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The irradiation of a finite electronic system by an ultrafast XUV pulse can lead to the ultrafast creation of core-holes with strong subsequent electronic rearrangements. A standard theoretical tool to investigate such excitations that avoids the cost of the simulation of the XUV irradiation is to start the dynamics with an instantaneous core-hole excitation. In a recent series of papers aiming at investigating in some detail the excitation processes induced by a short XUV pulse, we identified an unexpected scenario characterized by the development of large amplitude dipole oscillations [1,2,3]. Once irradiation by a far-off resonance laser pulse is over, the dipole signal normally shrinks down to zero. Total ionization rapidly levels off and stays constant. For a certain range of laser frequencies, we observe that after some time the dipole increases again to reach sizable values, thus generating extra ionization. We presently call this phenomenon a dipole instability as it cannot be mapped to well-known deexcitation mechanisms such as Auger. A dipole instability can be measured on time-resolved photo-electron spectra with the delayed appearance of an extra low energy peak, which could be used for experimental identification of the process. It has been shown by comparing various numerical approaches that this instability is not a numerical problem. But it is not yet excluded that it is a defect of the level of theory used to describe electrons, actually time-dependent density functional theory at the local density approximation level.
[1] P.-G. Reinhard, P.M. Dinh, D. Dundas, E. Suraud, M. Vincendon, On the stability of hole states in molecules and clusters, Eur. Phys. J. Spec. Top. 232 (2023) 2095
[2] P.-G. Reinhard, D. Dundas, P.M. Dinh, M. Vincendon, E. Suraud, Unexpected dipole instabilities in small molecules after ultrafast XUV irradiation, Phys. Rev. A 107 (2023) L020801
[3] D. Hughes, D. Dundas, P.M. Dinh, M. Vincendon, P.-G. Reinhard, E. Suraud, Dipole instability in molecules irradiated by XUV pulses, Eur. Phys. J. D 77 (2023) 177