Research of Prof. Dr. Bernhardt

Surface Femtochemistry

The influence of a solid support on the dynamics of a molecular encounter is of fundamental interest to all aspects of surface chemistry and catalysis. An issue of central importance in this respect is the geometrical alignment of the initial collision complex, which determines the passage through the transition state and ultimately the outcome of the desired chemical reaction. The motion involved in this process occurs on the ultrafast timescale of nuclear movement and its understanding is fundamental to the perception of chemical reaction mechanisms on surfaces of, e.g., catalytic materials.

However, in order to be able to unravel the decisive molecular dynamics, the averaging over an ensemble of impact parameters and trajectories originating from variations in the starting geometry has to be minimized.

In gas phase experiments this can be realized by employing van der Waals complexes of reactants with defined geometry that are prepared in a supersonic molecular beam. On surfaces a complementary approach has been pioneered by John C. Polanyi which was termed “surface-aligned reaction” and which relies on an ordered adsorbate structure on a solid surface. In our laboratory this idea was realized by a novel experimental approach for direct mass resolved monitoring of surface transition states and products.

Complementary two photon photoemission spectroscopy experiments reveal the electronic structure of the investigated substrates.

 

M. E. Vaida, T. M. Bernhardt:
Surface pump-probe femtosecond-laser mass spectrometry: Time-, mass-, and velocity-resolved detection of surface reaction dynamics,
Rev. Sci. Instrum. 81, 104103 (2010).

Z. Ning, J. C. Polanyi:
Surface-aligned reaction,
J. Chem. Phys. 137, 09170 (2012)

M. E. Vaida, T. M. Bernhardt:
Surface-aligned femtochemistry: Molecular reaction dynamics on oxide surfaces,
in “Ultrafast phenomena in molecular sciences”, Ed. R. de Nalda and L. Bañares, Springer Verlag, Berlin (2014), S. 231.

Research Projects

Methyl halide molecular photoreaction dynamics on oxide surfaces

The breaking and making of chemical bonds in molecules that are attached to a substrate constitute the elementary steps of a surface chemical reaction and occur on the ultrafast timescale of nuclear motion. Their understanding is fundamental to the perception of chemical reaction mechanisms on surfaces of, for example, catalytic materials. The key to a molecular level insight of a bimolecular reactive encounter, however, is the knowledge about the structure and the dynamics of the transition state of the reaction. We demonstrated that in contrast to other surface femtochemistry approaches, the transition state and the product formation dynamics of a bimolecular surface reaction can be directly probed by time-, mass-, and velocity-resolved multi-photon ionization on the surface.

Methyl iodide molecules adsorbed on an insulating magnesia thin film were chosen as a photochemical model system and the ultrafast bimolecular reaction of spin-orbit excited iodine atoms (I*) with ground-state iodine (I) emerging after photodissociation on the surface was probed by detection of molecular iodine in the electronically excited B-state and of CD3 radicals via multi-photon ionization.
 

M. E. Vaida, T. M. Bernhardt:
Surface-aligned femtochemistry: Real-time dynamics of photoinduced I2 formation from CD3I on MgO(100)
ChemPhysChem 11, 804 (2010).

M. E. Vaida, T. M. Bernhardt:
Surface-aligned femtochemistry: Photoinduced reaction dynamics of CH3I and CH3Br on MgO(100),
Faraday Disc. 157, 437 (2012).

Ultrafast view on surface chemistry: Methyl iodide molecules geometrically aligned through adsorption on an insulating magnesia surface form molecular iodine, if excited by a uv laser pulse. Femtosecond time-resolved detection of the very first steps of the bimolecular encounter reveals the surprisingly complex dynamics of this model type surface reaction

Electron dynamics on metal cluster lattices on graphene

The bound unoccupied electronic state structure of an Ir(111)/graphene surface covered by differently sized and spaced Ir clusters was investigated by means of two-photon photoemission spectroscopy. The cluster lattice was found to affect the image potential states of the substrate to a surprisingly large extend. This effect can be related to the influence of the cluster lattice on the screening of the image state electron trapped in front of the surface. The symmetric arrangement of Ir clusters considerably reduces the lateral extension of graphene areas with a homogeneous local work function and from a certain minimum area size the excitation of an electron to a stable state in a Coulomb-like potential is not possible anymore. Furthermore, lateral confinement effects could be observed due to the decreasing extension of bare graphene areas.

 

K. Jochmann, T. M. Bernhardt
The influence of metal cluster lattices on the screening of image potential state electrons on graphene
J. Chem. Phys. 149, 164706 (2018)

Iridium cluster arrays on graphene islands on Ir(111); STM image:125 x 125 nm

Tuning the interaction of molecules with metal substrates by ultrathin oxide layers

The femtosecond-laser induced photodissociation of CH3Br adsorbed at sub-monolayer coverage on a solid surface was investigated by time-resolved pump-probe mass spectrometry. To tune the interaction of the CH3Br molecules with the substrate, an Mo(100) surface was covered with ultrathin insulating MgO layers of variable thickness. By gradually decreasing the magnesia layer thickness to the 2D limit the photodissociation dynamics observed by detection of the methyl fragment indicates an energetic lowering of the relevant methyl bromide excited states due to the increasing spatial proximity of the metallic support. Potential orientational effects of the methyl bromide adsorption geometry are also considered.

M. E. Vaida, T. M. Bernhardt:
Tuning the ultrafast photodissociation dynamics of CH3Br on ultrathin MgO films by reducing the layer thickness to the 2D limit,
Chem. Phys. Lett. 688, 106 (2017).

L. A. Ciprinano, S. Tosoni, G. Pacchioni:
CH3Br adsorption on MgO/Mo ultrathin films: a DFT study,
Surf. Sci. 672-673, 1 (2018)

Schematic energy diagram of the orbitals of a hypothetical particle (atom or molecule) as a function of distance between the particle and a metallic surface. In addition, the proposed change of the methyl bromide adsorption geometry with the variation of the magnesia layer thickness on Mo(100) is depicted schematically.