Femtosecond transparency in the extreme-ultraviolet region

Electromagnetically-induced-transparency-like behavior in the extreme-ultraviolet region is studied theoretically, including the effect of intense 800 nm laser dressing of He 2s2p(¹-0.16em0exPᵒ) and 2p²{(}¹{{-0.16em}{0ex}}{S}ᵉ)$ autoionizing states. We present an ab initio solution of the time-dependent Schr\"odinger equation in an $LS$-coupling configuration-interaction basis set. The method enables a rigorous treatment of optical field ionization of these coupled autoionizing states into the $N=2$ continuum in addition to $N=1$. Our calculated transient absorption spectra show encouraging agreement with experiment.