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In this paper, we presented a model, describing transition scattering of ultrashort laser pulses (USLPs) in Maxwellian plasmas in terms of the full spectral-angular probability during the entire time of pulse action. We demonstrated that in the case of the USLP, probability dependence on pulse duration (τ-dependence) generally is a non-monotonic function. The main trends of the τ-dependence were defined. For quasi-elastic transition scattering, we derived analytical formulas of the full spectral-angular probability that describes non-monotonic areas of the τ-dependence with high accuracy. In the framework of the analytical approach, we established conditions, at which non-monotonic trends are realized, and assessed dynamics of local extremes at the variation of problem parameters. We compared τ-dependence, angular dependence of transition, and Thomson scattering probability and assessed limits of analytical approximations. The crucial similarities and differences of these two types of scattering were described.

The standard approach to calculate the Thomson scattering probability is reconsidered for the case of ultrashort incident laser pulses (USLPs). We established a new model for the interaction of USLP with plasmas that is based on Fermi's equivalent photon conception to calculate the spectral-angular differential Thomson scattering probability. The simulations demonstrate that the scattering probability for USLP is a non-monotonic function of pulse duration in contrast to the standard long-pulse model showing linear dependence. An analytical approach is developed to study the nonlinear behavior of the scattering probability as a function of pulse duration and other parameters.

X-ray free electron laser (XFEL) interaction with solids has been simulated to resolve simultaneously variable XFEL photon energy and x-ray spectral distribution of the target emission (2D-maps). It is discovered that the highly transient charge state distribution exhibits a characteristic target response due to the action of the sharply rising radiation field. Finally, we identify advantageous features for studies of dense plasma effects of two K-shell vacancy hollow ion x-ray emission excited via resonance excitation. These features and characteristics permit the global study of dense plasma effects via the simulation of the time-integrated joint distribution of pumped and fluorescence energies. It is shown that the simulation of these specific 2D-maps offers a global vision of the complex interplay between different processes or phenomena such as photoionization, resonance excitation, or ionization potential depression.

A study on the interaction of a 10 TW, 60 fs, Ti–Sapphire laser with a high-density long-scale pulsed nitrogen gas jet is reported. Experimental data on the laser propagation are analyzed with the help of a ray-tracing model. The plasma dynamics is investigated by means of time-resolved shadowgraphy and time-integrated high-resolution x-ray spectroscopy. Shadowgrams show that the plasma does not expand during the first 55 ps, while x-ray spectra exhibit an unusual continuum-like structure attributed to hollow atoms produced by charge exchange process between bare nuclei expelled from the plasma and molecules of the surrounding gas. The interpretation of the results is supported by particle-in-cell simulations. The question of x-ray lasing is also examined using a hydrodynamic code to simulate the long lasting regime of recombination.