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 and supported metal clusters

The detection of intermediate species and the correlation of their ultrafast dynamics with the morphology and electronic structure of a surface is crucial to fully understand and control heterogeneous photoinduced and photocatalytic reactions. In this work, the ultrafast photodissociation dynamics of CH₃Br molecules adsorbed on variable-size Au clusters on MgO/Mo(100) is investigated by monitoring the CH₃⁺ transient evolution using a pump–probe technique in conjunction with surface mass spectrometry. Furthermore, extreme-UV photoemission spectroscopy in combination with theoretical calculations is employed to study the electronic structure of the Au clusters on MgO/Mo(100). Changes in the ultrafast dynamics of the CH₃⁺ fragment are correlated with the electronic structure of Au as it evolves from monomers to small nonmetallic clusters to larger nanoparticles with a metallic character. This work provides a new avenue to a detailed understanding of how surface-photoinduced chemical reactions are influenced by the composition and electronic structure of the surface.

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).

M. E. Vaida, T. B. Rawal, T. M. Bernhardt, B. M. Marsh, T. S. Rahman, S. R. Leone
Nonmetal-to-Metal Transition of Magnesia Supported Au Clusters Affects the Ultrafast Dissociation Dynamics of Adsorbed CH₃Br Molecules
J. Phys. Chem. Lett. 13, 4747 (2022).

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)