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motivation Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño an important goal is to understand how solid surfaces can be used to promote gas-phase chemical reactions static properties (equilibrium) - adsorption sites and energies - chemical bonding - induced reconstructions - self-assembling experimental techniques: - LEED, STM, PE, etc. dynamical properties - reaction rates (adsorption, recombination, …) - diffusion - induced desorption - energy and charge exchange experimental techniques: - molecular beams, TPD, etc. dissociative adsorption molecular adsorption desorption

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dynamical properties - reaction rates (adsorption, recombination, …) - diffusion - induced desorption - energy and charge exchange experimental techniques: - molecular beams, TPD, etc. dissociative adsorption motivation Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño an important goal is to understand how solid surfaces can be used to promote gas-phase chemical reactions static properties (equilibrium) - adsorption sites and energies - chemical bonding - induced reconstructions - self-assembling experimental techniques: - LEED, STM, PE, etc. molecular adsorption desorption EiEi adsorption probability depends on: incidence kinetic energy initial rovibrational state incidence angle dissociative adsorption

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adiabatic approximation Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño SurfaceSurface We assume that the time-dependent potential is changing so slowly that the electronic wave function rearranges to the new ground state at any instant of time: The system remains in its instantaneous eigenstate. excited electronic states are not relevant

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electronic excitations Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño SurfaceSurface experimental evidence chemicurrents Exposure (ML) Gergen et al., Science 294, 2521 (2001). vibrational promotion of electron transfer vibrational promotion of electron transfer Huang et al., Science 290, 111 (2000) White et al., Nature 433, 503 (2005) role of electronic friction Trail et al., JCP 119, 4539 (2003) Luntz et al., JCP 123, 074704 (2005) Díaz et al., PRL 96, 096102 (2006) Nieto et al., Science 312, 86 (2006). sticking coefficient initial kinetic energy (eV) polar angle of incidence i =0 i =45 i =60 Juaristi et al., PRL 100, 116102 (2008) Luntz et al., PRL 102, 109601 (2009) (comment) Juaristi et al., PRL 102, 109602 (2009) (reply) electronic excitations at the surface can be considered as decoupled

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O 2 on metal surfaces Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño SurfaceSurface electronic excitations are created in the system Yourdshahyan et al., PRB 65, 075416 (2002) Behler et al., PRL 94, 036104 (2005) Carbogno et al. PRL 101, 096104 (2008) non-adiabatic effects in the incoming O 2 molecule Yourdshahyan et al., PRB 65, 075416 (2002) Behler et al., PRL 94, 036104 (2005) Carbogno et al. PRL 101, 096104 (2008)

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O 2 /Ag Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño EiEi molecular beam experiments on flat Ag surfaces Ts < 150K: O 2 adsorbs only molecularly (E i < 1eV) Ag (100)/Ag(110) L. Vattuone et al., Surf. Sci. 408, L698 (1998). A. Raukema et al., Surf. Sci. 347, 151 (1996). 70% Low probability dissociation of O 2 on Ag(100) possible ways to enhance dissociation: role of excited electronic states? Ag (111)

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O 2 /Ag(100) - theoretical calculations Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño EiEi incidence conditions are fixed: (E i, Monte-Carlo sampling on the internal degrees of freadom: (X, Y, , ) and on (parallel velocity) Born-Oppenheimer approximation frozen surface approximation 6D PES: V(X, Y, Z, r, , ) calculation of the Potential Energy Surface (PES) classical trajectory calculations X Y Z x y z surface unit cell r

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building the 6D PES Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño about 2300 spin-polarized DFT values interpolation of the DFT data: Corrugation reducing procedure [Busnengo et al., JCP 112, 7641 (2000)] numerical procedure O 2 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 DFT energy data z top hollow bridge x y top view front view

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relevant configurations Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño Molecular potential well Energy depth: E well ~ -0.25 eV Position: Over hollow θ=90 o Z≈1.6 Å r ≈ 1.4 Å Ag(100) surface unit cell Dissociative configuration Energy barrier: E ~ 1.1 eV Position: Over bridge θ=90 º Z≈1.5 Å Ag(100) surface unit cell

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Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño dissociation probability

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Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño dissociation probability

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dissociation probability Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño 2D Potential energy surface Energy barrier: E ~ 1.1 eV Position: Z≈1.5 Å General features: Activation energy: ~ 1.1eV Low dissociation probability Reason: Only configurations around bridge lead to dissociation

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the question Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño Gas phase O 2 3g3g 1 eV singlet to triplet excitation energy 1g1g can we enhance O 2 dissociation on clean Ag(100) ? 6D PES calculation: Non spin polarized DFT

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differences between SP and NSP PESs Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño Non spin polarized Spin polarized 1 eV singlet to triplet excitation energy Gas phase O 2 for Z < 2A, the SP and NSP PESs merge 3g3g 1g1g X Y Z x y z surface unit cell r

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dissociation is enhanced for singlet O 2 Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño spin-triplet O 2 spin-singlet O 2 dissociation occurs for E i < 1 eV dissociation can increase in one order of magnitude

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dissociation occurs for E i < 1 eV dissociation can increase in one order of magnitude dissociation is enhanced for singlet O 2 Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño spin-triplet O 2 spin-singlet O 2 Gas phase O 2 3g3g 1 eV singlet to triplet excitation energy 1g1g But there is a trick here! The total energy is 1 eV larger for the singlet O 2

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Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño spin-triplet O 2 spin-singlet O 2 1 eV dissociation is enhanced for singlet O 2 dissociation occurs for E i < 1 eV dissociation can increase in one order of magnitude for ≠ 0 o, singlet-O 2 is more efficient than triplet-O 2 with the same total energy

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why is that? Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño 1 eV spin-triplet O 2 spin-singlet O 2

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why is that? Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño available paths to dissociation are different (and more!) spin-triplet O 2 spin-singlet O 2

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it is not the same road Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño triplet O 2 singlet O 2 + 1 eV triplet O 2 singlet O 2

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conclusions Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño The low dissociation of O 2 on Ag(100) is due to two main factors: - The existence of large activation energy barriers of about 1eV. - Only a small number of configurations in phase space lead to dissociation. Dissociation increases in about one order of magnitud, if singlet–O 2 molecular beams are used. Under off-normal incidence angles, the efficiency of singlet-O 2 to dissociation is remarkable: it exceeds the reactivity of triplet-O 2 with an extra kinetic energy of 1eV.

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Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño

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it is not the same road Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaceson clean Ag(100) surfaces M. AlducinM. Alducin H. F. BusnengoH. F. Busnengo R. Díez MuiñoR. Díez Muiño triplet O 2 singlet O 2 + 1 eV triplet O 2 singlet O 2

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Why O 2 on Ag(100) ? EiEi Molecular beam experiments on flat Ag surfaces Ts < 150K: O 2 adsorbs only molecularly (E i < 1eV, ) Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008 Ag (100)/Ag(110) L. Vattuone et al., Surf. Sci. 408, L698 (1998). A. Raukema et al., Surf. Sci. 347, 151 (1996). 70% Low probability Dissociation of O 2 on Ag(100) reasons for the lack of dissociation possible ways to enhance dissociation: role of excited electronic states? Ag (111)

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Dissociative dynamics of O 2 /Ag(100): Classical trajectory calculations Z=3.5 Å Ag(100) unit cell Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008 E i =1.5 eV =0 o 3.5 Å

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Dissociative dynamics of O 2 /Ag(100): Classical trajectory calculations Z=2 Å Z=3.5 Å Ag(100) unit cell Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008 E i =1.5 eV =0 o 3.5 Å 2.0 Å

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Dissociative dynamics of O 2 /Ag(100): Classical trajectory calculations Z=2 Å Z=1.5 Å Z=3.5 Å Ag(100) unit cell Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008 E i =1.5 eV =0 o 3.5 Å 2.0 Å 1.5 Å

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Dissociative dynamics of O 2 /Ag(100): Classical trajectory calculations Z=2 Å Z=1.5 Å Z=3.5 Å Ag(100) unit cell Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008 E i =1.5 eV =0 o E i =2 eV =0 o 3.5 Å 2.0 Å 1.5 Å Ag(100) unit cell

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Non spin polarized Spin polarized Reactivity of spin-singlet O 2 on the Ag(100) surface 1 eV singlet to triplet excitation energy Gas phase O 2 PES calculation: Non spin polarized DFT Z < 2A: SP and NSP PESs merging Classical trajectory calculations NSP PES SurfaceSurface SP PES 3g3g 1g1g Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008

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Dissociative dynamics of spin-singlet O 2 on Ag(100) Dissociation occurs for E i < 1 eV Dissociation can increase in one order of magnitud spin-triplet O 2 spin-singlet O 2 Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008

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Dissociative dynamics of spin-singlet O 2 on Ag(100) Dissociation occurs for E i < 1 eV Dissociation can increase in one order of magnitud For ≠ , singlet-O 2 is more efficient than triplet-O 2 with higher E i spin-triplet O 2 spin-singlet O 2 1 eV Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008

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Dynamics of spin-triplet versus spin-singlet O 2 Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008 Dissociation is a direct process spin-triplet O 2 num. rebounds >3 num. rebounds <3 spin-singlet O 2 Dynamic trapping also important SurfaceSurface direct trapping

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Dynamics of spin-triplet versus spin-singlet O 2 Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008 1 eV

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Dynamics of spin-triplet versus spin-singlet O 2 Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008 1 eV Under off-normal incidence angles, the efficiency of singlet-O 2 to dissociation is due to the existence of more paths leading to dissociation

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Thank you for your attention ! Centro de Física de Materiales CFM Donostia International Physics Center

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Work in progress and open questions Experimental data from L. Vattuone et al., Surf. Sci. 408, L698 (1998) Molecular trapping vs Molecular sticking Molecular potential well No energy barriers in the entrance channel !! Energy depth: E well ~ -0.25 eV Position: Over hollow θ=90 o Z≈1.6 Å r ≈ 1.4 Å Ag(100) surface unit cell Dynamics of spin-triplet and spin-singlet O 2 on Ag(100)UAM, December 2008

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Dynamics on NSP PES

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Note: (Different scales in Y-axis)

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NSP vs ‘adiabatic’ singlet O2 i=0

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Direct vs indirect in ‘adiabatic singlet-PES’

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NSP PES: RPBE vs PW91

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Technical details: Ab initio SP PES Dependence of the difference between NSP and SP energies on the distance from the surface Z Filled symbols: DFT values Open symbols: Interpolated values

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Thank you for your attention ! CFM Centro de Física de Materiales, Centro Mixto CSIC-UPV/EHU

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