Analysing a two-electron wavepacket by semiclassically propagating its Fourier components in space

In the last few years, the development of high order harmonic generation sources and free electron lasers delivering ultra-intense and ultra-short VUV-XUV pulses has made it possible to study nonlinear processes in atoms and molecules on the electronic time scale. The theoretical support required by the ongoing experiments comes notably in the form of numerical tools intended to solve the time-dependent Schrodinger equation. The wavepacket produced in these approaches has a multichannel character and its analysis in terms of the observed physical channels is a problem in itself. Various solutions have been proposed so far, which all suffer from one or another inconvenience, ranging from very heavy computational costs to the inability to characterize differential cross sections. The purpose of this paper is to propose a new, low-cost and complete method of analysis. It consists in propagating the Fourier components of the wavepacket with respect to the hyper-radius all the way to the genuine asymptotic region where the various channels disentangle from each other based on their kinematics. We demonstrate the feasibility and versatility of this proposal by applying it to two different time-propagation codes in the case of one-photon double ionization of helium using short pulses.