A crystal is a regular, periodic arrangement of atoms or ions held together by forces between their electrons. The atomic nuclei can make oscillations around their equilibrium positions. The spatial displacement of nuclei in such vibrations is much smaller than the distance between adjacent atoms. Nevertheless, the vibrational motion has a retroactive effect on the electrons by modulating their spatial distribution and thus changing the electronic and optical properties of the crystal. These processes run on a time scale well below one picosecond (1 ps = 10-12 s). In order to understand and apply such effects, for example in acousto-optic devices, a direct representation of the filigree interaction between core and electron movements on the subpicosecond timescale is desirable.
Extremely small atomic motions are recorded using ultrashort X-ray flashes
Fig. 1: In an X-ray absorption experiment, light excites a strongly bound core electron in a conduction band state of the crystal, as shown on the left side of the figure. The core electron of the Li atom (green wave function) is excited into the conduction band (red wave function), which interacts with both the Li core and the borohydride group. This state is very sensitive to changes in the distance between the anions and cations (see also Fig. 2 (b) and 3 (d) in the main article). On the right side you can see the lithium K-edge X-ray absorption spectrum for different, exaggerated vibration excitations.
Researchers at the Max Planck Institute in Berlin (Germany), Empa (Swiss Federal Laboratories for Materials Science and Technology in Dübendorf (Switzerland)) and the National Institute of Standards report in the latest issue of Physical Review B (Rapid Communication) and Technology, Gaithersburg (USA) on a novel experiment that allows one to excite coherent atomic vibrations in small LiBH4 crystals, and on the other hand to read them out by means of the modified X-ray absorption [Fig. 1.] In the experiments, an optical light pulse (wavelength 800 nm) excited an optical phonon by means of impulsive Raman scattering [movie]. The atomic movements of this vibration periodically change the distances between Li + and (BH4) - ions. These distance changes in turn modulate the spatial distribution of the electrons in the crystal and thus the X-ray absorption spectrum Li + ions. In this way, the atomic vibrations transform into an oscillatory modulation of the X-ray absorption at the so-called Li K edge at photon energies of 60 eV. Ultra-short X-ray flashes measure the changes in X-ray absorption at different delay times between excitation and scanning pulses. The atomic movements can then be reconstructed from this series of snapshots.
What happens in the unit cell of LiBH4 crystals after an impulsive Raman excitation with a femtosecond laser pulse? Upper partial image: Measured, transient absorption change Δ A (t) (symbols) as a function of the delay time between infrared excitation light pulses and scanning pulses in the soft X-ray region at photon energies of ħω = 61.5 eV [see also Fig. 3 (a) in the main article] , The lower box shows the atoms in the unit cell of LiBH4 crystals with red boron atoms, gray hydrogen atoms, and green lithium atoms. The moving blue dot in the upper field is synchronized with the moving atoms in the lower box. The amplitude of the movement is oversubscribed by a factor of 30000 to visualize the concerted movement. The reddish color of the unit cell shows the intensity of the infrared excitation light pulses during the momentum overlap.
The new experimental concept is extremely sensitive and allowed for the first time to initiate and measure atomic vibrations with extremely small amplitudes. In the present case, the Li + ions moved only a distance of 3 femtomes = 3 x 10-15 m, a length which corresponds approximately to the diameter of a Li + atomic nucleus. This distance is thus 100,000 times smaller than the distance between the ions in the crystal. The experimental observations are in excellent agreement with a detailed theory of X-ray absorption. This new method on the femtosecond timescale holds a promising potential to map and understand the interaction between nuclear and electron motion in condensed matter, an essential prerequisite for more advanced theories and applications in different technologies.