Scientists using the Linac Coherent Light Source at SLAC National Accelerator Laboratory can localize excitation in a molecule and follow the subsequent ultrafast energy redistribution with atomic precision. The team produced pairs of ultrafast x-ray pulses of slightly different frequencies and hit the molecule with a one-two punch of x-rays, just quadrillionths of a second apart. By exciting an electron at one atomic site within the molecule and recording snapshots at another site, researchers obtain insights as to how excess energy flows within the molecule and causes it to break up.
This approach could provide a new understanding of ultrafast dynamics in a range of systems. For example, it could allow scientists to track the structural changes that occur in light-sensitive molecules, nanoparticles, and materials that are being developed to harvest solar energy.
New capabilities at x-ray free-electron lasers (XFELs), such as the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, enable the generation of two intense, femtosecond-duration x-ray pulses with different energies and controlled time delays (there are 1,000,000,000,000,000 femtoseconds in 1 second). Researchers can exploit this capability to selectively excite molecules at specific atomic sites and then follow the x-ray-induced dynamics at other sites. As a first application of the method, researchers studied the response of xenon difluoride molecules to x-ray absorption. An x-ray photon from the first "pump" pulse ejected an inner-shell electron from the xenon atom. The loss of the electron triggered a decay process in which several additional electrons were ejected, including delocalized valence electrons that are responsible for the chemical bonding between xenon and the fluorine atoms. Without these electrons to hold the molecule together, mutual repulsion of the neighboring atoms caused the molecule to dissociate into separate atomic ions. After time delays in the 4- to 54-femtosecond range, an x-ray photon of a slightly different frequency from the second "probe" pulse was absorbed by one of the emerging fluorine ions, increasing its charge. The scientists measured the charge and momentum of each of the fragment ions and used this information in conjunction with theoretical simulations to determine the ionization and dissociation pathways. The results provide a fundamental understanding of the underlying mechanism.
Stephen H. Southworth
Argonne National Laboratory
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under contract DE-AC02-06CH11357. Use of the Linac Coherent Light Source, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract DE-AC02-76SF00515.
A. Picón, C.S. Lehmann, C. Bostedt, A. Rudenko, A. Marinelli, T. Osipov, D. Rolles, N. Berrah, C. Bomme, M. Bucher, G. Doumy, B. Erk, K.R. Ferguson, T. Gorkhover, P.J. Ho, E.P. Kanter, B. Krässig, J. Krzywinski, A.A. Lutman, A.M. March, D. Moonshiram, D. Ray, L. Young, S.T. Pratt, and S.H. Southworth, "Hetero-site-specific x-ray pump-probe spectroscopy for femtosecond intramolecular dynamics." Nature Communications 7, 11652 (2016). [DOI: 10.1038/ncomms11652]
Argonne National Laboratory press release: New X-Ray Methods Allows Scientists to Probe Molecular Explosions