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. A limitation on the versatility of such electronic systems for optical interactions arises from the discrete nature of the electronic transitions and from the ionization threshold that constrains the energy scale of light-matter interactions with bound electrons. We show that shaping electron wavepackets as superpositions of extended states above the ionization threshold enables the control of optical emission 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 towards developing designer emitters at tunable energy and length scales.