Ultrafast dynamics in real time Julia Stähler Fritz-Haber...

Ultrafast dynamics in real time

http://www.fhi-berlin.mpg.de/pc/electrondynamix

Julia StählerFritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Physical Chemistry, Berlin, Germany

why ultrafast dynamics?

electricalenergy

light

light

electricalenergy

BLACK BOX

How do we see ?

example: photochemistry of vision

Rhodopsin

Eye Rod cells: A single rodcontains 108 rhodopsins(renewed every 10 daysthroughout a person’s life)

cis-trans isomerization

Retinal

hν

11-cis all-trans

ultrafast processes in excited solids

solids and interfaces exhibit various fundamental processes on atto- to picosecond timescales

adapted from Petek & Ogawa, Prog. Surf. Sci. 56, 239 (1997)

e - e scattering

e - phonon scattering

linewidth (eV)1 0.1 0.01 0.001

0.1 1 10 100 1000

molecular motions

screening

e - correlation

timescale (fs)

dephasing

femtoseconds

10-15 s 10-10 s 10-5 s 1 s 105 s 1010 s 1015 s

processorspeed

flash month end of lastice age

age ofuniverse

pump-probe experiment

outline

▪ introduction: elementary excitations in solids

▪ charge carrier dynamics in metals and semiconductors

▪ surface femtochemistry

coupling of electrons, spins, and phonons

electrons

Tel

lattice

Tph

spins

elementary scattering processes and energy flow:

hν (~eV)

electron-electron

scattering

electron-phonon

couplingτe-ph

τe-magn

electron-magnon

coupling,

‚spin flip‘ scattering

spin-lattice relaxation

phonon-magnon

scattering

τph-magn

phonon

decay

elementary excitations

▪ single (quasi)-particle excitations :e.g. photoholes, injected excess electrons,…

▪ two-particle excitations :e.g. excitons, Cooper pairs,…

▪ collective excitations :e.g. magnons, plasmons, phonons, polariton,…

spin wave

(assuming rapid decay of coherence)

dephasing/ optical absorption

elementary processes:electronic polarization

cooling by e-ph coupling, heat transport

electron thermalization

at high excitation densities: description by distribution function and time evolution, e.g., according Boltzmann equation

t = 0

electron thermalization

electrons in a metal

fs-laser excitation

transient electron distribution function

photoabsorption in a metal

elementary scattering processes:

electron-electron scattering electron-phonon, electron-magnon scattering, ….

lifetime of an excited electronabove the Fermi sea

▪ Fermi liquid theory (Landau 1957)

▪ solution for homogeneous electron gas,self energy (Quinn & Ferell 1958)

screened Coulomb interaction:

screening in metals

τ~ ωP-1

(10-16 s in metals)

time requiredfor screening

build-up?

quasiparticles

system of interacting real particles acts as if it were

composed of weakly interacting fictitous bodies (quasiparticles)

bareparticle

+‘cloud’ of

other particles

=

quasiparticle

fundamental concept in condensed-matter physics:

one many-body problem multiple one-body problems

quasiparticle lifetimes

▪ Interactions between quasiparticles limit how long the corresponding quantum states retain their identity, i.e., the lifetime of the excitation.

▪ In combination with the velocity, this lifetime determines the mean free path, a measure of the interaction in the solid.

hυ EF

lifetime-determining factors:▪ initial state (energy, momentum)▪ final state (“phase space”)▪ screening (response function)

outline



▪ molecule dynamics at surfaces

image potential states at metal surfaces

z

Echenique & Pendry (1978)classical image potential:

courtesy: Ulrich Höfer, Marburg

binding energy (Rydberg series):

lifetime:

probing the energy levels: photoemission

▪ photoelectron spectroscopy:absolute binding energiessurface sensitivityUHV conditions

Y. Gao. Mat. Sci. Eng. R 68, 39 (2010)

2PPE spectroscopy

2-photonphotoelectron spectroscopy

experimental setup I

NOPA< 20 fs,

laser setup

RegA 9050< 40 fs, 3 µJ/pulse

@ 300kHz

275 nm

200 nm

OPA 9450< 60 fs,

stretcher

compressorVerd

i V18

Micr

a < 40

fs,

4 nJ/p

ulse @

76 M

Hz

2PPE of image potential states

Berthold et al, PRL 88, (2002) 056805n = 1

n = 2


time-resolved 2PPE of image potential states


Cu(100)▪ direct measurement of the lifetime using 2PPE▪ excellent agreement with many-body theory(GW approximation)

Echenique et al., Surf. Sci. Rep. 52, 219 (2004)

time-resolved 2PPE of image potential states

populationdynamics

Berthold et al, PRL 88, 056805 (2002)

ferromagnetic surfaces: spin splitting

Passek et al., PRL 75, 2746 (1995)Aeschlimann et al., PRL 79, 5158 (1997)

origin: different density of states of majority and minority electrons

spin splitting

spin-, angle-, and energy-resolved 2PPE

spin-polarized LEED detector

minority spin

majority spin

parallel momentum (1/Å)

bind

ing

ener

gy(e

V)

example: 3ML Fe film on Cu(100)

Schmidt et al., PRL 95, 107402 (2005)

spin-dependent image state lifetimes

origin: different density of states

Passek et al., PRL 75, 2746 (1995)Aeschlimann et al., PRL 79, 5158 (1997)

Schmidt et al., PRL 95, 107402 (2005)

Magnon-enhanced intraband scattering

magnon emission

Schmidt et al., PRL 95, 107402 (2005)

VB

CB

semiconductor surfaces

EF

▪ potential step at the surface:electrons leaking into the vacuum

Φ

▪ band bending & adsorption:change of surface dipole

Evac

HOMO

LUMO

Evac

zincoxide: a promising challenge

▪ inorganic semiconductor: ZnO-wide band gap semiconductor-transparent-high carrier mobility-room temperature luminescence

-wurtzide structure-two polar, one non-polarlow-index surface

Is the pristine ZnO(10-10) surface stable?

Wang et al., PRL 95, 266104 (2005)

▪ hydrogen termination probable▪ potential metalicity of the

ZnO(10-10) surface:

hydrogen termination

Ozawa and Mase, PRB 83, 125406 (2011)

our work

▪ hyrogen-induced CB down-bending observed

Fermi edge

time-resolved spectroscopy

Tisdale et al., JPC C 112, 14682 (2008)


▪ fast initial dynamics▪ slow relaxation (~100 ps)

Tisdale et al.


▪ fast initial dynamics▪ slow relaxation (~100 ps)

Tisdale et al.

Tisdale et al., JPC C 112, 14682 (2008)

summary




Further reading:P.M. Echenique et al., Surf. Sci. Rep. 52, 219 (2004)

M. Weinelt et al., Prog. Surf. Sci. 82, 224 (2007)

outline




temporal evolution from reactants to products:

▪ dynamics of the transition state▪ goal: microscopic understanding of elementary

steps and dynamics of energy transfer

▪ femtosecond laser pulses for direct observationin time domain

▪ key concept: dynamics on Born-Oppenheimer potential energy surfaces

▪ non-adiabatic coupling between different electronic states

timescales of chemical reactions

▪ breakdown of the BO approximation near (avoided) crossings

surface reactions and heterogeneous catalysis

heterogeneous catalysis:▪ reduced reaction barrier at surfaces▪ prominent examples:

• automotive catalyst• ammonia synthesis

rate-limiting step:dissociative adsorption

ABgas

Aads Bads ABgas DE

Aads+Bads

dissociative desorption

ABgas

Aads BadsDEAads+Bads

EaABgas

goal: microscopic understanding of surface reactions

chemical reactions typically occurin the electronic ground state

role of electronic excitations?

femtochemistry at metal surfaces

ultrafast energy redistribution after optical excitation of a metal surface

fs-laser pulse

Frischkorn & Wolf, Chem. Rev. 106, 4207 (2006)

▪ optically thin monolayer▪ substrate-mediated excitations

dominate

coupling to adsorbate vibrations induces reaction

reminder: fs-laser excitation in metals

(assuming rapid decay of coherence)

dephasing/ optical absorption

elementary processes:electronic polarization

cooling by e-ph coupling, heat transportelectron thermalization

transient electron distribution functionfs-laser excitation ofelectrons in a metal

t = 0

Lisowski et al., Appl. Phys. A 78, 165 (2004)

coupling to adsorbate motions

t = 0

Desorption Induced byElectronic Transitions

DIET -

Desorption Induced byMultiple Electronic Transitions

DIMET -

* MGR model (Menzel, Gomer, Redhead)

*

fs-laser excitation ofelectrons in a metal

reaction mechanism

phonon-mediatedelectron-mediated

How to distinguish?

substrate

adsorbateE

reaction coordinate

phonons Edes

kBTph

2-pulse correlation

FWHM ~ 10 ps

FWHM ~ 1 ps

phonon-mediated: slow responseelectron-mediated: fast response

pulse-pulse delay Dt = 0 fs

How to distinguish electron and phonon mediated mechanism?

= 5 ps= 2 ps= 1 ps= 0.5 ps

Tel and Tph for 2 pulse excitation with time delay ∆t

experimental setup II

“Feulner cup“ detector

80 mm

O = 33 mm/

Ru(001)

QMSionisationvolume

H2

Ti:sapphire oscillator + amplifier 4 mJ 800 nm, 110 fs, 20-400 Hz,

2 equally intensepulses

Ru(001)

QMS

H2/D2

CCD

pulse profilediagnostics

/

UHV

(1x1) saturation coverage H/Ru(001)

fs-laser activationY = A <F>n

kBT

thermal activationY = A exp (-Ea/kBT)

H+H recombinative desorption

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

first s

hot y

ield [

a.u.]

pulse-pulse delay [ps]

H2exp. data

0-10 -5 0 5 10

model

FWHM = 1.1 ps

2-pulse correlation

2-pulse correlation

Energy transfer by coupling betweenadsorbate vibration and metal electrons

Electronic friction:

hotelectrons

EFermi

Eleveladsorbate

electronshot

metal

Coupling rate η = ~ 11Melτ

electronic friction model

Struck et al., PRL 77, 4576 (1996)

▪ energy transfer by coupling betweenadsorbate vibration and metal electrons

▪ example: CO desorption from Cu(100)

0

160140120100806040200

0

exp. data

model

power law fit

yield weighted fluence [J/m2]

first s

hot y

ield [

a.u.]

isotoperatio

H2

D2

Y ~ <F>3

yield weighted fluence (J/m2)

isot

ooe

ratio

Y(H

2)/Y

(D2)

2-pulse correlation

origin of isotope effect: DIMET

real-time probing of vibrational dynamics

~0.1-1 ps

~1 pselectrons

Tel

direct absorptionfs-excitation reaction

>1 ps

~100 fs

phononsTphm

etal

adsorbate vibrations Tads

▪ so far: only the (desorbing) reaction products probed→ surface-sensitive non-linear optics (SHG, SFG):

direct look inside the excited adlayer

vibrational sum-frequency generation (SFG)

▪ no background▪ visible detection▪ sensitivity: 0.001 ML▪ short pulses: time resolution▪ surface/interface sensitive

Pot.

Ener

g y

C-O distancev=0

v=1v=2

5 mm + 800 nm = 690 nm

IR

SFGVIS

▪ background▪ IR detection▪ optical linear technique

C-O distance

IRv=0

v=1v=2

Pot.

Ener

g y

IR absorption spectroscopy (FTIR)

CO

C-O stretch

vibrational spectroscopy

▪ no background▪ visible detection▪ sensitivity: 0.001 ML▪ short pulses: time resolution▪ surface/interface sensitive

Pot.

Ener

g y

C-O distancev=0

v=1v=2

5 mm + 800 nm = 690 nm

IR

SFGVIS

vibrational SFG spectroscopy

P E E = χ ω(2) (2)

IR ωVISω ωSFGSFG

time-resolved SFG as molecule-specific probe of surface reactions

second order non-linear optical process

→ surface sensitive method: SFG is symmetry forbidden in isotropic (bulk) media (dipole approximation)

broadband IR vibrational SFG spectroscopy

SFG spectrum ( 3x 3)-CO/Ru(001) √ √

SFG wavelength [nm]690 688 686 684694 692

SFG

inte

nsity

IR

20001950

FWHM9 cm-1

1900 2050IR wavenumber [cm ]-1

2100 2150

200 K

SFG

IR intensity

IR

VIS SFG

IR

VIS

Ru

SFG

IRIR

VIS

VIS

Γn

ΓSFG

01

2

~5 cm-1

~150 cm-1

Richter et al., Opt. Lett. 23, 1594 (1998)

spectrally broad fs-IR pulse + spectrally narrow VIS upconversion pulse

experimental setup III

Ti:sapphire oscillator + amplifier4 mJ, 800 nm , 110 fs, 20-400 Hz,

OPG/OPA (TOPAS), 20-30 J µ2.5-10 mµ

tunableIR pulse

pump pulse

spectrometer

CCD camera

multi-channel detectionRu(001)

QMS

SFGprobe

pulse shaperFWHM 7 cm , 7 J µ-1

narrow bandVIS pulse

SFG

inte

nsity

(a.u

.)

21002050200019501900IR wavenumber (cm-1)

3 ps

23 ps

0.5 ps×8

×8

×8

Pump-Probe delay –4.5 ps–2.0 ps–1.5 ps–1.0 ps–0.5 ps0.5 ps3.0 ps

23.0 ps68.0 ps

168.0 ps

CO

yie

ld (a

.u.)

100shot number

210020001900

T =340 K, F=55 J/m02

CO/Ru(001) optical probe

time-resolved SFG of CO/Ru during desorption

▪ pronounced transient red shift▪ linewidth broadening▪ decrease of intensity

problems:▪ dipole-dipole coupling in the adlayer▪ small concentration of „products“

Bonn et al., PRL 84, 4653 (2000)

calculation:▪ thermal excitation of frustratedtranslation▪ anharmonic coupling to CO stretch mode

high fluence:▪ excitation and coupling

to higher frequency modes

strong redshift indicates: frustrated rotation dominantes

low fluence: good fit

time-resolved SFG of CO/Ru during desorption

Bonn et al., PRL 84, 4653 (2000)

summary




Further reading:A.M. Wodke et.al., Prog. Surf. Sci. 83, 167 (2008)

M. Bonn et al., J. Phys.: Condens. Matter 17, 201 (2005)

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