DC and subcycle-resolved AC Stark shifts in Helium

We are developing a finite element discrete variable representation (FE-DVR) code to model the response of two-electron atoms to ultra-short pulses of EM radiation. Our first numerical results for the DC stark shift of helium deviate significantly from previous [1] single-active-electron (SAE), but are in close agreement with improved SAE calculations that include the effect of core polarization in the external field. For 3x10$^{14}$ W/cm$^{2}$ infra red fields, we calculate sub-IR-cycle- resolved instantaneous (AC) level shifts of low-lying bound He states that also strongly deviate from the SAE prediction [1]. We plan to apply our code to model recently measured subcycle time-resolved absorption spectra [2].\\[4pt] [1] F. He, C. Ruiz, A. Becker, and U. Thumm, J. Phys. B \textbf{44}, 211001 (2011).\\[0pt] [2] H. Wang, M. Chini, S. Chen, C.-H. Zhang, F. He, Y. Cheng, Y. Wu, U. Thumm, and Z. Chang, Phys. Rev. Lett. \textbf{105}, 143002 (2010); M. Chini, Z. Chang \textit{et al.}, to be published.