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Analytical Chemistry

X-ray pulses yield charge density movies

Technique is complementary to methods that measure electron energies and spins, such as time-resolved electron and X-ray spectroscopies

by Jyllian Kemsley
October 3, 2016 | A version of this story appeared in Volume 94, Issue 39

Taking advantage of molecules’ quantum mechanical ability to exist in two states at once, researchers at SLAC National Accelerator Laboratory have used X-ray pulses to create movies of charge density and nuclear motion in gas-phase iodine molecules with atomic spatial and femtosecond time resolution (Phys. Rev. Lett. 2016, arXiv: 1608.03039). The research team was led by Adi Natan, James M. Glownia, and Philip H. Bucksbaum, who are affiliated with the Stanford PULSE Institute. They excited the iodine molecules with a visible laser pulse to put the molecules into an excited state that exists simultaneously with the ground state. Then they scattered ultrafast X-ray pulses from both states to create a stop-action movie that tracks the molecules’ excited-state charge density and nuclear motion with unprecedented resolution. “This effect is almost always there in ultrafast X-ray diffraction measurements; we just need to look for it,” Natan says, raising the possibility of researchers extracting new information from old experiments. The team also hopes to be able to apply the technique to study photoreactions of small molecules incorporating lighter atoms, such as vitamin protection of DNA and retinal isomerization for vision.

Excited state charge distribution plotted as a function of distance between iodine atoms versus time.
Credit: Phys. Rev. Lett.
A filmstrip from a charge density shows the behavior of excited iodine molecules, with excitation going from maximum in yellow to minimum in dark blue.


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