Slow and fast multi-photon ionization of clusters in strong XUV and X-ray pulses

The photo-electron spectrum resulting from multi-photon absorption of an extended target, such as an atomic cluster or a large molecule, from an intense laser pulse with photon energies larger than the ionization potential of the atomic constituents is discussed. We develop an approximate analytical framework and provide simple analytical expressions for the shape of the photo-electron spectrum in the limit of sequential and parallel ionization, realized by long and short pulses, respectively. The width of the spectrum provides valuable information about the absorbed photons of the target in relation to its extension.

Highlights► Microscopic calculations of electron spectra as a function of the pulse length. ► Analytical expression for the spectra in the limits of parallel and sequential emission. ► Discussion of the absolute width of the spectra for experimental diagnostics.