Electron-hole pair excitations in the dissociative adsorption Liverpool, July 2008 Electron-hole pair excitations in the dissociative adsorption of diatomic molecules on metal surfaces Ricardo Díez Muiño Centro de Física de Materiales Centro Mixto CSIC-UPV/EHU San Sebastián (Spain)

gas/solid interfaces static properties (equilibrium) desorption dissociative adsorption molecular adsorption static properties (equilibrium) dynamical properties - adsorption sites and energies chemical bonding induced reconstructions self-assembling reaction rates (adsorption, recombination, …) - diffusion - induced desorption - energy and charge exchange experimental techniques: - LEED, STM, PE, etc. experimental techniques: - molecular beams, TPD, etc. an important goal is to understand how solid surfaces can be used to promote gas-phase chemical reactions

dynamics of diatomic molecules on metal surfaces theoretical model: two steps calculation of the Potential Energy Surface (PES) adiabatic approximation frozen surface approximation  6D PES: V(X, Y, Z, r, q, j) 6D PES construction • extended set of DFT energy values, V(X, Y, Z, r, q, j ) • interpolation of the DFT data: corrugation reduction method [Busnengo et al., JCP 112, 7641 (2000)] classical trajectory calculations: Monte Carlo sampling Q Ei  incidence conditions are fixed: (Ei, Q) sampling on the internal degrees of freedom: (X, Y, q, j) and on  (azimuthal angle of trajectory)

why N2 abundantly dissociate on W(100) and not on W(110) T=800K T=300K T=100K impact energy Ei (eV) W(110) normal incidence Rettner et al. (1988) Beutl et al. (1997) Pfnür et al. (1986) Rettner et al. (1990) sticking coefficient S0

Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006) why N2 abundantly dissociate on W(100) and not on W(110) Z=3.25Å Z=2Å probabiliity of reaching Z the amount of N2 molecules that are able to reach Z=3.25 Å is much smaller in the W(110) face impact energy Ei (meV) Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006) why N2 abundantly dissociate on W(100) and not on W(110) Pfnür et al. (1986) Rettner et al. (1990) theory sticking coefficient S0 impact energy Ei (eV) Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

We are assuming the validity of the Born-Oppenheimer approximation dynamics of diatomic molecules on metal surfaces theoretical model: two steps calculation of the Potential Energy Surface (PES) adiabatic approximation frozen surface approximation  6D PES: V(X, Y, Z, r, q, j) We are assuming the validity of the Born-Oppenheimer approximation 6D PES construction • extended set of DFT energy values, V(X, Y, Z, r, q, j ) • interpolation of the DFT data: corrugation reduction method [Busnengo et al., JCP 112, 7641 (2000)] classical trajectory calculations: Monte Carlo sampling Q Ei  incidence conditions are fixed: (Ei, Q) sampling on the internal degrees of freedom: (X, Y, q, j) and on  (azimuthal angle of trajectory)

vibrational promotion non-adiabatic effects: electron-hole pair excitations chemicurrents vibrational promotion of electron transfer Exposure (ML) NO on Cs/Au(111) electron emission as a function of initial vibrational state Gergen et al., Science 294, 2521 (2001). Huang et al., Science 290, 111 (2000) White et al., Nature 433, 503 (2005) friction in N2/Ru(0001) Luntz et al., JCP 123, 074704 (2005) Díaz et al., PRL 96, 096102 (2006) adiabatic approximation for H2 on Pt(111) Nieto et al., Science 312, 86 (2006).

description of electronic excitations by a friction coefficient previously used for: - damping of adsorbate vibrations: Persson and Hellsing, PRL49, 662 (1982) - dynamics of atomic adsorption Trail, Bird, et al., JCP119, 4539 (2003) dissociation dynamics (low dimensions) Luntz et al., JCP 123, 074704 (2005) classical equations of motion for each atom “i” in the molecule mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt) adiabatic force: 6D DFT PES friction coefficient

friction coefficient: effective medium approximation description of electronic excitations by a friction coefficient previously used for: friction coefficient: effective medium approximation - damping of adsorbate vibrations: Persson and Hellsing, PRL49, 662 (1982) - dynamics of atomic adsorption Trail, Bird, et al., JCP119, 4539 (2003) dissociation dynamics (low dimensions) Luntz et al., JCP 123, 074704 (2005) n(z) z n0 bulk metal classical equations of motion for each atom “i” in the molecule mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt) adiabatic force: 6D DFT PES friction coefficient n0 h=n0kFstr(kF) effective medium: FEG with electronic density n0

probability of dissociative adsorption: N2 on W(110) polar angle of incidence Qi=0 adiabatic sticking coefficient Qi=45 Qi=60 initial kinetic energy (eV)

probability of dissociative adsorption: N2 on W(110) non-adiabatic polar angle of incidence Qi=0 adiabatic but for this system, dissociation is roughly decided at Z=2.5A (low energies) sticking coefficient Qi=45 Qi=60 initial kinetic energy (eV) Juaristi et al., PRL 100, 116102 (2008)

probability of dissociative adsorption: H2 on Cu(110) non-adiabatic adiabatic sticking coefficient initial kinetic energy (eV) polar angle of incidence Qi=0 Qi=45 Qi=60 [Salin, JCP 124, 104704 (2006)] Juaristi et al., PRL 100, 116102 (2008)

why the excitation of electron-hole pairs is not relevant classical equations of motion mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt) for each atom “i” in the molecule n(z) z n0 bulk metal friction coefficient velocity

the Born-Oppenheimer approximation remains valid in summary a local description of the friction coefficient shows that electronic excitations play a minor role in the dissociation of diatomic molecules on metal surfaces: the Born-Oppenheimer approximation remains valid open questions still remain about the role of electron-hole pair excitations in other situations, in which nonadiabatic effects are due to the crossing of two or more potential energy curves, with possible transfer of charge included

gas/surface dynamics in San Sebastian Maite Alducin (CSIC-UPV/EHU) Gisela A. Bocan (DIPC) Ricardo Díez Muiño (CSIC-UPV/EHU) Itziar Goikoetxea (CSIC-UPV/EHU) J. Iñaki Juaristi (UPV/EHU) Fernando Mingo (CSIC-UPV/EHU) Collaborators H. Fabio Busnengo (Universidad de Rosario) Antoine Salin (Université de Bordeaux)

dissociation path for N2/W(110) Z(Å) q=0o Z(Å) q=45o j=270o elbow plots show that the N atoms start to become apart after dissociation has been decided r(Å) r(Å) Z(Å) q=90o j 54o Z(Å) q=90o j 125o r(Å) r(Å) Alducin et al., PRL 97, 056102 (2006)

Qi=0 Qi=45 Qi=60 energy loss of reflected molecules: N2 on W(110) Ei=1.5 eV energy losses in the reflected molecules due to electronic excitations are < 100 meV Juaristi et al., PRL 100, 116102 (2008)

Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006) adiabatic calculation of dissociative sticking Pfnür et al. (1986) Rettner et al. (1990) theory N2/W(110) Qi=60 sticking coefficient S0 impact energy Ei (eV) Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

Díez Muiño and Salin, PRB 62, 5207 (2000) two centers effects Díez Muiño and Salin, PRB 62, 5207 (2000)

embedding in a FEG

friction coefficients multiplied by a factor of 10 centro de física de materiales CFM friction coefficients multiplied by a factor of 10

In the dissociation path, friction coeffient can take large values centro de física de materiales CFM Why are friction effects so minor? Hydrogen Nitrogen Cu (110) Y(Å) Z(Å) W (110) Y(Å) Z(Å) In the dissociation path, friction coeffient can take large values