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## Challenges in reaction dynamics of gas-surface interactions and methodological advances in dissipative and non-adiabatic processes

#### CECAM-FR-GSO

#### Organisers

The role of dissipation in surface chemistry is central to a challenging and important area of current research. At short range the adsorbate motion can couple to either phonon modes or through electronically nonadiabatic coupling to electron-hole pairs (EHPs), which can influence the rates and mechanisms of molecule-solid energy transfer as well as the outcomes of chemical reactions. In a related context, how excitation of a solid becomes localized on an adsorbate, and vice versa, how quenching of vibrationally or electronically excited adsorbates occurs, are central questions in surface photochemistry, especially for metallic surfaces. The challenge is to understand the coupling of the elementary metal excitations to the elementary degrees-of-freedom (DOFs) (translation, rotation, vibration, electronic) of atoms and molecules at or near the surface of the metal. Translational motion of atoms or molecules is involved in elementary surface chemical processes like adsorption and diffusion. When translation is coupled to phonons or to EHPs, adsorption and diffusion can be dramatically altered. Molecular vibration is the elementary motion needed for dissociation. How molecular vibration is coupled to the surface determines how energy flows into and out of reactive bonds. Rotational motion determines the orientation of the adsorbate to the surface; molecular orientation can influence the coupling of the surface to the adsorbate. Also, excited surface modes can transfer their excitation energy to adsorbate molecules and thereby induce dissociation.

Experimental methods for probing and characterizing the coupling of atomic and molecular DOFs to surface excitations have improved in recent years. State-to-state molecular beam surface scattering employing time-of-flight methods and orientation can now comprehensively characterize the dynamics of molecules colliding with surfaces. Optical pumping can prepare molecules in vibrationally excited states and vibrational distributions of relaxed molecules can be measured. Rydberg atom tagging is now implemented in surface scattering. These methods have been applied extensively to the study of energy transfer between molecules and solids, providing insights about the mechanisms of vibrational and translational inelasticity. Coupling of the elementary motions of atoms and molecules to EHPs is now clearly seen in experiment. Consideration of an electronically excited molecule or of EHPs excitations require consideration of electronically non-adiabatic effects beyond the Born-Oppenheimer Approximation (BOA).

For simpler gas phase problems, the electronic structure of atoms or molecules can be treated via post Hartree Fock wave-function (WFT) methods (e.g. Configuration Interaction, Coupled-Cluster Response, theory or mixed variational perturbative methods). Beyond BOA, transitions between electronic states can also be calculated. For the more complex case of reactions at metal surfaces, the molecule-surface system has too many degrees of freedom, both electronic and vibrational, making WFT treatments unfeasible. Instead, Density Functional Theory (DFT) has become a standard approach to investigate molecule-surface interactions. Still, DFT must often be implemented along with reduced dimension approximations (e.g. frozen surface). Yet, full-dimensional, DFT-based on-the-fly calculations (e.g. AIMD) can be attempted for the ground electronic state, but the computation of excited states and of electronic processes remains a formidable challenge. Nevertheless, AIMD has recently been implemented with electronic friction employing a weak coupling approximation. More generally, excited states can, in principle, now be treated via Time-Dependent DFT (TD-DFT). Still, on-the-fly direct electron-nuclei dynamics of molecule-surface interactions is too time-consuming for simulations beyond a few typical trajectories, preventing meaningful comparison to experiment. Other schemes such as Independent Electron Surface Hopping (IESH) and full dimensional molecular dynamics with electronic friction have been developed. Approximations to DFT such as Density Functional Tight Binding (DFTB) or TD-DFTB, constitute another promising route and may allow for full-dimensional dynamical simulations.

The workshop aims to gather theoreticians from molecular physics and from surface physics and chemistry, and to focus on the ways theory can address the various challenges connected with the dynamical aspects of the coupling of a molecule with a surface and in particular the dissipative and non adiabatic processes, with special emphasis on metallic surfaces. Three experimentalists gathering complementary expertise have also been invited for depicting a survey of the experimental state of the art and critical challenges in the field.

The talks and discussions will focus on the following critical topics:

- Full or high dimensionality; reduced dimensionality or hierarchical treatment of the degrees of freedom in the simulations; mixed approach with relevant choice for the degrees of freedom that must be treated quantum-mechanically and those that can be treated in more approximate ways - How to test the validity of the partitioning ?

- Comparison or combined use of various levels of scattering dynamics: quantum, classical, mixed, ...

- Applicability of quantum molecular dynamics methods and the dimensionality problem: standard wavepacket propagation, (multi-layered) Multi-Configuration Time Dependent Hartree for molecule-surface dynamics

- Ground state dynamics: AIMD classical dynamics versus dynamics on pre-computed potential energy surfaces (PES); Fitting PESs for complex polyatomic molecules

- Current and future predictive power of non adiabatic TD-DFT and TD-DFTB approximate versions for molecule-surface dynamics; Treatment of the electronically excited adsorbate, treatment of surface excitations, explicit account of EHPs versus excitation degeneracy in metals; How to bypass the problems stemming from Trajectory Surface Hopping or Ehrenfenst dynamics ?

- Alternative methodologies to implement the coupling of the various molecular degrees of freedom with phonon modes or electron-hole pair states: Independent Electron Surface Hopping (IESH), AIMD with friction, modeling via thermal bath, ...; Validity and benchmarks

- Interplay of theory with experiment; Observables, accuracy and statistical significance of the theoretical data

## References

**France**

Didier Lemoine (CNRS and Université Toulouse III Paul Sabatier) - Organiser

Fernand Spiegelman (Université Paul Sabatier (UPS) and CNRS) - Organiser

**Italy**

Rocco Martinazzo (Università degli Studi di Milano) - Organiser

**United States**

Bret Jackson (University of Massachusetts Amherst) - Organiser