AVS 65th International Symposium & Exhibition
    Novel Trends in Synchrotron and FEL-Based Analysis Focus Topic Thursday Sessions
       Session SA+AS+HC+SS-ThA

Paper SA+AS+HC+SS-ThA6
Surface Action Spectroscopy Using FHI FEL Infrared Radiation

Thursday, October 25, 2018, 4:00 pm, Room 202A

Session: IoT Session: Multi-modal Characterization of Energy Materials & Device Processing
Presenter: Zongfang Wu, Fritz-Haber Institute of the Max Planck Society, Germany
Authors: Z. Wu, Fritz-Haber Institute of the Max Planck Society, Germany
H. Kuhlenbeck, Fritz-Haber Institute of the Max Planck Society, Germany
W. Schöllkopf, Fritz-Haber Institute of the Max Planck Society, Germany
H.J. Freund, Fritz-Haber Institute of the Max Planck Society, Germany
Correspondent: Click to Email
In model catalysis and general surface science studies, the vibrational characterization of a surface is usually performed with HREELS (high-resolution electron energy loss spectroscopy) or IRAS (infrared reflection absorption spectroscopy). Both methods have disadvantages: HREELS suffers from its low resolution and (in the case of ionic samples) the presence of intense multiple phonon losses, while IRAS requires normalization of the spectrum with the spectrum of a reference sample, which leads to a contamination of the spectrum with features of the reference sample. Action spectroscopy with rare-gas messenger atoms [A. Fielicke et al., Phys. Rev. Lett. 93, 023401 (2004)] is an established method for vibrational spectroscopy of gas-phase clusters that avoids these disadvantages. Rare-gas atoms attached to gas-phase clusters may desorb when the incident infrared (IR) radiation is in resonance with a cluster vibration. The fragmentation rate, as monitored with a mass spectrometer, represents a vibrational spectrum, which can be used to draw conclusions regarding the cluster structure. Clusters, in this case deposited on a surface, are also highly relevant in model catalysis since they greatly influence catalytic reaction paths of supported catalysts. Both cluster shape and size matter. With this topic in mind a new apparatus for the application of IR FEL radiation to action spectroscopy of solid surfaces was set up at the Fritz Haber Institute (FHI), the V2O3(0001)/Au(111) (~10 nm thick) and a TiO2(110) single crystal surface were studied as first test systems. The machine is connected to the FHI FEL, which is able to provide intense and widely tunable (3 – 60 µm) IR radiation. For V2O3(0001)/Au(111) the well-characterized vanadyl surface vibration and other surface vibrations could be detected, while bulk states turned out to be invisible to the method. Presumably, anharmonic vibrational coupling between the primary excited vibration and the rare-gas vs. surface vibration leads to desorption. This explains the surface sensitivity of the method as well as its insensitivity to bulk vibrations. In the case of the TiO2(110) single crystal also rare-gas desorption resulting from an IR induced increase of the crystal temperature could be observed. This led to a better understanding of polaritons in rutile.