We study the quasi-instantaneous change of electron density in the unit cells of LiH and ${\mathrm{NaBH}}_4$ in response to a nonresonant strong optical field. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The light-induced charge relocation in ${\mathrm{NaBH}}_4$ exhibits an electron transfer from the anion (${\mathrm{BH}}_4^{-}$) to the ${\mathrm{Na}}^{+}$ cation. In contrast, LiH displays the opposite behavior, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This behavior originates from strong electron correlations in LiH, as is evident from a comparison with quasiparticle band structures calculated within the Coulomb-hole-plus-screened-exchange formalism.

• Received 18 June 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.217401