Two-photon Ionization of Helium using the Complex Kohn Variational Method

The complex Kohn variational method [1,2] is extended to compute light-driven electronic transitions between continuum wavefunctions of an atomic or molecular system. This development enables the treatment of multiphoton processes in the perturbative regime. We present a proof of principles on two-photon ionization of ground and excited states of Helium induced by combining extreme ultra-violet (XUV) and near infrared (NIR) fields. The XUV pulse is tuned near the He($2s2p$)$^1P^o_1$ Feshbach resonance and the photoionization spectrum is compared with time-dependent calculations. The method is general and could for instance be used to study photoionization time-delay RABBITT experiments. [1] B. I. Schneider and T. N. Rescigno, Phys. Rev. A {\bf 37} 3749 (1988), [2] T. N. Rescigno, B. H. Lengsfield III, and C. W. McCurdy, {\it Modern Electronic Structure Theory 1} (World Scientific, Singapore, 1995).