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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. desorption . dissociative

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Electron-hole pair excitations in the dissociative adsorption

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  1. 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)

  2. gas/solid interfaces desorption dissociative adsorption molecular adsorption staticproperties (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

  3. dynamics of diatomic molecules on metal surfaces theoreticalmodel: twosteps 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) 

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

  5. why N2 abundantly dissociate on W(100) and not on W(110) Z=3.25Å Z=2Å probabiliity of reaching Z theamount of N2moleculesthat are abletoreach Z=3.25 Å is much smaller in the W(110) face impact energy Ei (meV) Alducinet al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

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

  7. dynamics of diatomic molecules on metal surfaces theoreticalmodel: twosteps calculation of the Potential Energy Surface (PES) • adiabatic approximation • frozen surface approximation  6D PES: V(X, Y, Z, r,q, j) 6D PES construction We are assuming the validity of the Born-Oppenheimer approximation • 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) 

  8. Exposure (ML) non-adiabaticeffects: electron-holepairexcitations chemicurrents vibrationalpromotion of electron transfer 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) adiabaticapproximation forH2on Pt(111) Luntzet al., JCP 123, 074704 (2005) Díaz et al., PRL 96, 096102 (2006) Nieto et al., Science312, 86 (2006).

  9. description of electronicexcitationsby a frictioncoefficient previouslyusedfor: • - damping of adsorbatevibrations: • Persson and Hellsing, PRL49, 662 (1982) • - dynamics of atomicadsorption • Trail, Bird, et al., JCP119, 4539 (2003) • dissociationdynamics (lowdimensions) • 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

  10. description of electronicexcitationsby a frictioncoefficient friction coefficient: effective medium approximation previouslyusedfor: • - damping of adsorbatevibrations: • Persson and Hellsing, PRL49, 662 (1982) • - dynamics of atomicadsorption • Trail, Bird, et al., JCP119, 4539 (2003) • dissociationdynamics (lowdimensions) • Luntz et al., JCP 123, 074704 (2005) bulk metal classical equations of motion for each atom “i” in the molecule n(z) mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt) n0 z adiabatic force: 6D DFT PES friction coefficient n0 h=n0kFstr(kF) effective medium: FEG with electronic density n0

  11. probability of dissociativeadsorption: N2on W(110) polar angle of incidence Qi=0 adiabatic sticking coefficient Qi=45 Qi=60 initial kinetic energy (eV)

  12. probability of dissociativeadsorption: N2on W(110) non-adiabatic polar angle of incidence Qi=0 adiabatic sticking coefficient Qi=45 butforthissystem, dissociationis roughlydecided at Z=2.5A (low energies) Qi=60 initial kinetic energy (eV) Juaristiet al., PRL 100, 116102 (2008)

  13. polar angle of incidence Qi=0 Qi=45 sticking coefficient [Salin, JCP 124, 104704 (2006)] Qi=60 non-adiabatic adiabatic initial kinetic energy (eV) probability of dissociativeadsorption: H2on Cu(110) Juaristiet al., PRL 100, 116102 (2008)

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

  15. in summary a local description of the friction coefficient shows that electronic excitations play a minor rolein 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

  16. 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) AntoineSalin(Université de Bordeaux)

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

  18. energyloss of reflectedmolecules: N2on W(110) Qi=0 Qi=45 Qi=60 Ei=1.5 eV energy losses in the reflected molecules due to electronic excitations are < 100 meV Juaristiet al., PRL 100, 116102 (2008)

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

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

  21. embedding in a FEG

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

  23. Cu (110) W (110) Z(Å) Z(Å) Y(Å) Y(Å) CFM centro de física de materiales Why are frictioneffects so minor? Hydrogen Nitrogen In thedissociationpath, frictioncoeffient can takelargevalues

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