Ultrafast soft-mode driven charge relocation in an ionic crystal

Abstract

Transient electron density maps of potassium dihydrogen phosphate (KH₂PO₄, KDP) are derived from femtosecond X-ray powder diffraction patterns. Upon photoexcitation, the low-frequency TO soft mode is elongated impulsively and modulates the electronic charge distribution on the length scale of interatomic distances, much larger than the vibrational amplitude. The results demonstrate a charge transfer from the volumes around the P-atoms and K⁺-ions to those containing the O–H⋯O units and a quadrupolar distortion of the K⁺ charge distribution. This behavior reflects the interplay of nuclear motions and electric polarizations in the ionic crystal lattice.