Study of the nonlinear interactions of atomic autoionizing states with ultrashort intense EUV pulses

We study theoretically the formation and decay of atomic autoionizing states by intense ultrashort EUV pulses. In the case of the $2s2p(^1P^o)$ resonance in helium, we examined how the electron yields and the shape of the electron spectra (the Fano $q$-parameter) change with the pulse intensity and pulse duration. Results obtained using a theory generalized to short pulses from the earlier models for long pulses [1,2] are compared to \textit{ab initio} calculations based on the numerical solution of the time-dependent Schr\"{o}dinger Equation for the two-electron helium. In view of the emerging intense light pulses from free-electron lasers, the theory will be used to evaluate the condition for observing nonlinear near-resonance photoabsorption for EUV and X-rays.\\[4pt] [1] P. Lambropoulos and P. Zoller, Phys. Rev. A 24, 379 (1981).\\[0pt] [2] L. B. Madsen and P. Lambropoulos, J. Phys. B: At. Mol. Opt. Phys. 34, 1855 (2001).