WIR SCHAFFEN WISSEN – HEUTE FÜR MORGEN Henrik Lemke :: SwissFEL :: Paul Scherrer Institut Charge, spin and structural dynamics in Spin-Cross-Over materials School on Synchrotron and Free-Electron-Laser Methods for Multidisciplinary Applications school, Trieste, 15. 5. 2018
Outline • Functions of materials and Free Electron Lasers • The ultrafast FEL Toolset • Ultrafast charge/spin/structure in molecular transition metal complexes The Problem The Findings • Ultrafast phenomena in solid state • The SwissFEL free electron laser Page 2
How do materials change their function? Out-of-equilibrium ? Models imprecise B A Transformation coordinate Page 3
Contribution of Free Electron Lasers Electronic and nuclear structure sensitivity • at atomic length scale • at the timescale of transitions Bostedt (2016), Rev. Mod. Phys. 88, 15007. Page 4
The Toolset How can we learn about functional transitions with FELs? Page 5
Pump/probe tracks ultrafast processes A Transition states are • ultrashort lived B • ultra-dilute at a given point in time ? Pump/probe “freezes” B intermediates A t 0 t 1 Page 6
Excitation mechanisms - pump Ultrafast femtosecond excitation by ultrafast laser light pulses • UV-Vis : excitation of unoccupied electronic state (valence) • Infrared/THz : vibrational states, phonons in solids wikipedia.org Page 7
Measure mechanisms – X-ray probe X-ray photons can interact with electrons, also from core shell levels. electronic state can interact with atom environment can interfere …and they often pass by. Page 8
X-ray Spectroscopy Emission spectrum • Occupied valence states Absorption spectrum • Chemical potential • Unoccupied states X-ray spectroscopy is element specific Page 9
Ultrafast x-ray Spectroscopy B A Ultrafast transitions • High variations in “chemistry” • Chemical potential • Electronic states • Local environment • Low variations in core electron levels A B Page 10
X-ray solution scattering Elastic scattering interference from all electrons in a sample approximated by atomic form factor and Debye scattering equation 𝑛 𝑟 sin(𝑟𝑠 𝑜𝑛 ) I q = 𝑔 𝑜 𝑟 𝑔 𝑟𝑠 𝑜𝑛 𝑜 𝑛 Page 11
General concepts in pump probe methods 𝑇 pumped = 𝑏 𝑇 excited + 1 − 𝑏 𝑇 static 𝑇 unpumped = 𝑇 static Reference measurements can correct drifts Signal Time Slow drifting parasitic signal 𝑇 pumped = 𝑏 𝑇 excited + 1 − 𝑏 𝑇 static + 𝑌 drift (𝑢) 𝑇 unpumped = 𝑇 static + 𝑌 drift 𝑢 Δ𝑇 = 𝑇 pumped − 𝑇 unpumped = 𝑏 [𝑇 𝑓𝑦𝑑𝑗𝑢𝑓𝑒 − 𝑇 𝑡𝑢𝑏𝑢𝑗𝑑 ] 𝑇 pumped = [𝑏 𝑇 excited + 1 − 𝑏 𝑇 static ] × 𝑌 drift (𝑢) Slow drifting sensitivity 𝑇 unpumped = 𝑇 static × 𝑌 drift 𝑢 𝑇 pumped − 1 = 𝑏 𝑇 excited − 1 𝑇 unpumped 𝑇 static Page 12
Pump/probe on liquids Circulating sample solution Optical pump X-ray probe Pump/probe group velocity mismatch scales with sample thickness tL-tX = d/c (n-1) d Page 13
Pump excitation High concentration of excitations high excitation fraction a Pump High pump fluence can lead to sequential excitation of already excited states. • Difficult to interpret Pump/probe • Usually not of interest signal Search for linear signal regime Pump fluence Page 14
Summary FEL toolset • Accumulation of non-equilibrium states by pump/probe • Element specific electronic states, chem. potential local structure by X-ray spectroscopy • Global structure by X-ray scattering Page 15
The Problem Charge transfer and spin transitions in transition metal complexes Page 16
Charge transfer in dye sensitized solar cells Electrolyte -e - +e - Dye Charge • Competition (= Race) between charge transfer separation and recombination Cond. • band Dye development Potential Nanoparticle • Absorption spectrum Energy • Price HOMO Page 17
Spin transition molecular system Towards e g Ligand Fe(bipy) 3 Transition metal complex in t 2 g Away from octahedral symmetry Ligand Charge Energy Transfer Intermed. Triplet High spin L( π *) Fe-3d(e g ) Fe-3d(t 2 g) Fe-N distance Page 18
Spin transition molecular system Fe(bipy) 3 Transition metal complex in octahedral symmetry ~120 fs @ unity quantum yield Charge Energy Transfer Intermed. Triplet High spin L( π *) Fe-3d(e g ) ~650 ps Fe-3d(t 2 g) Fe-N distance Page 19
Summary “The Problem” Charge transfer Spin + charge + structural transition Electronic transition High Spin L( π *) Fe-3d(e g ) Fe-3d(t 2 g) Bousseksou et al., Chem. Soc. Rev. 40, 3313, (2011). Low spin • Model system for • Fe-based solar cell materials • Molecular switches • Transitions interact on femtosecond time scale Page 20
The Findings Page 21
Ultrafast X ray E mission S pectroscopy Dispersive spectrometer Page 22
Transient species Fingerprinting L( π *) Fe-3d(e g ) Dispersive spectrometer Fe-3d(t 2 g) = Doublet - Singlet = Triplet - Singlet Interpretation through Emission spectra from = Quintet - Singlet reference samples Zhang et al., Nature 509, 345 – 348 (2014) Page 23
Global kinetic electronic state model Reconstruction of Diff. signal by linear combination of reference differences in a kinetic model. Δ𝑇(𝑢) = a MLCT (t) Δ𝑇 𝑁𝑀𝐷𝑈 + a 3T (t) Δ𝑇 3T + (1 − a MLCT −a T ) Δ𝑇 5T2 • Triplet state can be observed • Triplet state is relatively τ MLCT = 150.6 ± 50 fs τ 3T = 70.6 ± 30 fs short lived small population Zhang et al., Nature 509, 345 – 348 (2014) Page 24
Ultrafast X ray A bsorption N ear-edge S tructure Absorption spectroscopy in dilute Sample by total Emission Optical pump X-ray probe Charge transfer High Spin = “3+” Ox. state = larger distance Page 25
Time resolved XANES Time resolved measurement at Absorption edge • Strong modulation in MLCT and HS state • Opposite sign of both contributions Time resolution ~ 25 fs RMS Page 26
XANES with improved sample jet Fast pre-edge rise (~25 fs RMS) Mixed signals Delayed Oscillation Lemke et al. Nat. Comm . 8, 15342 (2017) Page 27
Structural signal analysis Simulation of Structural signal as fuction of Fe-N distance MXAN: Benfatto et al. J. Synchrotron Radiat. 10, 51 – 57 (2002). Page 28
Linear structure/XANES signal dependence Direct average structural information at linear signal dependence Structural signal as fuction of Fe-N distance Oscillation: 126(3) cm -1 Breathing mode, from DFT calculations 124.4 cm -1 / 121.4 cm -1 Sousa et al. Chem. - A Eur. J. 19, 17541 – 17551 (2013). MXAN: Benfatto et al. J. Synchrotron Radiat. 10, 51 – 57 (2002). Page 29
Nonlinear structure/XANES signal dep. Absorption signal Ligand distance Non-linear signal dependence Sensitivity to ensemble distribution Page 30
Dispersion of coherent wave packet 110 fs transition ≤25 fs • Transient measurement of ensemble distribution width • Dispersion of coherence faster (~320 fs) than narrowing of distribution (1.6 ps) due to vibrational cooling Lemke et al. Nat. Comm . 8, 15342 (2017) Page 31
Solution X -ray D iffuse S cattering Separation of scattering signal Δ𝐽 𝑟, 𝑢 = Δ𝐽 solute 𝑟, 𝑢 + Δ𝐽 cage 𝑟, 𝑢 + Δ𝐽 solvent 𝑟, 𝑢 𝜖𝐽 𝜖𝐽 Δ𝐽 𝑡𝑝𝑚𝑤𝑓𝑜𝑢 𝑟, 𝑢 = Δ𝑈(𝑢) + Δ𝜍(𝑢) 𝜖𝑈 𝜖𝜍 𝜍 𝑈 Page 32
Energy transfer to Solvent 𝜖𝐽 𝜖𝐽 Δ𝐽 𝑡𝑝𝑚𝑤𝑓𝑜𝑢 𝑟, 𝑢 = Δ𝑈(𝑢) + Δ𝜍(𝑢) 𝜖𝑈 𝜖𝜍 𝜍 𝑈 Timescale of solvent heating fits vibrational cooling time constant Haldrup et al. 2016, J. Phys. Chem. B
Summary molecular spin-transition Lifetime MLCT state Short population of Triple state Dephasing, intramolecular coupling Vibrational cooling with environment Page 34
Quantum efficiency High quantum efficiency can be explained by strong coupling to intramolecular vibrations van Veenendaal et al. PRL 104, 67401 (2010). Page 35
Solid state transitions Page 36
Spin transition in solid states In solids, neighbor excitations can efficiently excert • Pressure/stress • Electric field/polarisation • Magnetic moment/spin Higher switching efficiency by collaborative effect R. Bertoni et al, Nature Materials 15, 606 (2016) Page 37
Ultrafast information in correlated systems Charge Lattice Orbital Magnetic Nature Physics 8, 864 – 866 (2012) Temporal separation Causality between degrees of freedom Selective probe Bragg diffraction Resonant diffraction + Polarisation transfer time Page 38
Re sonant X -ray D iffraction • Orbital and magnetic contrast by Polarisation transfer Page 39
Selective ultrafast probe of degrees of freedom Charge order Insulator Insulator to metal transition in Excitation manganite Energy • Separation of charge from lattice Metal order by resonant diffraction • Destruction of order Metal • Lattice motion follows destruction of charge order Excitation Distortion from Lattice Orbital order Insulator Metal Pr 0.5 Ca 0.5 MnO 3 Beaud et al. Nat. Mater. 13, 923 – 927 (2014). Page 40
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