Multicomponent dynamics of coupled quantum subspaces and field-induced molecular ionizations.

To describe successive ionization steps of a many-electron atom or molecule driven by an ultrashort, intense laser pulse, we introduce a hierarchy of successive two-subspace Feshbach partitions of the N-electron Hilbert space, and solve the partitioned time-dependent Schrödinger equation by a short-time unitary algorithm. The partitioning scheme allows one to use different level of theory to treat the many-electron dynamics in different subspaces. We illustrate the procedure on a simple two-active-electron model molecular system subjected to a few-cycle extreme Ultra-Violet (XUV) pulse to study channel-resolved photoelectron spectra as a function of the pulse's central frequency and duration. We observe how the momentum and kinetic-energy distributions of photoelectrons accompanying the formation of the molecular cation in a given electronic state (channel) change as the XUV few-cycle pulse's width is varied, from a form characteristic of an impulsive ionization regime, corresponding to the limit of a delta-function pulse, to a form characteristic of multiphoton above-threshold ionization, often associated with continuous-wave infinitely long pulse.