Non-adiabatic molecular dynamics at surfaces

Non-adiabatic effects, i.e., the violation of the Born-Oppenheimer approximation due to strong electron-nuclear coupling, are ubiquitous in the dynamics of molecules near solid surfaces.

Nonadiabatic effects are particularly important in substrate-mediated or direct surface photochemistry and femtochemistry, but also for processes considered as proceeding typically in the electronic ground state, in particular at metal surfaces: The electron-mediated vibrational relaxation of adsorbed or scattering, vibrationally excited molecules is an example, the occurrence of “chemicurrents” and exoelectron emission another.

Fully quantum, multi-dimensional dynamical approaches to treat non-Born-Oppenheimer dynamics are unfeasible in most cases and / or rely on far-going model assumptions. Recent years have seen the emergence of powerful tools based on classical dynamics, however, which are feasible but nevertheless account for non-adiabatic effects.

One such approach is (Langevin) Molecular Dynamics with Electronic Friction (MDEF), either on precomputed potential energy surfaces or “on the fly”. When coupled with Density Functional Theory (DFT) for forces and the Local Density Friction Approximation (LDFA), this method, allows for parameter-free, multi-dimensional molecular dynamics accounting for non-adiabaticity in an approximate, indirect way. A more direct approach to classical non-adiabatic dynamics is so-called “surface hopping”, i.e. classical dynamics on multiple, non-Born-Oppenheimer coupled potential energy surfaces.

In the present contribution, we illustrate the methodologies for both approaches, MDEF and “surface hopping”, and apply them to selected problems of surface science. MDEF is applied to femtosecond-laser induced, associative desorption of oxygen molecules from an Ag(110) surface [1] or molecular hydrogen from Ru(0001) [2,3], and to femtosecond-laser induced processes in
CO/Ru(0001) [4] and CO/Cu(100). Surface hopping is realized within a semiempirical electronic structure method (AM1/CI), and applied to the photoisomerization of azobenzene molecules on solid substrates [5,6].

References

[1] I. Lončarić, M. Alducin, P. Saalfrank, J.I. Juaristi, Phys. Rev. B 93, 014301 (2016).
[2] G. Füchsel, T. Klamroth, S. Monturet, P. Saalfrank, Phys. Chem. Chem. Phys. 13, 8659 (2011).
[3] J.I. Iuaristi, M. Alducin, P. Saalfrank, Phys. Rev. B 95, 125439 (2017).
[4] R. Scholz, G. Floß, P. Saalfrank, G. Füchsel, I. Lončarić, J.I. Juaristi, Phys. Rev. B 94, 165447 (2016).
[5] G. Floss, G. Granucci, P. Saalfrank, J. Chem. Phys. 137, 234701 (2012).
[6] E. Titov, G. Granucci, M. Persico, J.P. Götze, P. Saalfrank, J. Phys. Chem. Lett. 7, 3591 (2016).