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  5. Ultrafast Chemistry with Free Electron Lasers

Ultrafast Chemistry with Free Electron Lasers

1) Free Electron Lasers

Taking a photo of a (fast) moving object requires a camera with a shutter speed that is faster than the timescale of the object's motion. The same is true when it comes to the investigation of ultra-fast chemical processes, which is one of the research activities in our group. For this activity we mainly utilize X-ray spectroscopic tools, namely X-ray absorption and emission spectroscopy as well as (X-ray) photoelectron spectroscopy in the soft X-ray regime. In the focus of our research are surface chemical reactions that are relevant for heterogeneous catalysis such as the CO oxidation reaction or ammonia synthesis from N2 and hydrogen gas on metal surfaces (see also ‘Ultrafast Surface Chemistry’ section). The fundamental steps of these chemical reactions such as the motion of atoms and the cleavage and formation of chemical bonds between atoms occur on timescales down to pico- and femtoseconds (1 ps = 10-12 s, 1 fs = 10-15 s). Hence, to capture these ultra-fast processes with X-ray spectroscopy, ultra-short femtosecond X-ray pulses are required.

These femtosecond X-ray pulses that are at the same time sufficiently intense are only available at X-ray free-electron lasers (XFELs). XFELs are linear accelerator based X-ray sources where a train of electron bunches passes through a long (tens to hundreds of meters) periodic magnetic structure (undulator). In this long undulator section the electron bunches experience so-called microbunching and each bunch produces an intense and coherent femtosecond X-ray pulse. The femtosecond X-ray pulses can be synchronized to femtosecond optical laser pulses, which we use in our experiments to trigger a chemical reaction. By varying the time delay between the optical trigger pulse and the X-ray probe pulse we can investigate the dynamics of the chemical reaction in real time. The time resolution is determined by the pulse durations as well as by the stability of the delay between both pulses and is currently around 100 fs. For our experiments we have so far mostly used the Linac Coherent Light Source (LCLS) at Stanford, USA and will now extend our activities to the free electron-laser FERMI in Trieste, Italy as well as potentially FLASH and the European XFEL in Hamburg, Germany.

Here are some relevant publications:

Probing the transition state region in catalytic CO oxidation on Ru H. Öström, H. Öberg, H. Xin, J. LaRue, M. Beye, M. Dell’Angela, J. Gladh, M. L. Ng, J. A. Sellberg, S. Kaya, G. Mercurio, D. Nordlund, M. Hantschmann, F. Hieke, D. Kühn, W. F. Schlotter, G. L. Dakovski, J. J. Turner, M. P. Minitti, A. Mitra, S. P. Moeller, A. Föhlisch, M. Wolf, W. Wurth, M. Persson, J. K. Nørskov, F. Abild-Pedersen, H. Ogasawara, L. G. M. Pettersson, and A. Nilsson, Science, vol. 347, no. 6225, pp. 978–982, 2015.

Real-time observation of surface bond breaking with an x-ray laser M. Dell’Angela, T. Anniyev, M. Beye, R. Coffee, A. Föhlisch, J. Gladh, T. Katayama, S. Kaya, O. Krupin, J. LaRue, A. Møgelhøj, D. Nordlund, J. K. Nørskov, H. Öberg, H. Ogasawara, H. Öström, L. G. M. Pettersson, W. F. Schlotter, J. A. Sellberg, F. Sorgenfrei, J. J. Turner, M. Wolf, W. Wurth, and A. Nilsson, Science, vol. 339, no. 6125, pp. 1302–5, Mar. 2013.

Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001) M. Beye, T. Anniyev, R. Coffee, M. Dell’Angela, A. Föhlisch, J. Gladh, T. Katayama, S. Kaya, O. Krupin, A. Møgelhøj, A. Nilsson, D. Nordlund, J. K. Nørskov, H. Öberg, H. Ogasawara, L. G. M. Pettersson, W. F. Schlotter, J. A. Sellberg, F. Sorgenfrei, J. J. Turner, M. Wolf, W. Wurth, and H. Öström, Phys. Rev. Lett., vol. 110, no. 18, p. 186101, 2013.

2) Ultrafast Surface Chemistry

Solid catalysts are frequently used to enhance the rate or selectivity of desirable chemical reactions. These reactions include CO oxidation which helps to reduce poisonous carbon monoxide emissions and ammonia production which is vital for agriculture. Both of these reactions use a solid catalyst to make the reactions easier and therefore more economical. We aim to study the underlying mechanisms of these reactions by examining model catalytic systems. To do this we clean a catalytic surface under ultra-high vacuum conditions before depositing reactants onto it. The reactants typically adsorb onto the surface either through chemical bonding or a physical interaction with the surface. The adsorption process causes changes in the electronic structure of the reactants and keeps them in close proximity with the surface and each other.

To perform our experiments we use a laser “pump” pulse to provide energy to the system in a controlled manner with well-defined temporal characteristics. The energy from the laser pulse creates energetic electrons and phonons in the surface of the catalyst. Energy transfer to the adsorbed reactants can place the reactants into transiently excited states which could allow a reaction or transition to occur. X-ray “probe” pulses from a FEL with a controlled delay from the initial laser pulse can then be used to produce detectable photons or electrons from the system. The energy distribution of the particles produced will be a function of the initial and final electronic states of the reactants. By analysing the energy distribution of the emitted particles as a function of the pump-probe delay time we can measure changes in the electronic states of the reactants on ultrashort timescales. This can help to reveal short-lived intermediate states that may be necessary for a reaction to proceed, this information could be used to uncover the mechanisms behind certain reactions and to potentially design more efficient catalysts.

Here are a few references apart from the ones listed in 'Free Electron Lasers'

The bonding of CO to metal surfaces A. Fohlisch, M. Nyberg, P. Bennich, L. Triguero, J. Hasselstrom, O. Karis, L. G. M. Pettersson and A. Nilsson,Journal of Chemical Physics, vol. 112, no. 4, p. 1946, 2000.

Chemical bonding on surfaces probed by X-ray emission spectroscopy and density functional theory A. Nilsson and L. G. M. Pettersson, Surface Science Reports, vol. 55, no. 2-5, p. 49, 2004.

The electronic structure effect in heterogeneous catalysis A. Nilsson, L. G. M. Pettersson, B. Hammer, T. Bligaard, C. H. Christensen and J. K. Norskov, Catalysis Letters, vol. 100, no. 3-4, p.111-114, 2005.


3) Terahertz radiation

The presence of a surface allows us to achieve control of the reaction coordinate in a manner that is different from chemistry in the solution phase. If we can use electromagnetic radiation to control the motion of molecules on surfaces via electronic polarization effects in specific directions, we can hypothesize that reactivity can be enhanced for important individual steps in a reaction sequence. We have recently demonstrated the use of intense, quasi-half-cycle THz pulses with an associated electric field component comparable to intramolecular electric fields to direct the reaction coordinate of a chemical reaction by stimulating the nuclear motions of the reactants. Using a strong electric field from a THz pulse generated via coherent transition radiation from an ultrashort electron bunch, we present evidence that CO oxidation on Ru(0001) is selectively induced, while not promoting the thermally induced CO desorption process. The reaction is initiated by the motion of the O atoms on the surface driven by the electric field component of the THz pulse, rather than thermal heating of the surface. Here we will investigate how general the usage of THz radiation to provide a level of coherent control of surface chemical reactivity. In future we foresee the combination of THz pump and x-ray laser probe of chemical reactions on surfaces.


THz-Pulse-Induced Selective Catalytic CO Oxidation on Ru J. LaRue, T. Katayama, A. Lindenberg, A. S. Fischer, H. Öström, A. Nilsson and H. Ogasawara Phys. Rev. Lett. 115, 036103 (2015).

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