Theoretical investigations of the quantum dynamics of adsorbed molecules – DYQUMA

We use existing methods in quantum chemistry and quantum dynamics and develop new methods.

The project has effectively started 12 months ago. Results are in progress and are being prepared for publication.

A fair number of tasks foreseen in the project will come to a successful conclusion.

None at the moment.

This project is concerned with the theoretical study of the dynamics of single molecules adsorbed on a metallic crystalline substrate. Since the pioneering work by Langmuir we know that the adsorption processes play a key role in the field of heterogeneous catalysis. The Nobel Prize of Chemistry has recently been attributed to G. Ertl for his experimental findings regarding, among others, the understanding of elementary steps in the synthesis of ammonia on iron. However, the understanding of these elementary steps remains mainly phenomenological: very few basic studies of the dynamics of molecular adsorption have been undertaken, to date, at the microscopic level. Several fundamental questions remain without proper answer. For instance: how do the adsorbed atoms and molecules move parallel to the substrate? Can this motion be appropriately described by a semi-classical approach or is it of full quantum nature ? How does this motion affect the kinetics of the adsorption and desorption processes?
Relevant answers to these questions should allow us to better exploit and guide the catalytic processes; they are therefore highly pertinent technologically both in the gas phase as well as in the domain of heterogeneous catalysis and surface chemistry. One of the aim of this project is to open new perspectives in this field. We will concentrate on a microscopic approach, and give quantitative and rigorous insight for a given class of important systems.
The various parameters which may influence the physics of the process will be taken into account as completely as possible. In this project, the dynamics of the adsorbed atoms and molecules will be treated quantum mechanically. The proper understanding of quantum effects is strongly susceptible to carry us to new technological applications in the aforementioned fields..
Theoretical challenges are therefore manifold: first, we need to obtain accurate descriptions of potential energy surfaces, preferably by the intermediate of global analytical representations, for large systems where London dispersion forces can play an important role. For that purpose, we propose the development of new electronic structure models which combine density-functional and wave function theories. Secondly, we need to calculate the quantum dynamics in many dimensional spaces; the largest difficulty that arises in this respect is the very rapidly increasing density of vibrational states due to the low energies of eigenstates pertaining to frustrated translations parallel to the substrate as well as hindered rotations of the adsorbed species; since first principle calculations of potential energy surfaces in the appropriate zero-coverage limit are extremely difficult, the modeling of the single particle quantum dynamics is a formidable challenge. Thirdly, even the single particle dynamics of adsorbed species cannot be treated in a fully realistic way without appropriate consideration of couplings to the quasi-continuum of electronic substrate states, of the coupling to substrate phonons and, potentially, of further couplings to electronic states of the adsorbate itself. In this project we will cope with all these challenges.