Shaping long-lived electron wavepackets for customizable optical spectra

Electrons in atoms and molecules are versatile physical systems allowing a vast range of light–matter interactions. Spontaneous emission, which appears in a wide variety of applications, depends crucially on the bound electron energy levels. The discrete nature of these electron energy levels and the ionization threshold constrain the energy scale of all light–matter interactions involving bound electrons. To bypass these constraints, we take ideas from optical and electronic beam shaping and propose creating new electron states as superpositions of extended states above the ionization threshold. We show that such superpositions enable the control of spontaneous emission with tunable spectra in the eV–keV range. We find that the specific shaping lengthens the diffraction and radiative lifetimes of the wavepackets in exchange for increasing their spatial spreads. Our approach could have applications toward developing novel kinds of light emitters at hard-to-access spectral ranges.