Core-hole Clock Methods
The idea behind the core-hole clock method is to use the decay time of a core excitation as a reference clock for other processes in the studied system. Typically, the decay of a core excitation yields a spectral signature that is characteristic for the state of a system. If this state is now altered before the core excitation decays, the spectral signature will be altered accordingly. Averaging over many such processes and relating the temporal evolution of the studied process to the exponential decay of the core excitation with a well-known decay constant enables the access to ultrafast processes. Typical core hole lifetimes in the soft X-ray range are on the order of several femtoseconds and the standard spectroscopic sensitivity allows to detect processes that are maximally one order of magnitude faster or slower than the core hole life time. Therefore, the core hole clock method is very well suited to study processes on the ultrashort time scales ranging from around 100 fs down to the attosecond regime, filling the gap for ultrafast processes that most pump-probe methods cannot address.
In general, the core hole clock method can be applied to all kinds of processes that show a characteristic signature in the decay of the core excitation and that are either persistent in the sample (e.g. electron-phonon scattering processes) or that are induced by the core excitation itself (e.g. charge-transfer processes, wave packet dynamics).