Theoretical mechanism of inner-shell resonant absorption effects

A Chinese group recently published a paper proposing a new interpretation of neon's absorption of extremely strong X-ray photons from an X-ray free electron laser, which was experimentally studied at Stanford in 2010 (see, L. Young et al., Nature, 466, 56 (2010)). Although the ordinary absorption edge of neon is around 867 eV, the energy becomes higher than usual because of multiple ionization. Therefore, detailed studies were done between 800 eV and 2000 eV at Stanford at that time. The main discussion here is the large discrepancies between theory and experiment found at 1050 eV, where the rates of K-shell absorption 1s → 4p of Ne⁶⁺ and 1s → 3p of Ne⁷⁺ are larger than the direct single-photon ionization rates by more than one order of magnitude. The authors of this paper propose that the inner-shell resonant absorption (IRA) effects be considered as the mechanism. They showed that resonant photopumping of K-shell electrons to the L, M, or even higher bound orbitals can provide an interaction strength that is two or three orders of magnitude larger than that in the continuum level. Only when the IRA effects were taken into account were the observed charge state distributions explained well. For more information, see the paper, "Inner-shell resonant absorption effects on evolution dynamics of the charge state distribution in a neon atom interacting with ultraintense x-ray pulses", W. Xiang et al., Phys. Rev. A86, 061401(R) (2012).