Dipole-forbidden atomic transitions induced by superintense x-ray laser fields

Abstract

A hydrogen atom, initially prepared in the $2s$ and/or $2p(m=\ifmmode\pm\else\textpm\fi{}1)$ states, is assumed irradiated by $0.8\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$ ($1.5$ nm) photons in pulses of $1--250$ fs duration and intensities in the range ${10}^{20}$ to ${10}^{23}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. Solving the corresponding time-dependent Schr\odinger equation from first principles, we show that the ionization and excitation dynamics of the laser-atom system is strongly influenced by interactions beyond the electric dipole approximation. A beyond-dipole two-photon Raman-like transition between the $2s$ and $2p(m=\ifmmode\pm\else\textpm\fi{}1)$ states is found to completely dominate the underlying laser-matter interaction. It turns out that the large difference in the ionization rates of the $2s$ and $2p(m=\ifmmode\pm\else\textpm\fi{}1)$ states is important in this context, effectively leading to a symmetry breaking in the corresponding (beyond-dipole) bound-bound dynamics with the result that a net population transfer between the states occurs throughout the laser-matter interaction period. Varying the x-ray exposure time as well as the laser intensity, we probe the phenomenon as the bound wave packet oscillates between having $2s$ and $2p(m=\ifmmode\pm\else\textpm\fi{}1)$ character, eventually giving rise to a Rabi-like oscillation pattern in the populations.