Magenta dashed lines with arrows illustrate diffraction of hard femtosecond x-ray pulses off the lattice planes of the Bi crystal. (IMAGE)
Caption
Magenta dashed lines with arrows illustrate diffraction of hard femtosecond x-ray pulses off the lattice planes of the Bi crystal. Red balls connected by red lines: unit cell of an unexcited bismuth crystal containing two Bi atoms with one atom at its origin. The second atom is shown as green balls in panel (b) and is indicated as small balls in panels (c) and (d). Blue balls connected by blue lines: unit cell of the photo-excited crystal with reduced symmetry containing four Bi atoms. (c) Orange curve: electric field of the optical excitation pulse. Weak and/or short-wavelength pulses can only excite coherent phonons with identical motions in all unit cells indicated by light-red balls and arrows. (d) Strong excitation with femtosecond mid-infrared pulses reduces the crystal symmetry and allows for opposite atomic motions (light-blue balls and arrows) in adjacent unit cells.
Credit
MBI/M. Runge
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