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Journal of Chemical Physics / Volume 133 / Issue 6 / ARTICLES / Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
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J. Chem. Phys. 133, 064509 (2010); http://dx.doi.org/10.1063/1.3469779 (8 pages)

Concerted electron and proton transfer in ionic crystals mapped by femtosecond x-ray powder diffraction

Michael Woerner, Flavio Zamponi, Zunaira Ansari, Jens Dreyer, Benjamin Freyer, Mirabelle Prémont-Schwarz, and Thomas Elsaesser

Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany

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(Received 3 June 2010; accepted 7 July 2010; published online 13 August 2010)

X-ray powder diffraction, a fundamental technique of structure research in physics, chemistry, and biology, is extended into the femtosecond time domain of atomic motions. This allows for mapping (macro)molecular structure generated by basic chemical and biological processes and for deriving transient electronic charge density maps. In the experiments, the transient intensity and angular positions of up to 20 Debye Scherrer reflections from a polycrystalline powder are measured and atomic positions and charge density maps are determined with a combined spatial and temporal resolutions of 30 pm and 100 fs. We present evidence for the so far unknown concerted transfer of electrons and protons in a prototype material, the hydrogen-bonded ionic ammonium sulfate [(NH4)2SO4]. Photoexcitation of ammonium sulfate induces a sub-100 fs electron transfer from the sulfate groups into a highly confined electron channel along the c-axis of the unit cell. The latter geometry is stabilized by transferring protons from the adjacent ammonium groups into the channel. Time-dependent charge density maps derived from the diffraction data display a periodic modulation of the channel’s charge density by low-frequency lattice motions with a concerted electron and proton motion between the channel and the initial proton binding site. Our results set the stage for femtosecond structure studies in a wide class of (bio)molecular materials.

© 2010 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. EXPERIMENTAL TECHNIQUES AND DATA ANALYSIS
    1. Ammonium sulfate
    2. Femtosecond x-ray powder diffraction
    3. Reconstructing the transient three-dimensional electron density from the measured femtosecond powder pattern
    4. Femtosecond vibrational spectroscopy
  3. RESULTS AND DISCUSSION
    1. Femtosecond powder diffraction
    2. Ultrafast vibrational spectroscopy
    3. Mechanisms of structural change
  4. CONCLUSIONS

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KEYWORDS and PACS

Keywords

ammonium compounds, charge exchange, high-speed optical techniques, hydrogen bonds, ion exchange, photochemistry, photoexcitation, powders, X-ray diffraction

PACS

  • 82.30.Fi

    Ion-molecule, ion-ion, and charge-transfer reactions

  • 82.30.Hk

    Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

  • 82.50.-m

    Photochemistry

  • 82.53.Xa

    Femtosecond probes of molecules in solids and of molecular solids

  • 78.47.J-

    Ultrafast spectroscopy (<1 psec)

  • 61.43.Gt

    Powders, porous materials

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.3469779

PUBLICATION DATA

ISSN

0021-9606 (print)  
1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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Figures (6) Tables (1)

Figures (click on thumbnails to view enlargements)

FIG.1
(a) Left: Unit cell of ammonium sulfate containing sulfate SO42− and ammonium NH4+ groups (yellow: sulfur atoms; red: oxygen; blue: nitrogen; gray: hydrogen). The dimensions are a = 0.778 nm, b = 1.064 nm, and c = 0.599 nm. The shaded area marks a plane defined by the c-axis and the line connecting two hydrogens of the NH4(I) groups (see text). Right: View of the lattice along the c-axis. (b) Femtosecond powder diffraction setup showing the path of the x-ray probe through the multilayer focusing optic, the optical pump (blue) at 400 nm incident onto the powder sample and the Debye–Scherrer rings captured onto a CCD detector. The powder sample is mounted vertically in the focus of the x-ray beam. A typical diffraction pattern obtained after a 7 min exposure is shown. (c) Top: Measured (black line) and calculated (red dots) intensity profile of the Debye–Scherrer ring pattern as a function of 2θ (θ: diffraction angle). The intensities are normalized to that of the (111) peak. Bottom: Intensity change of the rings observed 260 fs after excitation of the sample.

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FIG.2
Changes in intensity (circles) of particular Debye–Scherrer rings as a function of time delay between the optical pump and x-ray probe. The solid lines are guides to the eye. Bottom panel: transient bleach of the bending mode ν4 of the ammonium ion NH4+ (pump energy E = 12 μJ). Please note the logarithmic time scale after the break.

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FIG.3
Contour plots (right hand side) showing the electron density as a function of delay time, and the corresponding planes [(a) upper row and (b) bottom row] through the unit cell of AS (left hand side) for easy visualization. (a) The blue dashed line indicates a plane perpendicular to the Z direction, cutting the unit cell at z/c = 0.25. (b) The blue dashed line marks a plane parallel to the Z direction through the unit cell corresponding to the “zigzag” orientation of the oxygen atoms that belong to two different sulfate ions. An electron channel appears between these oxygens (last contour plot) along the position marked in red.

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FIG.4
(a) Distance between the O1 and O2 atoms of adjacent SO42− groups (arrow in the left inset) as a function of delay time as derived from the x-ray data. Insets: Crystal structure of AS before (left hand side) and after (right hand side) the concerted electron and proton transfer into the conducting channel along the z-direction. (b) Transient change of electron density Δρe(x,y,z,t) at the initial proton position within the ammonium ion (open symbols) and at the transitional channel position, i.e., x = a/2, y = b/2, z = c/2 (solid circles). (c) Time-dependent change of total charge in the electron channel calculated by integrating over a cylindrical volume along the z-axis with a Gaussian lateral envelope of 0.1 nm width.

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FIG.5
(a) Linear absorption of a ≈ 1 μm thick AS layer (black solid line) in the midinfrared spectral range (red solid line: measured with femtosecond probe pulses). Dashed line: absorption spectrum calculated from reflectivity data of Refs. 33 , 34. (b) The amplitude of the bleach of the NH4+-ion bending mode corresponds exactly to the expected excitation intensity dependence for the three-photon absorption process. (c) Loss function −ωmath[1/ϵ(ω)] (solid line) and spectrally integrated energy loss rate (dashed line) as a function of the frequency of polar excitations of AS.

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FIG.6
Calculated electronic ground state energy along the reaction coordinate of the proton NH4++(SO4)2−↔NH3+HSO4−. Solid line: transfer of one proton in the unit cell of AS; dashed line: same curve for an ion pair in free space.

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Tables

Table I. Calculated oscillator strengths of all relevant educt and product vibrations.

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