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Electronic excitations in elementary reactive processes at metal surfaces

Electronic excitations in elementary reactive processes at metal surfaces. Ricardo Díez Muiño Centro de Física de Materiales Centro Mixto CSIC-UPV/EHU San Sebastián (Spain). Contributors. Donostia - San Sebastián Maite Alducin (CSIC) Ricardo Díez Muiño (CSIC)

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Electronic excitations in elementary reactive processes at metal surfaces

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  1. Electronic excitations in elementary reactive processes at metal surfaces Ricardo Díez Muiño Centro de Física de Materiales Centro Mixto CSIC-UPV/EHU San Sebastián (Spain)

  2. Contributors Donostia - San Sebastián Maite Alducin (CSIC) Ricardo Díez Muiño (CSIC) Itziar Goikoetxea (PhD, CSIC) Iñaki Juaristi (UPV) Geetha Kanuvakkarai (Post-doc, CSIC) Ludovic Martin (Post-doc, DIPC) Others Fabio Busnengo (Rosario, Argentina) Antoine Salin (Bordeaux, France)

  3. outline - brief introduction: adiabatic processes - electronic excitations at the surface: friction - electronic excitations at the molecule

  4. gas/solid interfaces 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

  5. N2 + 3H2 2NH3 surface face and reactivity rates of ammonia synthesis over five iron single-crystal surfaces Fe (111) Fe (110) Fe (100)

  6. dissociative adsorption of N2 on W(100) and 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)

  7. dissociative adsorption of N2 on W(100) and on W(110) W(100) W(110) why the difference in the N2 dissociation rate at low energies between the (100) and (110) faces of W? 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)

  8. dynamics of diatomic molecules on metal surfaces: theory theoreticalmodel: twosteps 1.- 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)] 2.- 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) 

  9. theory Pfnür et al. (1986) Rettner et al. (1990) dissociation of N2 on W(110) sticking coefficient S0 impact energy Ei (eV) Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

  10. Z(Å) q=0o r(Å) why N2 abundantly dissociate on W(100) and not on W(110) Z=3.25Å potential well Z=2Å probabiliity of reaching Z impact energy Ei (meV) Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

  11. Difficult access to the precursor well Once in the precursor well, molecules easily dissociate in summary, dynamics matters

  12. outline - brief introduction: adiabatic processes - electronic excitations at the surface: friction - electronic excitations at the molecule

  13. 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) 

  14. Exposure (ML) non-adiabatic effects: electron-hole pair excitations 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).

  15. 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

  16. description of electronic excitations by a friction coefficient friction coefficient: effective medium approximation 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) 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

  17. 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)

  18. probability of dissociative adsorption: N2 on 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) Juaristi et al., PRL 100, 116102 (2008)

  19. 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 dissociative adsorption: H2 on Cu(110) Juaristi et al., PRL 100, 116102 (2008)

  20. 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 friction coefficient velocity

  21. energy loss of reflected molecules: N2 on 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 Juaristi et al., PRL 100, 116102 (2008)

  22. Energy loss of reflected molecules: N2 on W(110) Experimental conditions Experimental data by Hanisco et al. J.Vac.Sci.Technol. A 11 ,1907 (1993) • Ts=1200K • Trot <5K (J=0) • Normal incidence • and detection

  23. Energy loss of reflected molecules: N2 on W(110) Experimental conditions Experimental data by Hanisco et al. J.Vac.Sci.Technol. A 11 ,1907 (1993) • Ts=1200K • Trot <5K (J=0) • Normal incidence • and detection adiabatic electronic friction

  24. Energy loss of reflected molecules: N2 on W(110) Experimental conditions Experimental data by Hanisco et al. J.Vac.Sci.Technol. A 11 ,1907 (1993) • Ts=1200K • Trot <5K (J=0) • Normal incidence • and detection adiabatic GLO: phonons Phonon excitations are responsible for the energy transfer observed experimentally

  25. conclusions • A local description of the friction coefficient shows that electronic excitations play a minor role in the dissociation of N2/W and H2/Cu: • The Born-Oppenheimer approximation remains valid in these systems. • Open questions still remain about the role of electron-hole pair excitations in other situations, in which non-adiabatic effects are due to the crossing of two or more potential energy curves, with possible transfer of charge included.

  26. outline - brief introduction: adiabatic processes - electronic excitations at the surface: friction - electronic excitations at the molecule

  27. : can we enhance dissociation? O2 on metal surfaces non-adiabatic effects in the incoming O2 molecule singlet O2 Yourdshahyan et al., PRB 65, 075416 (2002) Behler et al., PRL 94, 036104 (2005) Carbogno et al. PRL 101, 096104 (2008) triplet O2 we prepare the incoming O2 molecule in an excited state excited state Surface 1Dg 1 eV triplet to singlet excitation energy (gas phase O2) electronic excited states may play a role 3Sg

  28. adsorption of O2 on flat Ag surfaces Q Ei • Ts < 150K: O2 adsorbs only molecularly (Ei < 1eV) molecular beam experiments  • Ag (111) • Ag (100)/Ag(110) Low probability 70% A. Raukema et al., Surf. Sci. 347, 151 (1996). L. Vattuone et al., Surf. Sci. 408, L698 (1998).

  29. O2/Ag(100) - theoretical calculations Q Ei calculation of the Potential Energy Surface (PES) z • Born-Oppenheimer approximation • frozen surface approximation 6D PES: V(X, Y, Z, r, q, j) q Z  r Y y X j classical trajectory calculations surface unit cell x • incidence conditions are fixed: • (Ei, Q) • Monte-Carlo sampling on the • internal degrees of freedom: • (X, Y, q, j) and on  (parallel velocity)

  30. building the 6D PES numerical procedure DFT energy data • O2 in vacuum spin-triplet ground state: • DFT - GGA (PW91) calculation with VASP • plane-wave basis set and US pseudopotentials • periodic supercell: (2 x 2) and 5-layer slab y hollow top view front view x top bridge z • about 2300 spin-polarized DFTvalues • interpolation of the DFT data: Corrugation reducing procedure [Busnengo et al., JCP 112, 7641 (2000)]

  31. dissociation probability of spin-triplet O2/Ag(100) Q=0o Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

  32. dissociation probability of spin-triplet O2/Ag(100) Q=30o Q=0o Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

  33. dissociation probability of spin-triplet O2/Ag(100) Q=30o Q=0o Q=45o • General features: • Activation energy: ~1.1eV • Low dissociation probability Q=60o 2D Potential energy surface Reason: Only configurations around bridge lead to dissociation • Energy barrier: • E~ 1.1 eV • Position: • Z≈1.5 Å Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

  34. the question can we enhance O2 dissociation on clean Ag(100) ? Gas phase O2 6D PES calculation: Non spin polarized DFT 1Dg 1 eV triplet to singlet excitation energy 3Sg

  35. differences between SP and NSP PESs z q Z r Gas phase O2 Y 1Dg Non spin polarized y X j 1 eV triplet to singlet excitation energy surface unit cell x Spin polarized 3Sg

  36. dissociation is enhanced for singlet O2 Q=0o Q=30o Q=45o spin-triplet O2 spin-triplet O2 spin-triplet O2 spin-singlet O2 spin-singlet O2 spin-singlet O2 dissociation occurs for Ei < 1 eV dissociation can increase in one order of magnitude Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

  37. dissociation is enhanced for singlet O2 Q=0o Q=30o Q=45o But there is a trick here! The total energy is 1 eV larger for the singlet O2 spin-triplet O2 spin-triplet O2 spin-triplet O2 spin-singlet O2 spin-singlet O2 spin-singlet O2 Gas phase O2 1Dg 1 eV triplet to singlet excitation energy 3Sg dissociation occurs for Ei < 1 eV dissociation can increase in one order of magnitude Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

  38. dissociation is enhanced for singlet O2 Q=0o Q=30o Q=45o spin-triplet O2 spin-triplet O2 spin-triplet O2 spin-singlet O2 spin-singlet O2 spin-singlet O2 1 eV 1 eV 1 eV dissociation occurs for Ei < 1 eV dissociation can increase in one order of magnitude for Q ≠ 0o, singlet-O2 is more efficient than triplet-O2 with the same total energy

  39. why is that? spin-triplet O2 spin-singlet O2 1 eV 1 eV

  40. why is that? spin-triplet O2 spin-singlet O2 available paths to dissociation are different (and more!)

  41. it is not the same road triplet O2 singlet O2 + 1 eV

  42. conclusions • Dissociation increases in about one order of magnitude, • if singlet–O2 molecular beams are used. • The enhancement of the dissociation rate is not only due • to the extra energy that we are adding to the system. • A different spin state in the incoming O2 molecule opens • new paths to dissociation.

  43. thank you for your attention

  44. Korta UPV/EHU (2007) chemistry faculty UPV/EHU (80’s) DIPC (2000) CIC Nanogune (June 2007) CFM (Febr. 2008)

  45. differences between SP and NSP PESs z q Z r Gas phase O2 Y 1Dg Non spin polarized y X j 1 eV triplet to singlet excitation energy surface unit cell x Spin polarized 3Sg for Z < 2A, the SP and NSP PESs merge

  46. why N2 abundantly dissociate on W(100) and not on W(110) probabiliity of reaching Z impact energy Ei (meV) Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

  47. it is not the same road singlet O2 triplet O2 triplet O2 singlet O2 + 1 eV

  48. R z axis -calculation from the energy: -calculation from the force: Time-dependent density functional theory: Energy loss and friction • TDDFT

  49. time-dependent evolution of the electronic density TDDFT calculation of the induced electronic density cluster: N=254 rs=2 Rcl=12,67 a.u. antiproton velocity: v=1,5 a.u. Borisov et al., Chem. Phys. Lett. 387, 95 (2004) Quijada et al., Phys. Rev. A 75, 042902 (2007)

  50. Time-dependent density functional theory: Energy loss and friction antiproton with v=2 au interacting with a metal cluster N=18 induced dissipative electronic density Dndiss=nTDDFT-nadiab

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