Martin Wolf Department 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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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
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