Theory and computation of the attosecond dynamics of pairs of electrons excited by high-frequency short light pulses

By defining and solving from first principles, using the state-specific expansion approach, a time-dependent pump-probe problem with real atomic states, we show computationally that, if time resolution reaches the attosecond regime, strongly correlated electronic "motion" can be probed and can manifest itself in terms of time-dependent mixing of symmetry-adapted configurations. For the system that was chosen in this study, these configurations, the He 2s2p,2p3d, and 3s3p P-1(o), whose radials are computed by solving multiconfigurational Hartree-Fock equations, label doubly excited states (DES) of He inside the 1sepsilonp P-1(o) scattering continuum and act as nonstationary states that mix, and simultaneously decay exponentially to 1sepsilonp P-1(o) via the atomic Hamiltonian, H-A. The herein presented theory and analysis permitted the computation of attosecond snapshots of pairs of electrons in terms of time-dependent probability distributions of the angle between the position vectors of the two electrons. The physical processes were determined by solving ab initio the time-dependent Schrodinger equation, using as initial states either the He 1s(2) or the 1s2s S-1 discrete states and two femtosecond Gaussian pulses of 86 fs full width at half-maximum, having frequencies in resonance with the energies of the correlated states represented by the 2s2p and 2p3d configurations. We calculated the probability of photoabsorption and of two-photon resonance ionization and of the simultaneous oscillatory mixing of the configurations 2s2p,2p3d,3s3p, and 1sepsilonp P-1(o), within the attosecond scale, via the interactions present in H-A. Among the possible channels for observing the attosecond oscillations of the occupation probabilities of the DES, is the de-excitation path of the transition to the He 1s3d D-1 discrete state, which emits at 6680 Angstrom.