Surface scattering at vacuum ultra-violet (VUV) free electron lasers

Surface scattering at vacuum ultra-violet (VUV) free electron lasers

The “Mobile Beamer” is designed to make use of the advantageous properties of the radiation produced by the VUV free electron laser (FEL) in Dalian (DCLS) for surface scattering experiments on atoms and molecules. The VUV photolysis of small molecules will be used as a novel source of atomic beams, providing fine control over the electronic state and kinetic energy. In addition, the tunable VUV light source will be used for sensitive, isomer-specific detection of molecular scattering products and reaction intermediates at surfaces, via photoionization mass spectrometry (PIMS) with high temporal time of flight (TOF) and spatial velocity map imaging (VMI) resolution.

Fundamental Questions

Open-shell atoms can interact strongly with the electrons of a metal surface. For example, it has recently been shown that nonadiabatic "electronic friction" is the dominant energy dissipation mechanism that allows hydrogen atoms to stick to metals.[1]

  1. To what extent do such forces govern the dynamics of heavier high-electron-affinity atoms (O, C, N) at surfaces? Will current theories for non-Born-Oppenheimer dynamics succeed in describing the scattering process of the atom?
  2. Can electronic excitation of atoms lead to an increased reactivity with surface adsorbates or is the electronic energy lost to surface degrees of freedom without enhancing a reaction
       

 Although elementary kinetic models provide invaluable predictive power for gas-phase reactions, they are practically non-existent in surface chemistry due to the complex structure of interfaces. Recent work in the Wodtke group has provided the first transferrable kinetic model to treat the elementary site-specific processes in CO oxidation on platinum.[2]

        3. By probing surface intermediates, can models based on elementary reaction steps be built for the predictive
           description of more complex catalytic systems?

 The Free Electron Laser in Dalian

The Dalian coherent light source is currently the world's brightest FEL in the vacuum ultra violet range, offering sub-picosecond light pulses with pulse energies well above 100 μJ in a wavelength range from 50-150 nm at a bandwidth of 0.1% of the photon energy. The “Mobile Beamer” will be afforded 20% of the beam time at the second beamline which offers a full control of the polarization (vertical, horizontal, circular).[3]

 The molecular beam surface scattering apparatus as an end station at the DCLS

The vacuum setup currently under construction is shown in the upper panel in Figure 1. The source chamber shown is designed for the production of atomic beams: A molecular beam of small molecules (CO, NO, O2) is expanded. The free electron laser is used to dissociate these molecules to produce a beam of atoms (O, C, N). A particular dissociative channel can be selected by changing the wavelength and the polarization of the free electron laser, allowing fine control over the atom’s velocity and electronic state. Laser ionization followed by ion imaging will be used to detect the scattering projectiles after collision with the surface. See also the lower panel of Figure 1.

 

Figure 1 : Sketch of the “Mobile Beamer”. Upper panel: the overview shows the vacuum apparatus, which is mounted on a translational stage so that free electron laser radiation can be used in either the source or scattering chamber. Lower panel: The atom beam is produced by dissociating small molecules at the position indicated by the red dot. Atoms traveling along the blue line can be used for scattering experiments and are detected via ionization at the position indicated by the orange dot.           

 

In addition, another source chamber can be coupled to the apparatus, featuring two molecular beam sources for the study of bimolecular surface reactions. In this experimental configuration, the FEL will be used for isomer-specific detection of reaction products and reaction intermediates by near threshold ionization. The tunable polarization properties of the FEL can also be used to distinguish chiral enantiomers via sensitive, state-of-the-art techniques such as photoelectron circular dichroism (PECD).

 

In order to fulfill these goals the end station will be equipped with:

 

  1. A nanosecond dye laser providing radiation for REMPI spectroscopy over a broad range of  wavelengths
  2. A femtosecond laser for non-resonant multiphoton ionization
  3. A picosecond  laser tunable in the IR for synchronization with the free electron laser to study ultra-fast dynamics at surfaces      
  4. A F2-Laser producing high pulse energy radiation at 157 nm giving rise to intense O-atom beams via dissociation of O2.

 

Even more importantly, we are looking for enthusiastic PhD candidates who will have the opportunity to use these powerful experimental tools at the worlds brightest VUV free electron laser to unravel fundamental processes in the interaction of atoms and molecules at surfaces.       

 

[1] O. Bünermann et al., 2015 Science 350 1346.

[2] J. Neugebohren et al., 2018 Nature 558 280.

[3] Deng Hai-Xiao et al., 2014 Chinese Phys. C 38 028101.

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