Dipole instability after an ultrashort XUV pulse in N₂: population inversion and timescales

We study the response of N₂ to a range of XUV laser pulses using real-time time-dependent density functional theory on real-space grids in order to better understand and quantify the internal processes that lead to dipole instabilities. These instabilities, documented in a series of recent papers, develop following the generation of a population inversion in the molecule, in this case induced by the laser pulse. In the current paper, a series of laser pulses having durations of one femtosecond are considered and we explore how the growth rate, total ionization and amount of population inversion changes. This reveals a new aspect to the instability in which the growth rate starts decreasing with increasing pulse intensity. In addition, by using a range of different pseudopotential descriptions of the electron-ion interactions, we find that the observed behaviour is qualitatively independent of the pseudopotential used.