Research Focus

Research Focus

Overview and summary

Dynamics at surfaces is the field where atomic-scale understanding emerges concerning the elementary events important to interfacial energy transfer and surface chemistry. Following the tradition of gas-phase chemical dynamics, a well proven strategy is employed, where quantum-state, angle (both incident and recoil) and speed resolved experiments are compared with first-principle theoretical simulations. Eventually, theoretical advances can be obtained that are sufficient to quantitatively describe a variety of highly sophisticated experimental results obtained over a wide range of experimental conditions. Where such agreement can be achieved, the validated first principle theory can be used to reveal an atomic level movie of interactions important in the molecule surface encounters. In a sense, this approach becomes our capable or informing us of the complete range of atomic motions contributing to the processes of interest.

Research concepts and strategies

In defining research concepts and strategies it is helpful to first ask the question: what distinguishes dynamics at surfaces from the well-studied field of dynamics in gas-phase chemical reactions. Many points come immediately to mind. First, the interactions present at a solid interface may dramatically alter the potential energy surface (PES), for example raising or lowering barriers to reactions in comparison to their gas phase counterparts. This is the principle of heterogeneous catalysis. The theoretical methods for calculating these changes in the PES are popular but not well tested. A second essential aspect of dynamics at surfaces concerns the fact that, in contrast to gas phase phenomena, the solid is an energy bath that can influence the outcome of chemical processes by supplying or draining energy from the reaction center in ways that are not possible in the gas phase. How translational energy is taken up by the solid during a molecular collision can influence the molecular trapping probability, the first step in all catalysis. How energy in molecular vibration exchanges with the solid can control the energy directly available to make and break chemical bonds. The research strategies used in the Department of Dynamics at Surfaces are designed to gain quantitative information about both of these essential defining aspects of the field.

State-to-state molecular-beam surface-scattering

A UHV surface scattering machine using optical excitation and detection techniques as well as molecular beam methods.

In order to observe how a molecule exchanges energy with a surface we perform collision experiments. In such experiments, the molecule is first prepared in a specific initial state prior to the collision with the surface. Here, the speed, direction, degree of orientation, rotational, vibrational, and electronic motion must all be defined. After the collision, all of these quantities, i.e. speed, direction, degree of orientation, rotational, vibrational, and electronic motion, should be re-measured. By amassing an enormous "before and after data set", one may use theoretical methods to determine what must have happened to the molecule during the collision.

The Nobel Prize for chemistry was awarded to Yuan T. Lee, Dudley Herschbach and John C. Polanyi for their development and implementation of the molecular beams method for the study of gas-phase chemical dynamics. This approach is now used routinely in physical chemistry and its importance in the study of Dynamics at Surfaces is profound. By controlling the experimental conditions, one is able to adjust and manipulate the speed and precisely define the direction of a molecular sample. Combining molecular beams with laser-based optical pumping methods, one can also define the initial quantum state of the molecules, essentially turning the rotational, vibrational and electronic knobs on the molecule. Thus, the initial conditions under which a molecule finds itself prior to colliding with a surface may be perfectly defined. Using quantum-state-selective detection, for example Resonance Enhanced Multiphoton Ionization (REMPI), one can interrogate the molecule, determining which quantum states are produced in the collision. Using pulsed lasers separated over a distance, one may perform state-to-state, time-of-flight (TOF) measurements, which reveal the speed and direction of the recoiling molecule after the collision with the surface.

We have built two instruments for these types of experiments and we are building a third (for studies under extreme conditions).

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