Theory of x-ray absorption by laser-dressed atoms*

We present an \emph{ab initio} theory for the x-ray photoabsorption cross section of atoms in the field of a moderately intense optical laser ($800 \, \mathrm{nm}$, $10^{13} \, \mathrm{\frac{W}{cm^2}}$). The laser dresses the core-excited atomic states, which introduces a dependence of the cross section on the angle between the polarization vectors of the two linearly polarized radiation sources. The strong interaction due to the laser-dressing is treated by diagonalization of a Floquet-type matrix; the weak coupling between x-rays and the atom is described by non-Hermitian perturbation theory. We apply our theory to study the photoabsorption cross section of neon and krypton atoms near the K~edge. A pronounced modification of the cross section is found in the presence of the optical laser --- \textbf{reference:} arXiv:physics/0611122.