Experimentally, the time-resolved studies of ultrafast dynamics are built on the quite general “pump-probe” scheme. To study the temporal evolution of an excited system over the characteristic time Δtdyn of the dynamics, one proceeds in two steps:
- first, one induces excitation of the system – of an electronic or nuclear wavepacket in the system - over a time interval Δtexc much shorter than Δtdyn. The excitation step determines time “zero” from which the system evolves under action of internal/intrinsic forces (may be external force, as exerted by field). As a “pump” tool, one should use a steep enough temporal gradient (a physical quantity which significantly varies over time Δtexc). They are ideally provided by light electric fields : either the “envelope” of the ultrashort light pulse, i.e., the variation throughout the pulse of the electric field amplitude averaged over one optical cycle, or the variation of the electric field within one optical cycle, can serve as “pump” gradients.
- second, using a “probe” gradient of Δtprb duration one probes and monitors the time-dependent system at successive instants after excitation, the delay between the “pump” and “probe” gradients being perfectly controlled (with an accuracy better than Δtprb, Δtexc). There is a large variety of physical processes which can act as a “probe”. In the case of a probe ultrashort light pulse of the same form as the above “pump” one, all the channels of elastic and inelastic scattering of light – diffraction, photoemission, absorption – can serve as a probe.
 note that this is not trivial since the duration of the excitation, e.g., using a light pulse, determines the spectral width of the electronic/nuclear wavepacket which is excited and therefore determines partially the characteristic time of its coherent evolution, independently of the dynamical process under study – see, e.g., A. H. Zewail, Chemistry at the Uncertainty Limit, Angew. Chem. 40, 4371 (2001).