# Secondary process detection

## Definition

It is experimentally often favourable to use methods other than transmission detection to obtain the linear attenuation coefficient $\mu(E)_{PE}$ or $\mu(E)$. This requires recording a signal that arises from a process that occurs with a probability that is proportional to $\mu_{PE}$. This can be the direct photoemission channel. The core hole that is created in the photoabsorption process decays with a lifetime $\tau$. The energy that is released in the decay or secondary process can either be carried by an outgoing electron (e.g. Auger) or a photon (fluorescence). Weak processes, such as excitations of phonons, are neglected here. Detection of the outgoing electrons of all kinetic energies is called Total Electron Yield (TEY) and that of all photons of all energies Total Fluorescence Yield (TFY).

If the secondary process detection is realized with an energy or wavelength dispersive instrument it is possible to further discriminate between the decay channels, e.g. only the $K\alpha$ fluorescence lines. The techniques are then referred to as partial yield detection. An instrumental resolution in the secondary process detection that is on the order of the core hole lifetime broadening or even below may enable one to observe resonance phenomena in the decay channel (e.g. resonant inelastic X-ray scattering, resonant Auger spectroscopy).

Detection of the intensity of a secondary process as a function of the incident energy that is tuned across an absorption edge may be proportional to $\mu_{PE}$ to sufficient accuracy. This assumption is the prerequisite for all secondary process detection schemes which aim to measure the absorption cross section. This may be a good approximation when the dominant decay channel is chosen for the secondary process detection, for example, the fluorescence lines in the hard X-ray range when not detected in high-resolution mode (fluorescence-detected absorption spectroscopy).

Electron yield detection is surface sensitive due to the short mean free path of the electrons.