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Laser photofragment spectroscopy has a close analogy with atom-ion scattering [73] and thus shares much of the same terminology. Indeed, the fragmentation process can be considered as the second half of a full collision, with exact controls over input energy. The term `resonance' is often used to describe levels which are quasibound with respect to the continua of the separated atoms.
Three mechanisms by which resonances may arise are considered here:
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Levels may lie at energies above the dissociation continuum but become trapped behind a barrier lying at intermediate internuclear separations. The origin of this barrier can be due to an avoided crossing between states of the same symmetry, or alternatively can be caused by a centrifugal term in the effective potential due to the high rotation of the molecule (shape resonance).
- Levels which are bound with respect to a potential surface lie embedded in the continuum of a state (or states) correlating to a lower dissociation asymptote. This case is known as in scattering terminology as a Feshbach resonance.
- A bound and a repulsive curve correlating to a lower dissociation asymptote cross. Radiationless transfer occurs between the rotational and vibrational states to the continuum. The wavefunctions of the two states are mixed to an extent that the wavefunction for the bound state has all the characteristics of a free state. Dissociation of the bound surface occurs in close proximity to the crossing due to the increased mixing in this region.
Next: Born Oppenheimer Approximation
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Tim Gibbon
1999-09-06