Photoionisation occurs whenever the photon energy is greater than the binding energy of the weakest bound atomic electron. At energies greater than this ionisation threshold, the photon is absorbed and the excess energy is simply the kinetic energy of the escaping electron:

These processes are described as ‘direct’ ionisation as, once you are above the state threshold, ionisation can occur at all photon energies.

In addition to the direct process there is the ‘indirect’ or ‘resonant’ process, which only occurs at specific photon energies. This corresponds to two (or more) electrons being excited to vacant orbitals so producing a excited neutral state. This temporary state is energetically above the ionisation threshold so the system generally decays quickly by the ejection of an electron - leaving a ground or excited state ion.

Generally, the excited neutral states decay selectively (i.e subject to certain angular momentum constraints) to all the energetically available ionic states. At resonance energies there are therefore two ways of producing the final ionic state. These two ionisation routes interfere with each other producing characteristic constructive or destructive interference patterns. This is analogous to the Young’s double slit experiment in optics. If, for some reason, the excited ionic state has virtually no direct photoionisation route then the indirect mechanism is the only available (or dominant) excitation route and one observes excitation ‘peaks’ rather than interference patterns.

Although indirect photoionisation can only occur at specific photon energies, the net indirect contribution is a significant part of the total photoionisation cross section. Therefore, apart from being interesting quantum mechanically, it is an important mechanism affecting, for example. upper atmosphere physics and chemistry.