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Photodissociation of molecular ions

Due to the use of high power lasers to fragment ions and the possibility of almost unit detection of fragments, photodissociation is a particularly favourable technique for ion studies. Photofragment detection not only yields information about the spectroscopy of the electronic states, but also the dissociation mechanism. The process of photodissociation is entirely analogous to that of atom-ion collisional scattering and shares much of the same terminology.

To fully understand a collision between an atom and an ion in which both fragments have non-zero spin, all the different couplings between the potential curves must be considered. Direct studies of collisional processes average over such couplings, whereas ion photofragment spectroscopy techniques can prepare the molecule in one dissociative state. Measurements can be made of the kinetic energy of the fragment, (and the angular distribution of the photofragments) and lifetime of such a state.

Since the advent of high power lasers, it has proved possible to probe the region between the `molecular' and `atomic' limits (the `re-coupling region') using laser photofragment spectroscopy and it thus offers significant advantages over scattering experiments, where initial reactant energies are poorly defined and control over the impact parameters is limited. The spectroscopy of molecular ions close to the equilibrium bond length is easily probed using absorption and emission techniques, and is generally thought to be well understood. The atomic limit is also well defined. Between these `chemical' and `physical' limits (in the `re-coupling region'), the spectroscopy is more difficult to define and generally less well developed.

Through studying molecular ions at rotational resolution, weak interactions (such as spin-orbit splittings, hyperfine interactions and lambda doubling) can be observed, giving a detailed characterization of the electronic wavefunction.


next up previous contents
Next: Ion Beam Techniques Up: Introduction Previous: Direct absorption techniques   Contents
Tim Gibbon
1999-09-06