Exploring the electron-induced vibrational linewidths of adsorbates on metal surfaces

Novko1, M. Blanco-Rey1,2, J. I. Juaristi1,2,3 and M. Alducin1,3

1.Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, Spain

2.Departamento de Física de Materiales, Facultad de Químicas UPV/EHU, Apartado 1072, 20080 Donostia-San Sebastián, Spain

3.Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), Paseo Manuel de Lardizabal 5, 20018 Donostia-San Sebastián, Spain

The essential part of describing the energy loss mechanism in gas-metal dynamics is to understand the role of non-adiabatic effects that come from the coupling of the adsorbates ionic degrees of freedom and the electronic continuum of the metal surface. Although these effects have been experimentally known to exist for a long time [1], some parts of the theoretical and computational insight have not achieved the desired depth. For example, it is not completely clear whether the vibrational linewidths of the adsorbates on the metal surfaces obtained in infrared absorption spectroscopy (IRAS) [2] can be explained through classical equations of motion corrected with an electronic friction term [3]. In many-body perturbation theory this problem of vibrational linewidths is well established by means of the dynamical phonon self-energy. Therefore, to understand the problem of the electronic friction term, it would be very helpful to establish the connection between the former and the latter physical quantities.

In our work we explore the non-adiabatic coupling in three different gas-metal systems: the H atom on the Pd(111) surface, the N2 molecule on Fe(110), and CO on Cu(100). To do so, we employ two different computational methods: on the one side we use ab-initio molecular dynamics with electronic friction (AIMDEF) [4], together with the local density friction approximation (LDFA) [5], while on the other side we use the phonon self-energy due to the electron-phonon coupling based on the density functional theory (DFT). Through the results of our simulations we show how both of the mentioned methods give similar qualitative behaviors, and we further explore to what extent the LDFA friction term quantitatively agrees with the DFT-based phonon self-energy. The crucial part of this exploration is to keep track of the approximations done in both formulations.

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[2] R. Ryberg, Phys. Rev. B 32, 2671 (1985).

[3] M. Head-Gordon and J. C. Tully, J. Chem. Phys. 103, 10137 (1995).

[4] M. Blanco-Rey et al., Phys. Rev. Lett. 112, 103203 (2014); D. Novko et al., Phys. Rev. B 92, 201411(R) (2015); D. Novko et al., to be published in Phys. Rev. B, (2016).

[5] J. I. Juaristi et al., Phys. Rev. Lett. 100, 116102 (2008).