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 of the nuclear motion. The results demonstrate a charge transfer from the volumes around the P-atoms 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.
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