On the basis of the quantum-statistical density-matrix equation [H.K.,
J. Phys. B 43, 115601 (2010)], we simulate ultrafast kinetics
of electron excitation and relaxation in a metallic target irradiated
by an intense X-ray free-electron laser (XFEL) pulse. Numerical
results for metallic lithium demonstrate that a near-edge multiple
photoexcitation of core electrons to the conduction band gives rise to
a lowering of core energy levels, which induces a blue shift of the
absorption edge and a reduction of the absorption coefficient within
approximately 10 fs; the Auger decay and Coulomb explosion can safely
be neglected during such a short timescale. In light of these
observations, we investigate the possibility of nonlinear transmission
of an XFEL pulse through a thin solid target, which was detected in
recent VUV laser experiments [H. Yoneda et al., Opt. Express
17, 23443 (2009)].
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