Ultrafast X-ray Spectroscopy of Solvated Transition-metal Complexes and Oxide Materials

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1 Ultrafast X-ray Spectroscopy of Solvated Transition-metal Complexes and Oxide Materials Robert Schoenlein Materials Sciences Division Chemical Sciences Division - UXSL Matteo Rini ils Huse F. Reboani & S. Rini LCLS Soft X-ray Workshop Oct. 18, 2008

2 Fundamental Challenge Condensed Matter Understand the Interplay between Atomic and Electronic Structure - Valence electronic structure energy levels, charge distribution, bonding, spin - Atomic structure coordination, atomic arrangements, bond distances Particularly in systems that challenge the standard paradigms : - beyond single-electron band structure models, Landau-Fermi Liquid Theory complex materials exhibiting strong correlation among charges, and between charge, spin, orbit, and lattice T lifetime (E-E F ) -1/2 - beyond simple adiabatic potential energy surfaces Born-Oppenheimer approximation ψ total ψ nuclear ψ electronic Atomic Structural Dynamics atomic vibrational period: T vib = 2π(k/m) -1/2 ~ 100 fs k~ev/a 2 m~10-25 Kg structural phase transitions chemical reactions Electronic Structural Dynamics electron-phonon interaction ~ 1 ps e-e scattering ~10 fs e - correlation time ~100 attoseconds (a/v Fermi ) bond dynamics, valence charge flow electronic phase transitions correlated electron systems

3 Ultrafast X-rays: Quantitative Information on Electronic and Atomic Structure time-resolved x-ray spectroscopy XAES local electronic structure, bonding geometry (x-ray absorption near-edge structure) XMCD, XMLD spin,magnetization dichroism (x-ray magnetic/linear dichroism) EXAFS local atomic structure and coordination (extended x-ray absorption fine structure) element specific symmetry/spin selective ferromagnetic/antiferromagnetic order molecular systems and reactions interfaces, complex/disordered materials r Spin Moment Orbital Moment visible pump x-ray probe absorption f(r) delay ħω opt time K edge energy L 3 l+s L 2 l-s ħω x J. Stohr time-resolved x-ray diffraction time delay atomic structure in systems with long-range order/periodicity phase transitions, coherent phonons x-ray probe visible pump time delay detector diffraction angle

4 Outline Structural Dynamics in Solvated Transition-Metal Complexes spin-crossover transition Fe(II) complex EXAFS, Fe K-edge (atomic structure) spin-crossover Fe(II) XAES, Fe L-edge (electronic structure) Fe Structural Dynamics in Colossal Magnetoresistive (CMR) Manganites ultrafast photo- and vibrationally-induced insulator-metal transition in Pr 1-x Ca x MnO 3 electronic structure time-resolved XAES (O K-edge, Mn L-edge) Future Directions, Ultrafast Soft X-ray Spectroscopy LCLS

5 Fe II Spin-Crossover Molecules MLCT (charge-transfer state) hν low-spin state τ~300 fs high-spin state 1 A 1g 5 T 2g S=2 (t 2g ) 6 (t 2g ) 4 (e g ) 2 Fe Fe[tren(py) 3 ] 2+ hν Fe low spin high spin ~10-15% increase in metal-ligand bond distances trigonal cage distortion? Motivation: ligand field strength (10Dq) ~ electron pairing energy relationship between structure, electronic, and magnetic properties Do the dynamic structural distortions facilitate the spin-crossover reaction? electron transfer mechanistic role in biochemical processes (cytochrome P450) molecular electronics, opto-magnetic storage material Collaborator: J.K. McCusker group Michigan State

6 Fe II Time-resolved EXAFS 0.2 Å change in Fe- bond distance within 70 picoseconds pump (3.1 ev) τ = 330 ps probe (7.1 KeV) b c transmission detector synchrotron (70 ps, 7 kev) delay Ge crystal mono ALS BL µ x mm solution laser 75 fs, 400 nm chopper X 0.1 chemically synthesized high spin low spin a XAS at τ = 0 ps (low spin) high spin Fe energy (ev) energy (ev) µ x10-3 τ delay = 330 ps Reactant 1 A 1 (τ τ = 0) 6 ±0.5 R (Å) 1.94 ± 0.01 Photoexcited 5 T 2 (τ= τ=330 ps) 6.5 ± ±0.03 µ x10-3 E o = 7140 ev σ (Å 2 ) k (Å -1 ) M. Khalil et al., J. Phys. Chem. A, 110, 38 (2006) delay (ps)

7 Electronic Structure - Fe II Spin-Crossover Molecules Charge Transfer Ligand Bonding Spin State Electronic structure: L-edge spectroscopy Metal-ligand bonding (high-spin vs. low spin) - Fe 3d-electrons (-2p) - spin state (S-O split Fe-2p) Soft X-ray XAES - transmission Fe Solvent influence - dielectric interaction with charge-transfer process - steric interaction with ligand distortions? MLCT (charge-transfer state) hν S=2 low-spin state high-spin state Fe-3d (t 2g ) 6 (t 2g ) 4 (e g ) 2 Fe-2p 3/2 Fe-2p 1/2 absorption high spin Fe[tren(py) 3 ] 2+ in acetonitrile L III L II Lee, et al., JACS, 122, 5742, energy (ev)

8 Electronic Structure - FeII Spin-Crossover Molecules Soft X-ray XAES - transmission Interferrogram sample thickness ~0.5 µm thick liquid sample cell Fe high spin 500 µm 100 nm Si34 windows Solvent Transmission Transmission 0.8 2µm of CH3C UXSL Soft X-ray Endstation Energy / ev

9 Electronic Structure - Fe II Spin-Crossover L-edge XAES Soft X-ray XAES Fe L III Fe[tren(py) 3 ] 2+ in acetonitrile Differential Spectra 0.04 Time-Dependence ps delay ev 5 ns delay ev ev 0.02 T/T (- α) T/T (- α) energy (ev) delay (ps) Ligand field strength ~ electron pairing energy reduction in ligand field parameter 10Dq electronic structure evolution within 70 ps resolution

10 Future Directions - Fe II Spin-Crossover L-edge XAES absorption L III Transient Absorption Spectra 1.6 ev Low-spin ground state Photo-excited high-spin state absorption L II Transient Absorption Spectra energy (ev) Low-spin Photohigh-spin energy (ev) T/T normalized ev high-spin Theory: (Brois et al., JACS, 117, 1995) ~0.55 ev reduction in 10Dq contribution from core-hole correlations low-spin delay (ps)

11 Outline Structural Dynamics in Solvated Transition-Metal Complexes spin-crossover transition Fe(II) complex EXAFS, Fe K-edge (atomic structure) spin-crossover Fe(II) XAES, Fe L-edge (electronic structure) Fe Structural Dynamics in Colossal Magnetoresistive (CMR) Manganites ultrafast photo- and vibrationally-induced insulator-metal transition in Pr 1-x Ca x MnO 3 electronic structure time-resolved XAES (O K-edge, Mn L-edge) Future Directions, Ultrafast Soft X-ray Spectroscopy LCLS

12 Colossal Magnetoresistive Manganites Pr 1-x Ca x MnO Pr 1-x Ca x MnO 3 Pr/Ca Mn O Temperature [K] T CI PI T CO T T C FI COI AFI CAFI COI - charge ordered insulator PI paramagnetic insulator AFI antiferromagnetic insulator CAFI canted antiferromagnetic insulator x Tokura et al., PRB, 1996 Distorted Perovskite structure Rich Phase Diagram - magnetic control of electronic structure (magneto-transport) - rich phase diagram order/symmetry breaking competing ground states - new physics, correlation effects other complex materials Mn 3d orbitals e g t 2g O Insulating Metallic Mn 3+ Mn 4+ Mn 3+ Charge-localizing Electron-delocalizing real-space ordering Jahn-Teller double-exchange Instability

13 Vibrationally-Driven Phase Transition in Manganites Tolerance Factor: Orthorhombic Distortion of Cubic Perovskite Structure Γ = d Pr O 2 d small tolerance factor Mn O Related to the Mn-Mn electron hopping rate tendency to charge localization mid-ir µm 1µJ, 200 fs 1 mj/cm 2 Pr 1-x Ca x MnO 3 Mn-O bend Mn-O stretch Conductivity [Ω -1 cm -1 ] I PEAK = 2 µa 1E-3 T = 40 K 1E-4 ρ PEAK = Ω cm 1E-5 1E-6 1E-7 ρ IIT = Ω cm 1E Delay [ns] M. Rini, et al. ature, 2007 Ground-state vibrational pumping Pr 0.7 Ca 0.3 MnO 3 - transient sample conductivity measurements Changes of sample resistance - depend on resonant phonon excitation o current is observed when pumping at 8.5 µm Collaborator: A. Cavalleri Oxford, DESY CFEL

14 Vibrationally Driven I-M Transition in a Manganite via coherent THz excitation mid-ir µm 1µJ, 200 fs Transient Reflectivity Phase Transition vibrational excitation - long-lived changes in reflectivity well-defined fluence threshold and saturation Vibrational Resonance Dependence Mn-O bend Mn-O stretch M. Rini, et al., ature, 2007 Collaborator: A. Cavalleri Oxford, DESY CFEL

15 Static XAS - Insulator/Metal Transition in Manganites Mn 4sp, 3d La, Ca, Pr (Mn-3d/O-2p) E F Mn-3d O K-edge: 1s 2p Pre-edge: unoccupied states of mixed O-2p and Mn-3d character Mn 3d La, Ca, Pr Mn 4sp Mn L-edge: 2p 3d Predominantly metal-3d character O-1s Mn-2p 3/2 Mn-2p 1/2 Mannella et al. PRB (2005)

16 Time-resolved XAS Insulator/Metal Transition in Manganite (PCMO) M. Rini, Y. Zhu, LBL Materials Sciences S. Wall, R. Tobey, A. Cavalleri - Oxford O Mn 3+ Mn 4+ Mn nm, 100 fs 30 mj/cm 2, s-pol X-Rays, 70 ps ev delay channeltron Pr 1-x Ca x MnO 3 Mn 3d orbitals e g t 2g Insulator Pr 0.7 Ca 0.3 MnO 3 O K-edge 543 nm O 1s Mn 3d XAS evidence of I-M transition, DOS spectral weight transferred to absorption threshold Mn-3d/O-2p hybridization Modification of 10Dq crystal field splitting

17 Photoinduced XAS Changes - Evidence of IM Transition Photo-induced vs. Thermally-induced Phase Transition: The DOS change in the conduction band appears in the O 1s XAS spectrum and spectral weight is transferred to the absorption threshold. PHOTOIDUCED: THERMALLY IDUCED: α 300ps - α LASER OFF α METALLIC - α ISULATIG α 25K - α 150K α25k - α 150K Pr 0.7 Ca 0.3 MnO 3 La 0.7 Sr 0.3 MnO 3 α 80K - α 370K La 1.2 Sr 1.8 MnO 3 (T C =130 K) La 1.3 Sr 1.7 MnO 3 (T C =130 K) (T C =360 K) Energy (ev) XAES experiments on the thermally-induced IM phase transition From: J.-H. Park et al., Phys. Rev. B 58, R13330 (1998)

18 Summary Structural Dynamics in Solvated Transition-Metal Complexes Ligand field coupling between molecular structure and electronic properties spin-crossover transition Fe(II) complex EXAFS, Fe K-edge (atomic structure) - atomic/molecular dynamics EXAFS (0.2 Å change in ligand bond within 70 ps) spin-crossover Fe(II) XAES, Fe L-edge (electronic structure) - electronic structural dynamics Fe L-edge XAES - ~0.55 ev reduction in 10Dq Fe Structural Dynamics in Colossal Magnetoresistive (CMR) Manganites THz vibrational control of correlated electron phases tolerance factor ultrafast photo- and vibrationally-induced insulator-metal transition in Pr 1-x Ca x MnO 3 - targeting specific vibrational modes - Mn-O stretch, modulation of the tolerance factor electronic structure time-resolved XAES (O K-edge, Mn L-edge) - changes in DOS spectrum ultrafast insulator-metal transition mid-ir