How to overtake electron-vibrating atomic nuclei - the X-ray film

Researchers at the Max Born Institute in Berlin followed the spatial vibrational motion of electrons in a crystal in real time by turning a film using ultrashort X-ray flashes. The outer electrons move back and forth on the length scale of a chemical bond and thus modulate the electrical properties, while the inner electrons and the atomic nuclei move only by 1% of this distance.

A crystal consists of a regular arrangement of atoms in space, also called crystal lattice, which is held together by the mutual electrostatic attraction of the electron clouds of neighboring atoms. Most of the electrons are strongly bound to an individual, positively charged nucleus. The outermost electrons of an atom are called valence electrons and build up the bond to neighboring atoms. These bonds determine the atomic spacing in the crystal as well as essential properties, such as its electrical conductivity or mechanical stability.

The atoms in a crystal lattice are not at rest, but vibrate about their respective equilibrium position. The spatial displacement of the movement of the atomic nuclei together with their electrons in the inner shells is typically only one percent of the distance between the atoms. How the outer valence electrons behave during this lattice vibration was not yet clear and the magnitude of their displacement was completely unknown. Direct measurement of this movement in real time is very important for a basic understanding of the static and dynamic electrical properties of the crystal.

Fig. 1 (A) Unit cell of the KDP crystal [yellow spheres: phosphorus atoms (P), pink: potassium (K), red: oxygen (O), white: hydrogen. (B) Electron density "map" in the drawn rectangle ρ0 (r) before the laser excitation. The black lines indicate the boxes for different atoms, in which the charge quantity and the center of gravity of the charge cloud are measured. (C) and (D) change in charge density after laser excitation (red: charge increase, blue: decrease). (E) Positions of the atoms in this plane and the charge exchange between phosphorus and oxygen. The electron cloud of the potassium atom shows distortions between a cigar or pancake shape.

To clarify this open question, Flavio Zamponi, Philip Rothhardt, Johannes Stingl, Michael Wörner and Thomas Elsässer have built an X-ray reaction microscope, which allows a recording of the electron movement in real time in a crystal. As reported in the latest issue of the journal PNAS (doi: 10.1073 / pnas.1108206109), lattice vibrations in a potassium dihydrogen phosphate (KDP) crystal are triggered by a laser flash lasting only 50 femtoseconds (1 fs = 10^-15 seconds) , The momentary positions of the atoms and electrons are measured with high spatial resolution using 100 fs X-ray flashes, which are diffracted by the vibrating atoms. X-ray photos shot at different times after the start of the oscillation together form the desired X-ray film.

It was a big surprise for the researchers from Berlin that after excitation of a special vibration in KDP, the so-called "soft" oscillation (soft mode), the outer valence electrons moved about 30 times greater distance during the oscillation than the oscillation Atomic nuclei and their electrons in the inner shells. This behavior can be observed directly in the electron density "maps" in Figure 1. During soft-mode oscillation, an electron originally located on the phosphorus (P) atom moves to one of its oxygen (O) neighbors (PO bond length: 160 picometers (10^-12m)) and returns to P after half an oscillation period Back. Surprisingly, the atoms involved move only a few picometers, in stark contrast to textbook knowledge, according to which one expects a common movement of all the electrons of an atom with its nucleus. The surprisingly wide movement of the valence electrons can be understood by means of the electrostatic forces exerted by the vibrating ion crystal lattice on the electrons during soft-mode oscillation. In the 1960s, theories were already being developed that predicted such behavior. Now finally the experimental proof has been achieved. The attached movie shows the iso-electron density surface of potassium ion and phosphate ion during soft-mode oscillation in KDP.

The newly developed powder method of femtosecond X-ray diffraction can be applied to many other systems to map ultrafast chemical and physical structure changes.

Explanation of the film:

Surface of constant electron density r (r, t) = 6000 e- / nm³ in the range of the potassium ion (sphere) and the phosphate ion. The phosphorus atom is in the middle and the 4 oxygen atoms are located outside in the corners of a tetrahedron. With the delay time t = 0, the soft-mode oscillation is excited and the electronic charge flows back and forth between P and O. As a smaller effect one sees the distortion of the electron cloud of the potassium atom between a cigar or pancake form.

Original publication

Ultrafast large-amplitude relocation of electronic charge in ionic crystals

F. Zamponi, P. Rothhardt, J. Stingl, M. Woerner, T Elsaesser

Proceedings of the National Academy of Sciences of the United States of America 109 (2012) 5207-5212

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