizing an Overtone-Preexcitation Infrared
  Multiphoton Dissociation (OP-IRMPD) approach.

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Overtone-Preexcitation Infrared Multiphoton
Dissociation (OP-IRMPD)
200 ns later Third laser causes LIF for
detection of SiCl2
Net Reaction
400 ns later - Multiphoton excitation of (v4)
bending vibration beyond dissociation by an IR
NH3 laser.
IR laser pulse excites 30Si-H stretch to (2v1)
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Experimental Apparatus
3-4 mJ /cm2 of IR radiation at 4450.5 cm-1 by
difference frequency mixing is used to excite
30SiH stretch. LiNbO3 crystal
4-6 J/cm2 of IR radiation at 797cm-1 used for
multiphoton dissociation.
LIF beam at 325.084 nm from second harmonic of
dye laser.
Semiconductor grade SiHCl3 (and Scavenger)
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Results
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Scavenger Study

• A scavenger is used to react with the products of
  the photodissociation to form a secondary product
  which can be efficiently collected.
• In this study, BCl3 , was the only scavenger that
  was found to produce a gas that could be
  collected. (SiCl3BCl2)
• It was fairly inefficient however. Only about 40
  of the dissociated SiCl2 could be scavenged and
  only at low pressures
• (lt0.5 Torr)

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Conlclusions

• While optimal conditions could be found for the
  isotopic separation of Silicone isotopes, an
  efficient means of collecting those isotopically
  pure products was not found.
• Additional research is needed to locate a more
  suitable scavenger.

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References

• Letokhov, V.S. Nature 1979, 277(22), 605-610.
• Brenner, D.M. Datta, S. Zare, R.N. J. Am. Chem.
  Soc. 1977, 99(14), 4554-4561
• Rao, P.R. Current Science 2003, 85(5), 615-633.

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Thank You

• Szczepura Research Group
• My Audience

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Test Question

• What is the first step in any LIS method?

All LIS techniques begin with the selective
photo-excitation of an atom or molecule with the
correct isotopic composition