Publications by authors named "Ankush Mitra"

9 Publications

  • Page 1 of 1

Femtosecond X-ray induced changes of the electronic and magnetic response of solids from electron redistribution.

Nat Commun 2019 11 21;10(1):5289. Epub 2019 Nov 21.

SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA, 94025, USA.

Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. With increasing X-ray intensity, the X-ray absorption strength is expected to become nonlinear. Here, we report the onset of such a nonlinearity in the resonant X-ray absorption of magnetic Co/Pd multilayers near the Co L[Formula: see text] edge. The nonlinearity is directly observed through the change of the absorption spectrum, which is modified in less than 40 fs within 2 eV of its threshold. This is interpreted as a redistribution of valence electrons near the Fermi level. For our magnetic sample this also involves mixing of majority and minority spins, due to sample demagnetization. Our findings reveal that nonlinear X-ray responses of materials may already occur at relatively low intensities, where the macroscopic sample is not destroyed, providing insight into ultrafast charge and spin dynamics.
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http://dx.doi.org/10.1038/s41467-019-13272-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6872582PMC
November 2019

Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering.

J Phys Chem Lett 2018 Jun 14;9(12):3538-3543. Epub 2018 Jun 14.

Institut für Physik und Astronomie , Universität Potsdam , 14476 Potsdam , Germany.

Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time-resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal-ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. π-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences σ-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes.
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http://dx.doi.org/10.1021/acs.jpclett.8b01429DOI Listing
June 2018

Chemical Bond Activation Observed with an X-ray Laser.

J Phys Chem Lett 2016 Sep 2;7(18):3647-51. Epub 2016 Sep 2.

DESY Photon Science , 22607 Hamburg, Germany.

The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free-electron laser to directly observe the decreased bonding-antibonding splitting following bond-activation using an ultrashort optical laser pulse.
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http://dx.doi.org/10.1021/acs.jpclett.6b01543DOI Listing
September 2016

Femtosecond X-ray magnetic circular dichroism absorption spectroscopy at an X-ray free electron laser.

Rev Sci Instrum 2016 Mar;87(3):033110

SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.

X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L(3,2)-edges. The high brightness of the X-ray free electron laser combined with high accuracy detection of incident and transmitted X-rays enables ultrafast X-ray magnetic circular dichroism studies of unprecedented sensitivity. This new capability is applied to a study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature.
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http://dx.doi.org/10.1063/1.4944410DOI Listing
March 2016

Indirect excitation of ultrafast demagnetization.

Sci Rep 2016 Jan 6;6:18970. Epub 2016 Jan 6.

Sorbone Universités, UPMC Univ Paris 06, UMR 7614, LCPMR, 75005 Paris, France.

Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions.
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http://dx.doi.org/10.1038/srep18970DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4702181PMC
January 2016

Combining THz laser excitation with resonant soft X-ray scattering at the Linac Coherent Light Source.

J Synchrotron Radiat 2015 May 11;22(3):621-5. Epub 2015 Apr 11.

Linac Coherent Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.

This paper describes the development of new instrumentation at the Linac Coherent Light Source for conducting THz excitation experiments in an ultra high vacuum environment probed by soft X-ray diffraction. This consists of a cantilevered, fully motorized mirror system which can provide 600 kV cm(-1) electric field strengths across the sample and an X-ray detector that can span the full Ewald sphere with in-vacuum motion. The scientific applications motivated by this development, the details of the instrument, and spectra demonstrating the field strengths achieved using this newly developed system are discussed.
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http://dx.doi.org/10.1107/S1600577515005998DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4416678PMC
May 2015

The Soft X-ray Research instrument at the Linac Coherent Light Source.

J Synchrotron Radiat 2015 May 2;22(3):498-502. Epub 2015 Apr 2.

Linac Coherent Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.

The Soft X-ray Research instrument provides intense ultrashort X-ray pulses in the energy range 280-2000 eV. A diverse set of experimental stations may be installed to investigate a broad range of scientific topics such as ultrafast chemistry, highly correlated materials, magnetism, surface science, and matter under extreme conditions. A brief description of the main instrument components will be given, followed by some selected scientific highlights.
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http://dx.doi.org/10.1107/S160057751500301XDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4416666PMC
May 2015

The Atomic, Molecular and Optical Science instrument at the Linac Coherent Light Source.

J Synchrotron Radiat 2015 May 17;22(3):492-7. Epub 2015 Apr 17.

Linac Coherent Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.

The Atomic, Molecular and Optical Science (AMO) instrument at the Linac Coherent Light Source (LCLS) provides a tight soft X-ray focus into one of three experimental endstations. The flexible instrument design is optimized for studying a wide variety of phenomena requiring peak intensity. There is a suite of spectrometers and two photon area detectors available. An optional mirror-based split-and-delay unit can be used for X-ray pump-probe experiments. Recent scientific highlights illustrate the imaging, time-resolved spectroscopy and high-power density capabilities of the AMO instrument.
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http://dx.doi.org/10.1107/S1600577515004646DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4416665PMC
May 2015
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