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.