IMPRS Block Course “Dynamic Processes on...
Transcript of IMPRS Block Course “Dynamic Processes on...
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
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
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
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 ?
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
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 ?
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
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)
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
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
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
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)
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
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
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)
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
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)
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 ?
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)
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
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 ?
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
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
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