Abstract
Currently available X-ray and extreme ultraviolet free electron laser (FEL) sources provide intense ultrashort photon pulses. Those sources open new exciting perspectives for experimental studies of ultrafast non-equilibrium processes at the nanoscale in condensed matter. Theoretical approaches and computer simulations are being developed to understand the complicated dynamical processes associated with the interaction of FEL pulses with matter. In this work, we present the results of the application of a simplified three-channel model to the non-equilibrium dynamics of ultrathin aluminum films excited by FEL radiation at 33.3, 37 and 92 eV photon energy. The model includes semi-classical rate equations coupled with the equation of propagation of the photon wave packets. X-ray transmission measurements are found to be in agreement with present simulations, which are also able to shed light on temporal dynamics (in the fs range) in nano-sized Al films strongly interacting with the photon pulse. We also expanded our non-linear model, explicitly including the two-photon absorption cross-section and the effect of including electron heating for reproducing transmission measurements.
Highlights
Over the past decade, X-ray and extreme ultraviolet free electron laser (FEL) sources have been developed, providing a source of extremely brilliant and ultrafast photon pulses
In this work we have shown the results of the application of a simplified three-channel model to the non-equilibrium dynamics of ultrathin aluminum films excited by FEL radiation at 33.3, 37 and 92 eV
Simulations were shown to be able to shed light on temporal dynamics for nano-sized Al films strongly interacting with the photon pulse
Summary
X-ray and extreme ultraviolet free electron laser (FEL) sources have been developed, providing a source of extremely brilliant and ultrafast photon pulses. The importance of developing proper models can be appreciated by looking first at the results of Nagler et al [7], in which saturable soft X-ray absorption (92 eV) of an ultrathin aluminum foil was obtained Those results were followed by other experiments at lower (23.7–37 eV) [8] and higher photon energy (1540–1870 eV) [9], both indicating the importance of accounting for electron heating phenomena at high fluence. Further experiments in the hard X-ray range (7.1 keV) confirmed the existence of important non-linear phenomena with increasing transmission of a factor of 10 and substantial shifts of the absorption edge in solid iron [10] Those results indicate that the description of photon–matter interaction at high intensities requires specific models to be devised, accounting for the various effects contributing to a modification of the X-ray and EUV absorption cross-section (relaxation of final state, ultrafast electron heating, and so on). We show the details of the temporal dynamics of the photon–matter interaction at fs resolution, including possible multi-photon absorption and electron-heating effects
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