IMPRS Block Course “Dynamic Processes on...
Transcript of IMPRS Block Course “Dynamic Processes on...
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Martin WolfDepartment of Physics, Freie Universität Berlin
IMPRS Block Course “Dynamic Processes on Surfaces”
Ultrafast laser spectroscopy and surface dynamics
Question: How can we catch the motion of objects?
…a fast camera is enough!
Stroboscopic investigation of motion and structural changes
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Surface femtochemistry
Surface dynamics with electronic friction
Outline
IntroductionNon-adiabatic processes at surfaces: Chemicurrents
Example: Associative desorption of H2 from Ru(0001)
Bi
Attosecond laser spectroscopy
Electron streak camera and Auger decay
High harmonic and attoseond pulse generation
Time-resolved probe of structural dynamics
Time-resolved x-ray diffraction
Femtosecond laser spectroscopy
Non-thermal melting and coherent phonon excitation
Time-resolved photoelectron spectroscopy
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Timescales of chemical reactions
Typical timescale: 10-100 fs
Temporal evolution from reactants to products:
Dynamics of the transition state
Key concept: Dynamics on Born-Oppenheimer potential energy surface
Non-adiabatic coupling between electronic states near (avoided) crossings
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Gas surface interaction
Role of non-adiabatic processes in surface reactions ?
Example: Adsorption at a metal surface
E
R
„forced-oscillator model“
Energy dissipation via phonon excitation
Coupling to electron-hole pairexcitations in the substrate
Coupling between nuclear motion of adsorbate and electronic excitations ?
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Chemicurrents
See review article by H. Nienhaus, Surf. Sci. Rep. 45, 3 (2002)
Evidence for non-adiabaticcoupling between adsorbatemotion electronic excitations
Direct observation of e-h pair excitation
Adsorption on thin metal film
Charge separation acrosssmall Schottky barrier
Chemicurrent
Current provides lower limitof excitation probability
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Chemicurrents in gas-surface interaction
metal semiconductor
e-h pair
e-
Gergen et.al., Science 294, 2521 (2001)
Chemicurrent observed forvarious adsorbates
Energy dissipation via e-h pair excitation
electronic excitationsplay an important roleduring adsorption
mechanism of e-h pair excitation ?
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Mechanism
Newns-Anderson model
Chemiluminescence Exoelectron emission
Unoccupied affinity level ispulled down by adsorbatesurface interaction
Filling of hole below EFermiby substrate electrons
broadening of resonance linewidth
adsorbate substratecharge transfer
e-h excitation:- Chemicurrent- Exoelectron emission
Chemiluminescence
EF
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Electronic friction
Electronic friction of adsorbates
Example: Increase of electric resistivityin a thin metal film upon Xe adsorption
-V Xe
~1 ML Xe/Ag film
Newns-Anderson picture providesa mechanism for electronic friction
vibrational damping at metals
surface femtochemistry
Brandbyge, et al, PRB 52, 6042 (1995); B. Persson, in Sliding Friction (Springer, 1998)
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Femtochemistry at metal surfaces
substrate-mediated excitation mechanism dominates
Mechanism and timescales of energy transfer after optical excitation
transient non-equilibrium between electrons and phonons: Tel >> Tph
~0.1-1 ps
~1 pselectrons
Tel
direct absorptionfs-excitation reaction
>1 ps
~100 fs
phononsTphm
etal
adsorbate vibrations Tads
non-thermal reaction mechanism:separation of time scales of energy flow
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Reaction mechanism
electron-mediated energy flow
Desorption Induced byMultiple Electronic Transitions
DIMET -
fs-laser excitation
Lisowski et al., Appl. Phys. A 78, 165 (2004)
electron thermalization
τthermalization << 100 fs * MGR model (Menzel, Gomer, Redhead)
*
electrons in a metal
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Experimental InvestigationsDesorption:• NO and O2/Pd• CO/Cu, CO/Pt• O2/Pt • CO/NiOReactions:• CO + O2/Pt• CO + O, C + O, H+H/Ru
Misewich, Loy, HeinzTom, PrybylaHo MazurStephenson, Richter, CavanaghBonn, Wolf, ErtlZacharias, Al-Shamery, FreundDomen
YieldFluence dependenceTranslational energyVibrational energy Rotational energyUltrafast dynamicsInfluence of laser pulse durationInfluence of photon energyDependence on adsorption siteDependence on coverageCompetitive reaction pathwaysIsotope effects
Surface femtochemistry
Systems
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associative H2 desorption from H/Ru(0001)
ultrashort
laser pulse
Femtosecond laser induced desorption of H2 by 100 fs, 10 mJ/cm2 pulses
Remarkably high translational energy (<Etrans>/2k ~ 2000 K)
1x1 H/Ru(001)
Denzler et al. Phys. Rev. Lett. 91, 226102 (2003) & J. Phys.Chem. B 108, 14503 (2004)
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2 pulse correlation measurements
Ultrafast response indicates coupling to hot electron transient
FWHM = 1.1 ps
first
shot
yiel
d[a
.u.]
pulse-pulse delay [ps]
H2exp. data
0
-10 -5 0 5 10
phonon-mediated: slow responseelectron-mediated: fast response
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0
160140120100806040200
0
exp. data
model
power law fit
yield weighted fluence [J/m2]fir
stsh
otyi
eld
[a.u
.]
isotoperatio
H2
D2
Y ~ <F>3fir
stsh
otyi
eld
[a.u
.]
pulse-pulse delay [ps]
H2exp. data
0
-10 -5 0 5 10
model
FWHM = 1.1 ps
Electronic 1D friction model provides consistent description:• 2-pulse correlation• fluence dependence• isotope effects
Ea = 1.35 eV
ηel = τel = 180 fs for H2-1
Coupling rates: τel τph τel-ph-1 -1 -1
., ,
met
al
τph
τel-ph Tph
τel
elTheat diffusion
adsT
ads-E /k Ta bRate ~ e
Two-Temperature Model
Coupled heat baths: el ph ads T , T , T
Brandbyge et al., PRB 52, 6042 (1995)
Coupled heat baths with Tel, Tph, Tads
friction coefficient activation energy
ηel
Tads∫Reaction rate ~ Ea exp(– Ea / kTads)
Electronic friction model
ηel = τel-1
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Energy partitioning
Energy partitioningwithin the D2 product
Resonance EnhancedMultiphoton Ionization (REMPI)
helicoper molecule cartwheel molecule
|i>
|r>
IP
VUV
UVscan VUVrovibrational population distribution
change VUV polarizationmolecular alignment
Wagner et al., Phys. Rev. B 72, 205404 (2005)
cooperation with H. Zacharias (Münster)
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Experimental setup
VUV conversion efficiency 10-6 330 nm: 8.5 mJ/pulse
532 nm: 330 mJ/pulse266 nm: 4.5 mJ/pulse
Reaction trigger: fs-laser + state-selective detection: tunable VUV sourceReaction trigger: fs-laser
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State-resolved detection of desorbing D2
No Boltzmann-like rotational population distribution
⟨Erot⟩ = 80 meV⟨Evib⟩ = 100 meV
⟨Etrans⟩ = 430 meV
N ~ exp-E kBT
Wagner et al., Phys. Rev. B 72, 205404 (2005)
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Reaction dynamics Detailed picture of reaction mechanism and energy flow
molecular alignment
A0 = 0.3preferentiallyhelicoper-like
(2)
quadrupole alignment parameter -1 ≤ A0 ≤ 2(2)
coupling times and activation energy
hot electron driven high translational energy
unequal energy partitioning
predominantly translational excitation
⟨Etrans⟩ : ⟨Evib⟩ : ⟨Erot⟩ = 5.5 : 1.5 : 1
energy balance
good agreement with 1D friction model
⟨ED2_flux⟩ = 160 meV = kB Tadsfriction
PES topology (barrier location)
multidimensional friction / energy transfer
1D model fits energy balance: Why ?
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Theory: Molecular dynamics with electronic friction
Head-Gordon & Tully, JCP 103, 10137 (1995)
µd = – ηdd d – ηdz z + Fd(t)∂V∂d
.. . .
µz = – ηzz z – ηdz d + Fz(t)∂V∂z
.. . .
µq = – ηq q + Fd(t)∂V∂q
.. .
Equations of motion:
cooperation with A. Luntz (Odense) and M. Persson (Liverpool)
Fluctuating forces F(t) by frictionalcoupling to electron temperature
( ) ( ) ( )2i i B e iiF t F t k T t tη δ′ ′= −
(fluctation dissipation theorem)
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Friction coefficients and PES
2x2 unit cell for DFT (LDA)
≈ 60 meV
PES:Friction coefficients (TDDFT):
Phys. Scr. 29, 360 (1984)JCP 119, 4539 (2003)
reaction path S
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2-pulse correlation & fluence dependence
Luntz, Persson, Wagner, Frischkorn, Wolf, JCP (submitted)
“First principle” model: V2D(z,d) AND η(z,d) no adjustable parameter
EXCELLENT agreement with experimenttime scale of energy transferfluence nonlinearity of yieldisotope effect
success of 1D model ?
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multi-dimensional friction
small S: ~ isotropic coupling
near V*: ηdd ≈ 3 ηzz
origin of unequal energy partitioning ???
crucial time span0 < t < 1 ps
excitation occurs only whileelectrons are still hot
time evolution
reaction path S
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Trajectories: dynamics on ground PES
F = 140 J/m2 F = 140 J/m2
rapid interchange between z and d(”thermalization”)
sufficient energy is a prerequisite,
... but not everything
dynamics on ground state PES
reaction
reaction no reaction
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Summary I
IntroductionNon-adiabatic surface dynamics: Chemicurrents
Bi
Time-resolved photoelectron spectroscopy
Surface femtochemistry
Femtochemistry driven by hot electron excitation
Example: Associative desorption of H2 from Ru(0001)
Multidimensional dynamics with electronic frictions
metal semiconductor
e-h pair
e-
Time-resolved probe of structural dynamics
Time-resolved x-ray diffraction
Femtosecond laser spectroscopy
Non-thermal melting and coherent phonon excitation