Above-ionization synchrotron-excited silicon-based nanograins by x-ray and EUV radiation: Implications to grain stability and shielding in planetary nebulae

We use silicon-based nanograins as model nanodust in planetary nebulae and present photo-excitation and stability studies using synchrotron extreme ultraviolet radiation, while monitoring the induced cascade of visible/UV luminescence. We also conduct theoretical studies (atomistic simulations and classical Mie scattering), as well as stability studies of the grains under long-wavelength UV excitation using lasers or discharge lamps or under thermal treatments. We report that the luminescence of 1-nm grains remains stable for above ionization limit excitation [5-22 eV, 10¹² (photons/cm²)/s]. Under excitation below the ionization limit, using lasers or discharge lamps (3.5 eV, 10¹⁴/cm²/s) or under thermal treatment, the luminescence exhibits partial stability at a steady ∼50%, with slow partial recovery. Time-dependent density functional theory shows the structural stability of neutral or ionized ultrasmall nanograins, while organic dye molecules are fully quenched with no recovery. Computations also show the enhancement of scattering of soft x rays over the geometrical cross section. We analyze the results in terms of quantum confinement induced effects, including inhibition of e-h and e-Coulomb scattering, enhancement of e-e correlation, and relativistic e-vibration coupling. These effects lead to multi-electron excitation, singlet-triplet intersystem conversion, and plasmon-type Mie “polarizmon scattering” by valence electrons. Such novel characteristics point to the survivability of ultrasmall grains in x-ray or UV environments, which may serve as a UV shield for large interstellar molecules, necessary to life.