Time Resolved XAS Research

Within this project we use X-ray absorption spectroscopy to investigate local atomic and electronic structure with time resolution, for example to catch intermediate state of catalysts and excited state of photosensitizers. There are a few advantages of X-ray absorption spectroscopy:
  • Selectivity to the type of atoms allows to probe individual components of photocatalytic mixtures.
  • X-ray absorption near edge structure (XANES) is sensitive to the 3D arrangements of atoms around the absorbing center at the distances up to ~4.5 Angstroms
  • Time-resolved methodology allows to track changes in the system triggered by light and to separate them from slow processes that can occur simultaneously in the system.

In combination with X-ray emission spectroscopy the method allows to get complete picture about the electronic structure changes, in particular, spin and oxidation state of 3d metal centers.

The pump-probe project is hosted at the SuperXAS beamline of Swiss Light Source (SLS) synchrotron (Paul Scherrer Institut, Villigen, Switzerland). The setups at SuperXAS are typically used in the time range 100 ns - 1ms and in more specific cases can be also used for experiments with 100 ps resolution. Faster processes in the time range 1 -50 ps we investigate at X-ray free electron lasers, in particular at Alvra beamline of SwissFEL and FXE instrument of European XFEL.
Principal component of the instrument are versatile laser systems, efficient XAS data acquisition system and chamber for anaerobic measurements. Available laser systems include
  • Femtosecond laser system based on Wyvern 500 laser amplifier from KM labs (Wavelength: 800 nm, SHG at 400nm, Repetition Rate: 50-150 kHz, Power: 8 W)
  • Nanosecond laser DS20HE-532 from Photonics Industries (Wavelength: 532 nm, Repetition Rate: 10-150 kHz, Power: 30 W)
  • Nanosecond laser IDOL from XITON Photonics (wavelength 447 and 671 nm, Repetition rate 10-40 kHz, Power 1.5 W)
  • CW laser from CNI (wavelength 447, Power 1.8W)
Nanosecond XITON laser system.

The chamber at SuperXAS beamline allows pump-probe measurements at anaerobic conditions. It allows to measure XAS spectra, XES spectra using von Hamos and Johann-type spectrometers.

The data acquisition system works in time-tagged photon counting mode, which allows to measure kinetics in the time range 30 ns -100 microseconds simultaneously with XAS spectra. Details about the setup can be found in the article: G. Smolentsev, A. Guda, M. Janousch, C. Frieh, G. Jud, F. Zamponi, M. Chavarot-Kerlidou, V. Artero, J. van Bokhoven and M. Nachtegaal “X-ray absorption spectroscopy with time-tagged photon counting. Application to study the structure of Co(I) intermediate of H2 evolving photo-catalysts" Faraday Discuss. 171 (2014) 259
A technology to use the energy of sunlight to produce fuels is highly desirable for replacing fossil fuels with a renew-able energy source. The promising approach for achieving this goal is to mimic photo-synthesis and develop molecular catalysts for water splitting and hydrogen evolution. This field is known as artificial photosynthesis. The most simple artificial photosynthesis process is splitting of water into molecular oxygen and hydrogen. It consists of two half-reactions – the oxidative O2 evolving reaction and reduction of protons generating H2. With time-resolved X-ray absorption spectroscopy we selectively look either at photosensitizer or on catalyst in the photocatalytic system. Since two half-reactions require different catalysts, we study them separately.

G. Smolentsev and V. Sundström “Time-resolved X-ray absorption spectroscopy for the study of molecular systems relevant for artificial photosynthesis” Coord. Chem. Rev. 304-305 (2015) 117-132
The focus of this project is on molecular catalysts with single 3d metal center, in particular Co. The goal is to catch Co(I), Co(III)H and other relevant intermediates of hydrogen evolution reaction which are formed in the time range from hundreds of nanoseconds to seconds. We determine the structure of such intermediates based on quantitative analysis of time-resolved XANES spectra, for example using FitIt code. Example of such application can be found in G. Smolentsev, B. Cecconi, A. Guda, M. Chavarot-Kerlidou, J. A. van Bokhoven, M. Nachtegaal, V. Artero “Microsecond X-ray Absorption Spectroscopy Identification of CoI Intermediates in Cobaloxime-Catalyzed Hydrogen Evolution” Chem. Eur. J. 21 (2015) 15158-15162
Within this project we investigate Co-based catalysts with cubane-type structure using time-resolved X-ray absorption spectroscopy. We have performed in-situ measurements of this catalyst during oxygen production reaction. The project is supported by NCCR MARVEL.

Co4 cubane catalyst for water splitting.
F. Song, R. Moré, M. Schilling, G. Smolentsev, N. Azzaroli, T. Fox, S. Luber, G.R. Patzke “{Co4O4} and {CoxNi4-xO4} cubane water oxidation catalysts as surface cut-outs of cobalt oxides. J. Am Chem. Soc 139 (2017) 14198–14208

Within this project we investigate Cu-based metalloorganic materials which are promising for applications in Organic Light Emitting Diodes. Their high efficiency is probably due to temperature-activated delayed fluorescence (TADF) effect. The goal of our investigation is to catch short lived singlet excited states and to determine its local atomic and electronic structure. This singlet state is short lived (picoseconds), but is relevant for emissive properties of the material also in the microsecond time range. We have received beamtime at European X-ray Free Electron Laser. We also recently performed first pilot pump-probe experiment at Alvra beamline of SwissFEL.
Pilot experiment at Alvra beamline of SwissFEL. View inside the chamber. Green fluorescence from Cu-based OLED complexes excited with UV light.