Development of a fs X-ray diffractometer for time resolved X-ray diffraction experiments

M. Wörner, M. Bargheer, Z. Ansari, F. Zamponi, P. Rothhardt

After the successful implementation of a home-built, table-top femtosecond hard X-ray source in our laboratory for time resolved X-ray diffraction studies, funding was obtained within the framework of the ProFIT program to develop a prototype laboratory with modular building blocks, each of which may become a commercial product.
A commercially available laser system (35 fsec, 1 kHz, 800 nm centre wavelength, 5.2 mJ per pulse) was combined with an interaction chamber, designed and built by the Institut für Gerätebau (IfG), Berlin, and its partners, for the generation of femtosecond X-ray bursts. A goniometer and a large area X-ray detector are also part of the system.
Focussing the laser beam to a spot size of 5 microns results in intensities larger than 10¹⁷ W/cm². Irradiating a 20 micron thick moving copper tape target with the focussed laser beam generates bursts of copper K_α and K_β radiation. Figure 2 shows the spectrum of the X-ray source with a yield of 2 x 10¹⁰ photons/ second. This is comparable to the yield obtained from the home-built X-ray source.

Fig. 1: The new femtosecond X-ray laboratory with the goniometer, movable lead shielding, interaction chamber and laser system

The large area CCD (4026 x 4026 pixels) will enable us to explore techniques such as the Debye-Scherrer and Laue methods that have hitherto not been investigated with plasma X-ray sources. We will now use this system to perform time resolved X-ray diffraction studies, complementing and extending on-going research.

Fig. 2: Energy spectrum of the femtosecond hard X-ray source

Ultrafast magnetostriction in the itinerant ferromagnet SrRuO₃ studied by femtosecond x-ray diffraction

M. Wörner, N. Zhavoronkov, M. Bargheer, C. v. Korff-Schmising, Z. Ansari, A. Harpoeth, C. Aku-Ley

Many materials with a perovskite crystal structure display both strongly correlated electrons leading to electronic phase transitions and structural changes of the crystal lattice in response to electronic correlations. A prominent example is the itinerant ferromagnet SrRuO₃ (SRO) which becomes ferromagnetic below the Curie temperature of T_c=165 K. SRO exhibits the ”Invar effect”, i.e., the itinerant ferromagnet has a negative contribution to thermal expansion and compensates the normal lattice expansion due to anharmonic phonon-phonon interactions. Nanolayers of SRO show an extraordinarily strong dependence of the saturation magnetization as a function of the tetragonal distortion of the crystal, i.e., at T = 0 the crystal exhibits a huge magneto-elastic effect. Recent femtosecond magneto-optical Kerr measurements show that the macroscopic magnetization of SRO can be reduced almost instantaneously (200 fs) by optical excitation with pump pulses of moderate fluence (30 mJ cm^-2). This raises the question if and how fast an ultrafast manipulation of the magnetic system causes mechanical stress in the crystal.

Fig. 5. (a) Measured transients of the x-ray reflectivity with λex = 2.2 μm for two different lattice temperatures. (b) Lattice temperature dependence of the amplitude of the SL phonon oscillation normalized to its value at room temperature.

We performed an ultrafast time resolved x-ray structure analysis to directly measure the femtosecond build-up of optically induced uniaxial stress in the ferromagnetic nanolayers of a SRO/SrTiO₃ superlattice (SL). In the femtosecond experiments, the sample is excited by a 50 fs pump pulse which interacts exclusively with the SRO layers, and the resulting lattice response is probed by an ultrashort hard x-ray pulse (Cu Kα , photon energy 8.05 keV) which is diffracted from the excited sample. Changes of the diffracted intensity are measured as a function of pump-probe delay. We placed the sample in a cryostat allowing for sample temperatures between T=20 and 300 K. Pump-probe measurements were performed in a broad range of excitation wavelengths between 800 nm and 2.2 mm. In Fig. 5(a), we present the transient change of the x-ray reflectivity ΔR=R₀ of the (0 0 116) SL Bragg reflection for two different lattice temperatures after ultrafast optical excitation at λ_ex = 2.2 μm with the same pump fluence. The particular SL Bragg reflection was chosen to be most sensitive to the particular SL phonon which modulates the layer thicknesses d_SRO and d_STO while keeping the SL period d_SL =d_SRO+d_STO constant. The transients show a delayed rise and oscillations with a period determined by d_SL and the velocity of sound. In the paramagnetic phase, i.e., T>T_C (Curie temperature), the amplitude of the oscillation has constant value which is exclusively determined by the fluence of absorbed pump photons.
The situation changes drastically if we cool the sample down to temperatures T < T_C at which the electronic system is in the ferromagnetic phase. Results from a series of measurements with a constant pump fluence are presented in Fig. 5(b). The amplitude of the SL phonon show a strong reduction below T_C. Thus, the observed behavior gives evidence of additional contributions to the photogenerated stress caused by the strong magneto-elastic effect existing in the ferromagnetic phase of SRO. As a result, a reduction of the magnetization leads to a contraction of SRO. Recent femtosecond studies have revealed a quasi-instantaneous reduction of the magnetization upon optical excitation at 800 nm. The data points in Fig. 5(b) follow the temperature dependent magnetization (solid line), revealing a contractive magneto-elastic contribution to the stress driving the SL phonon. Thus, our results demonstrate for the first time magnetostriction on a femtosecond time scale.

Ultrafast changes of molecular crystal structure induced by dipole solvation

M. Bargheer, N. Zavoronkov, C. v. Korff - Schmising, M. Kiel, J. Dreyer, M. Wörner, T. Elsässer , MBI, Berlin

M.Braun, C. Root, T.E. Schrader, P. Gilch, W. Zinth, BioMolekulare Optik and MAP, Ludwig-Maximilian- Universität München

Publication: M. Braun et al. Phys. Rev. Lett. 2007, 98, 248301.

Press release: Molecular pirouettes

Interactive animation of DIABN (monomer)

Interactive animation of DIABN crystal structure

In a joint project with the group of W. Zinth, University of Munich, we demonstrate that the ultrafast rearrangements in a molecular crystal in response to a local dipole change consist in an angular reorientation of molecules leaving the spacing of lattice planes unchanged. A combination of visible pump mid-infrared (IR) probe studies confirming the occurrence of intramolecular charge transfer in 4-(diiso-propylamino)benzonitrile (DIABN) crystals
and ultrafast x-ray diffraction/diffuse scattering experiments give direct evidence that the x-ray signals are dominated by solvation-related geometry changes of the crystal lattice, i.e, a collective response of many molecules, rather than local geometry changes of excited chromophores.
In the optical pump – x-ray probe experiments femtosecond photoexcitation of organic chromophores in a DIABN single crystal induces strong changes of the electronic dipole moment via intramolecular charge transfer as is evident from transient vibrational spectra [Fig. 3(a)]. The structural response of the crystal to the dipole change is directly mapped for the first time by ultrafast x-ray diffraction/diffuse scattering. Changes of diffracted and transmitted x-ray intensity [Fig. 3(b,c)] demonstrate an angular rearrangement of molecules around excited dipoles. Such motions are driven by the inhomogeneous electric field change originating from the dipole change of excited chromophores and resulting in a torque exerted on the unexcited molecules in the surrounding. The reorientation process displays a rise time of 10 ps, the intramolecular charge transfer time. Transient x-ray scattering is clearly governed
by such solvation processes, masking local changes of the chromophore molecular structure. Under the conditions of weak femtosecond excitation applied here, a giant modulation of x-ray extinction around low order reflections is found, an effect which is probably present in many crystals made up of a complex unit cell consisting of light elements (Z<10).

Fig. 3. Various femtosecond pump-probe data measured on single DIABN crystals. In each case the crystal is excited with a 50 fs pulse at λ=400 nm. (a) Transient rise (time constant: 11 ps) of an absorption band at 2104 cm-1 stemming from the CN vibration (see inset) of molecules in the ICT state. (b,c) Time-resolved change of the x-ray reflectivity ΔR/R0 measured at the angles of the 004 (red circles) and 006 Bragg reflections (blue circles), respectively. The respective transmission change ΔT/T0 (open symbols) is in both cases identical to the reflectivity change, i.e.ΔT/T0 = ΔR/R0.

Ultrafast structure and polarization dynamics in nanolayered perovskites studied by femtosecond x-ray diffraction

C. v. Korff - Schmising, M. Bargheer, M. Kiel, N. Zhavoronkov, M. Woerner, T. Elsaesser,
I. Vrejoiu, D. Hesse, M. Alexe vom Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany

Publikation:C. von Korff Schmising et al. Phys. Rev. Lett. 98 (2007) 257601/1-4

In this application of femtosecond hard x-rays we study the polarization and lattice dynamics in a metal / ferroelectric /metal nanolayer system by means of femtosecond x-ray diffraction. We experimentally demonstrate that optically induced giant stress in the metal layers can switch off the ferroelectric polarization within 2 ps. In ferroelectric materials the lattice energy displays a double-minimum potential along the soft-mode distortion, i.e., the relative displacement of anions and cations ξ within the unit cell. This goes along with a tetragonal distortion η in the ferroelectric phase. Ultrafast time resolved X-ray structure analysis serves for directly measuring the polarization dynamics (i.e. ξ) of PZT in a PZT/SRO superlattice, which is triggered by optically induced uniaxial stress (i.e. η) in the metallic SRO layers.
In the femtosecond experiments, superlattice samples grown by the group of D. Hesse, MPI for Microstructure Physics, Halle, are studied. The 50 fs pump pulse at 800 nm interacts exclusively with the SRO layers, and the resulting lattice response is probed by an ultrashort hard x-ray pulse (Cu Kα, photon energy 8.05 keV, λ=0.154 nm) which is diffracted from the excited sample. Changes of the diffracted intensity are measured as a function of pump-probe de-lay. In Fig. 4(a) we show the transient change of the x-ray reflectivity ΔR/R₀ of the (0 0 56) (blue circles) and (0 0 55) (red circles) superlattice Bragg reflections of the PZT/SRO sample after ultrafast optical excitation of the SRO layers (pump fluence of 5 mJ/cm². (0 0 56) data on a longer timescale (not shown) exhibit an oscillatory intensity modulation with a period of 2 ps.
The 800 nm pump pulse generates an electronic excitation in the SRO layers with a spatial periodicity 1/d_SL where d_SL is the pe-riod of the SRO/PZT superlattice. Electron-phonon coupling results in the generation of coherent acoustic phonon motions with wavevector g=2π/d_SL. Such elongations along a SL mode periodically modulate the SRO and PZT layer thicknesses (i.e. tetragonal distortions η_PZT and <η_SRO with a period determined by d_SL and the respective velocity of sound.
The anharmonic coupling of the directly driven tetragonal distortion η and the soft mode coordinate ξ results in a simultaneous elongation of the latter and a change of the polarization P. Our measurements for two different Bragg peaks allow for a quantitative analysis of the microscopic lattice dynamics, i.e., the time-dependent longations along the two coordinates ξ and η.
The derived time-dependence of ηPZT and x/x₀ = P/P₀ is plotted in Fig. 4(b) and shows an ultrafast and slightly delayed reduction of the polarization by 50 %. For the highest excitation fluence we measured a maximal ΔRmax/R₀ = 3 of the (0 0 56) peak (not shown), which corresponds to a peak strain Δη/η₀ = 2 % and a complete switch-off of the polarization P.

Fig. 4. (a) Transient change of the (0 0 56) (blue) and (0 0 55) (red) reflectivities of the PZT/SRO superlattice for a moderate fluence of 5 mJ/cm2 . (b) Derived transient change of the tetragonality η and polarization P(t).