1. Elizabeth Ryland ISMS 2016 6/21/16
High-Harmonic Generation XUV Spectroscopy for Studying Ultrafast Photophysics of Coordination Complexes
Elizabeth Ryland
ISMS 2016
6/21/16

2. ultrafast dynamics of coordination complexes
Charge-Transfer dynamics
Probing the
ultrafast dynamics of coordination complexes
MMCT
LMCT
Catalytic steps
Spin-flip mechanisms
Knowledge of the ultrafast dynamics of coordination complexes is pivotal to understanding the greater scheme of the energy transfer mechanisms in many catalytic systems and biological processes.
Understanding the electron transfer mechanism in multimetallic enzymes is currently inhibited by the lack of a readily accessible ultrafast technique that is able to monitor the metal centers separately. Transient Uvvis and IR studies are restricted to studying vibrational tags. XAS is specific and ideal for probing these interactions, however current XAS techniques based at synchrotorns and free electron lasers have limited availability and harsh time constraints, as well as a limiting time resolution of around 100fs.
Development of efficient processes for the creation of storable fuels for viable energy alternatives is one of the main challenges faced by the scientific community today. Many have turned towards imitating natural catalysts, such as mimicking the oxygen evolving complex for water-splitting applications. However, these catalytic reaction mechanisms are poorly understood due to the complexity of the excited-state electronic structure of multimetallic systems. Understanding of these systems has been limited by the capabilities of existing spectroscopic techniques; current high-specificity options such as EPR and X-ray absorption are too slow to watch the transient intermediate states. Conversely, high time-resolution options such as transient IR are not specific enough to definitively show the progression of electronic states. While time-resolved synchrotron usage is an option, the limited availability and high expense of these instruments reduces their practicality as an efficient tool for chemical research. Transient extreme ultraviolet spectroscopy is a developing technique that is aptly suited to studying these types of ultrafast multielectron systems.1 This technique, which excites 3p3d transitions, is element-, oxidation state-, spin state-, and ligand field-specific. The femtosecond time resolution provided by this laser-based method will allow us to study the changes in the electronic structure of each metal in the cluster, composing an accurate image of the short-lived intermediates that occur during charge transfer and catalysis.
Fe2 center of Methane monooxygenase
[Ni2+Fe2+]-  [Ni?Fe?]0
[Ni2+Fe2+]0  [Ni?Fe?]- 
Step 1
Step 2
Step 3
Step 4

3. HHG XAS spectroscopy probes absorptions in the XUV energy range
Continuum
Valence 3d
M-edge absorption hv
3p63dN
3p53dN+1
K-edge
L-edge
M-edge
X-ray
EXAFS only happens when the photoelctron kinetic energy is >50,000eV (around >10^4 eV, the wavelength is on the scale of 1angstrom, the same order as atom-atom separations.
3s or 3p
2s or 2p
Inner-shell 1s
XAS

4. High Harmonic Generation high energy photons are produced by the interaction of gas atoms with a strong electromagnetic field
Residual IR light
Exciting laser pulse
Neon gas
XUV laser
Tunneling
ionization
Propagation
Recombination
Three step picture of harmonic generation: (1) The coulombic potential of a gas atom is lowered by the laser field allowing for an electron to tunnel out of the barrier to the vacuum level. (2) Once free, the electron is accelerated away from the atom and back again by the E-field of the laser. (3) The electron recombines with the ionized atom emitting radiation
Maximum harmonic photon energy (the cutoff energy) Ec=Ip+3.17Up where Ip is the ionizing potential of the atom (for Ne =21.6,40.5,63eV, Ar= 15.8, 27.6, 40.7 He=24.6, 54.4) and Up is the pondermotive potential (Up=Eo^2/4wo^2=9.337e10^-14I)
3 step model first introduced in 1992 by Krause et al. HHG first “discovered” in 1987 my Mcphearson et al. E Ip
XUV 1 2 3
Corkum. Phys. Rev. Lett. 71, Krause et al. Phys Rev. Lett. 68, 992

5. Energy Profile of Probe Beam Transmission spectrum of Ne harmonics through 100nm Si3N4
Normalized counts

6. Applying HHG XUV spectroscopy to coordnation complexes

7. Understanding XUV transitions
The energy absorbed is sensitive to the element identity, ligand field, oxidation state, and spin state of the Metal.
Essentially, XUV spectroscopy is probing the d-orbitial occupation and energies.
XUV hv
3p63dN
3p53dN+1

8. Understanding XUV transitions through symmetry analysis: FeIII HS… 4H
e-e repulsion 4I 6P 6P
3p53d6
2S+1L 6D
Selection Rules
ΔL=0, ±1
ΔS=0 6F …
R-S term symbols describe the potential electronic states from electronic transitions, holding to LS (R-S coupling). [aka In this scheme it is assumed that:  spin-spin coupling > orbit-orbit coupling > spin-orbit coupling and works best for 1st row transition metals. ] GS is determined by Hund’s rules. 2I 4D
3p63d5 4P 4G 6S

9. Understanding XUV transitions through symmetry analysis: FeIII HS… … 4H
Oh ligand field splitting … 4I 6P
6T1
3p53d6
6T2 6D 6E
6T1 6F
6T2
6A2 …
Method of descending symmetry. Why? In the lower state all are gerade, in the core-hole excited state, all are ungerade due to perturbation by p-orbital. Mulliken labels for irreducible representations: A, B, E, T. A and B are singly degenerate and symmetric or antisymmetric to Cn, respectively. E is 2D, T is 3D. Subscripts: 1/2 symmetric/antisymmetric to C2 (perp to Cn). g/u symmetric/antisymmetric w respect to i.
Based on Hund’s rules, the higher degeneracy states are usually lowest in energy. However, from the HF-based LFMT CTM4XAS calculations, the orbitals are ordered this way in terms of energy. … 2I 4D …
4A1 …
3p63d5 4P 4E 4G
4T1
4T2 6S
6A1

10. Understanding XUV transitions through symmetry analysis: FeIII HS… … 4H … 4I 6P
6T1
3p53d6
6T2 6D 6E
6T1 6F
6T2
6A2 …
Completed transition diagram for FeIII HS. Note: spin-orbit coupling for 1st row transition metals is weak and will effectively broaden the six allowed transitions. … 2I 4D …
4A1 …
3p63d5 4P 4E 4G
4T1
4T2 6S
6A1

11. XUV transition diagram shows agreement of LFMT simulation with experimental data… … 4H … 4I 6P
6T1
6T1
3p53d6
6T2
6T2 6D 6E 6E
6T1
6T1 6F
6T2
6T2
6A2
6A2 …
Animate in sticks, then curve.
Completed transition diagram for FeIII HS. Note: spin-orbit coupling for 1st row transition metals is weak and will effectively broaden the six allowed transitions. Remember that the kaili program DOES encorparate spin-orbit coupling … 2I 4D …
4T2
4A1 …
3p63d5 4P
4T1 4E 4G 4E
4T1
4A1
4T2 6S
6A1
6A1

12. Ground state M-edge spectra of Model complexes

13. XUV absorption is diagnostic of the metal electronic structure
Experiment
Simulation
Oxidation state specificity:
Co(II) vs Co(III)
Ligand field specificity:
Co(II) octahedral vs square planar
Spin specificity:
Fe(II) high-spin vs low-spin

14. Experimental XUV spectra of Fe3+ showing environmental sensitivity
Acknowledge Micahela for FeOEPClO4 synthesis

15. Relaxation of metalloporphyrin
Predictions and preliminary transient spectra of FeOEPCl
Relaxation of metalloporphyrin

16. Time-resolved dynamics of FeIIITPPCl
FeTPPCl ground state
Simulated XUV absorption difference spectra
Relaxation mechanism
S2(π,π*)
S1(π,π*)
LMCT (π, d?)
d-d XS GS
3𝑑𝑥𝑦
3 𝑑 𝑦𝑧 , 𝑑 𝑥𝑧
3 𝑑 𝑧 2
3 𝑑 𝑥 2 − 𝑦 2
b1g
a1g eg
b2g
d-d XS
LMCT (π, d?)
LMCT (π, d?)
LMCT (π, d?)
6 𝐴 1𝑔
π π*
π* dA
dB  π
dA  dB
Rise of LMCT
Rise of d-d state
Decay of d-d state
100fs
600fs
2.6ps
18ps

17. Transient XUV absorption of FeIIITPPCl
Pumped at 400nm, broadband probe ~40-75eV
π π*
π* dA
dB  π
dA  dB
Rise of LMCT
Rise of d-d state
Decay of d-d state
100fs
600fs
2.6ps
18ps

18. Conclusions Acknowledgements:
High time resolution, metal electronic state specificity, and accessibility make HHG XUV spectroscopy a powerful tool for studying charge transfer in coordination complexes.
Ground state M2,3-edge spectra of model coordination complexes show a good match to theoretical simulations
Acknowledgements:
Prof. Josh Vura-Weis Dr. Ming-Fu Lin Max, Kaili, Michaela, and Kristin of the Vura-Weis group
Funding: AFOSR

19. Back up slides

20. Detailed Overview

21. LFMT simulations predict experimental specificity
Element-, oxidation state-, spin state-, and ligand field specific
Spin state specificity Oxidation state specificity
Make the oxidation state one more different colors.

22. Non-resonant absorption of Zn porphyrin
Non-resonant background subtraction
Resonant absorption peak of Fe porphyrin
Non-resonant absorption of Zn porphyrin
3𝑝𝑥,𝑝𝑦,𝑝𝑧
3𝑑𝑥𝑦
3 𝑑 𝑦𝑧 , 𝑑 𝑥𝑧
3 𝑑 𝑧 2
3 𝑑 𝑥 2 − 𝑦 2
b1g
a1g eg
b2g
Absorption (OD)
Fix axis, and replace 1 image

23. FeIII HS XUV transition diagram… … 4H z xy … 4I 6P
6T1
6A2 + 6E
3p53d6
6T2
6B2 + 6E 6D 6E
6A1 + 6B1
6T1
6A2 + 6E 6F
6T2
6B2 + 6E
6A2
6B1 …
Completed transition diagram for FeIII HS. Note: spin-orbit coupling for 1st row transition metals is weak and will effectively broaden the six allowed transitions. … 2I 4D …
4A1
4A1 …
3p63d5 4P 4E
4A1 + 4B1 4G
4T1
4A2 + 4E
4T2
4A2+ 4E 6S
6A1
6A1

24. XUV transition diagram shows agreement of LFMT simulation with experimental data… … 4H z xy … 4I 6P
6T1
6A2 + 6E
3p53d6
6T2
6B2 + 6E 6D 6E
6A1 + 6B1
6T1
6A2 + 6E 6F
6T2
6B2 + 6E
6A2
6B1 …
Animate in sticks, then curve.
Completed transition diagram for FeIII HS. Note: spin-orbit coupling for 1st row transition metals is weak and will effectively broaden the six allowed transitions. Remember that the kaili program DOES encorparate spin-orbit coupling … 2I 4D …
4T2
4A1 4P …
3p63d5
4T1
4A1 + 4B1 4G 4E
4A2 + 4E
4A1
4A2+ 4E 6S
6A1
6A1

25. Matching data to expected timeframe
Condensed phase
Ultrafast TR-XUV transient absorption spec, 400nm pump, broadband XUV probe generated using HHG
FeTPPCl thin film
Initial (pi,pi*) state forms instantly and lasts ~100fs. XUV dark.
Formation of probable LMCT state within 300fs.then lasts ~2ps. Possible cascade of LMCT states.
Decay of state with fast and slow component(s) Possible cascade of d-d XSs.
π π*
π* dA
dB  π
dA  dB
Rise of LMCT
Rise of d-d state
Decay of d-d state
100fs
600fs
2.6ps
18ps

26. Detailed schematic Optics Chamber Sample Chamber HHG Chamber
Spectrometer
Chamber
Probe
Reflectivities: Silicon- 10% for NIR and 50% for XUV, Al reflects NIR, not absorbs. Al is 10^-5 for NIR, .5 for XUV. Au-coated torroidal mirrors are 80% efficient. grating efficiency is 20%
Pump
Delay Stage

27. HHG Noise (artifacts)
Figures by Dr. Ming-Fu Lin for a publication under review (Optics Express)

28. Deposition methods Spin coating Thermal Evaporation
Poor thickness control
Shapes on membrane
Must sample map
Spatially inhomogeous thickness
XUV collection time: 1-2 hrs per sample
nm thickness control
Spatially homogenous films
XUV collection time: 10 min per sample
TED built by Kristin Benke

29. Future work: probing multimetallic photophysics
Directly-bonded MM’ complexes
Bridged porphyrin-corrole complexes
Modified figure from Lu, C.; Vura-Weis, J. Porphyrin proposal.