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17 Jun 2002
Shining lasers on a gas containing two isotopes of the same element, Athos Kasapi of Stanford University has made one isotope highly transparent to the laser light while causing the other to become highly opaque, enabling a trace isotope to be detected amidst a far more abundantisotope.
Isotopes of the same element will absorb light at very slightly different energies because the electron-nucleus interaction inside an atom depends subtly on the spin of the nucleus, which in turn is different for each isotope. Demonstrating the isotope discrimination technique in a gas containing 99.97% lead-208 and 0.03% lead-207, Kasapi employs a "probe" laser which ordinarily sends the abundant lead-208 atoms from a low-energy ground state to a high-energy excited state, and a "coupling" laser which normally sends the lead-208 from an intermediate "metastable" state to an excited state. When the probe and coupling lasers are both turned on, quantum interference between the two pathways to the excited state prevents the atom from absorbing any light.
Kasapi made the other isotope, lead-207, highly absorbing by adjusting the coupling laser's intensity. Although this technique cannot currently compete with conventional methods of isotope separation, it does represent another interesting application of "laser-induced transparency. Results of Kasapi's research will be presented in an upcoming paper in Physical Review Letters.
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