Abstract
We describe a first-principles NonEquilibrium Green's Function (NEGF) approach to time-resolved photoabsortion spectroscopy in atomic and nanoscale systems. The method is used to highlight a recently discovered dynamical correlation effect in the spectrum of a Krypton gas subject to a strong ionizing pump pulse. We propose a minimal model that captures the effect, and study the performance of time-local approximations versus time-nonlocal ones. In particular we implement the time-local Hartree-Fock and Markovian second Born (2B) approximation as well as the exact adiabatic approximation within the Time-Dependent Density Functional Theory framework. For the time-nonlocal approximation we instead use the 2B one. We provide enough convincing evidence for the fact that a proper description of the spectrum of an evolving admixture of ionizing atoms requires the simultaneous occurrence of correlation and memory effects.
Highlights
Time-resolved photoabsorption spectroscopy is a cutting edge experimental technique to probe quantum systems in nonstationary states [1,2,3,4,5]
We show that to the HF and the M2B approximations the exact adiabatic approximation of Time-Dependent Density Functional Theory (TDDFT) fails in reproducing the absorption peaks of multiply ionized atoms
We argue that the difference between time-local approximations, like HF and M2B, and the 2B approximation in describing the double ionization process is due to the absence of multiple excitations in the formers
Summary
Time-resolved photoabsorption spectroscopy is a cutting edge experimental technique to probe quantum systems in nonstationary states [1,2,3,4,5]. By measuring the energy per unit frequency carried by the transmitted probe as a function of the delay between the pump and probe pulses one obtains the time-dependent photoabsorption spectrum. For optically thin samples (atoms, molecules and more generally nanostructures) the transmitted probe pulse can be expressed in terms of the time-dependent probe-induced change of the dipole moment [34,35,36,37,38,39,40,41] The latter is defined as the difference between the time-dependent dipole moment originating from the pump+probe fields and the time-dependent dipole moment originating from the pump only.
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