Ultrafast Dynamics of Nonequilibrium Electrons in Metal/Adsorbate Nanosystems

Christophe Bauer, Jean-Pierre Abid and Hubert GiraultLaboratoire d’Electrochimie Physique et Analytique,

ISIC, SB, EPFL, CH-1015 Lausanne, Switzerland

Summary

Metal/molecule interfaces play a crucial role in topics such asmolecular electronics, surface femtochemistry, organic light emitting diodes,catalysis and solar cells. Here, we use adsorbates-covered metalnanoparticles to investigate by femtosecond transient absorptionspectroscopy the dynamical behaviour of nonequilibrum electrons (NEs) atmetal/molecule interfaces. This approach allows: the separation of internalfrom external thermalization, the investigation of the size behaviour ofinternal thermalization and hot electron cooling dynamics, the study ofmolecular vibrations effect on electron transfer process, the identification ofthe mechanism of internal thermalization retardation, the observation ofcomplex nonlinear dynamics with feedback loops.

Ultrafast Electron Dynamics in Metals

Dynamical events: The picture

Spectral Map:Nonthermal regime and hot electron gas

A: Energy redistribution processes in a metal. B: Metal/molecule/metal interfaces: Heart ofmolecular electronics

Femtosecond Pump-Probe Spectroscopy

Metal moleculeMetal molecule Metal

e-

ph

e-

MV

Light

Metal moleculeMetal molecule Metal

e-

ph

e-

MV

Light

e-

ph

e-

MV

LightP

I

Nascent nonthermal electrons (NNEs)

Hot electron gas

Hot lattice

Surrounding medium

electron-electron scattering

electron-Molecular Vibration scattering

PII

PIII

electron-phonon interaction

phonon-phonon interaction

PI

Nascent nonthermal electrons (NNEs)

Hot electron gas

Hot lattice

Surrounding medium

electron-electron scattering

electron-Molecular Vibration scattering

PII

PIII

electron-phonon interaction

phonon-phonon interaction

500 550 600 650

-1.0

-0.5

0.0

0.5

Absorb

ance C

hange (

a.

u.)

Wavelength (nm)

R e g i o n I R e g i o n I I R e g i o n I I I

200 fs

4 ps

500 5 5 0 6 0 0 6 5 0

-1.0

- 0 . 5

0 . 0

0 . 5

Absorb

ance C

hange (

a.

u.)

Wavelength (nm)

Region I Region II Region III

200 fs

4 ps

Gold NP Sulfate

Excitation

Feedback Energy dissipation

toward surrounding medium

Gold NP Sulfate

Excitation

Feedback Energy dissipation

toward surrounding medium

0 2 4 6 8 10 12

1.0

1.5

2.0Lifetim

e (

ps)

Nanoparticles Diameter (nm)

0 2 4 6 8 10 12

1.0

1.5

2.0

L i f e t i m e ( p s )

Nanoparticles Diameter (nm)

E = Ei- Ef

LUMO

h

EF

Adsorbates

ini

fin

Density of

states

Nonthermal

electrons

LUMO

Metal

h

EF

ini

fin

Density of

states

Nonthermal

electrons

E = Ei- Ef

LUMO

h

EF

Adsorbates

ini

fin

Density of

states

Nonthermal

electrons

LUMO

Metal

h

EF

ini

fin

Density of

states

Nonthermal

electrons

E = Ei- Ef

LUMO

h

EF

Adsorbates

ini

fin

Density of

states

Nonthermal

electrons

LUMO

Metal

h

EF

ini

fin

Density of

states

Nonthermal

electrons

-1 0 1 2 3 4 5 6 7

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Ab

so

rba

nc

e C

ha

ng

e (

a.

u.)

Delay Time (ps)

50 100 150 200 250 3001.5

2.0

2.5

3.0

3.5

4.0

Life

tim

e (

ps)

Pump Fluence (nJ/pulse)

Inelastic electron tunneling at metal/molecule interface

Build-up of a population of hot adsorbates (highly vibrationnaly excited adsorbates)

Size effect on hot electron cooling

LW

k k

d-band

Conduction

band

Ener

gy

EF

Emax

=

Emax=

LW

k k

d-band

Conduction

band

Ener

gy

EF

Emax

=

Emax=Emax=

NNEs (Metal) Adsorbates Hot adsorbates

Electron tunneling e-MV interaction

Negative feedback

NNEs (Metal) Adsorbates Hot adsorbates

Electron tunneling e-MV interaction

Hot electron emission

Negative feedback

Retardation

Internal

thermalization

External

thermalizationUCIS

e-e scatteringAdsorbates

e-phinteraction

e-e scatteringAdsorbates

e-ph

interaction

Retardation

Build-up hot

adsorbates

Retardation

Internal

thermalization

External

thermalizationUCIS

e-e scatteringAdsorbates

e-phinteraction

e-e scatteringAdsorbates

e-ph

interaction

Retardation

Build-up hot

adsorbates

BA

Gold band structure around L pointof Brillouin zone

Nonthermal

Thermal

Nonthermal regime controls external thermalization

530 nm

Probing the d-band to Fermi surface transition:Accordance with Fermi liquid theory

Non-adiabatic surface reaction:Break-down of Born-Oppenheimer

approximation

Low-perturbation regime

Nonlinear dynamics

Interconnection between the dynamical processes

625 nm

615 nm

605 nm

590 nm

0 2 4 6 8 10 12

-0.04

-0.02

0.00

0.02

0.04

0.06

Ab

so

rba

nce

Ch

an

ge

(a

. u

.)

Delay Time (ps)

6 2 5 n m

6 1 5 n m

6 0 5 n m

5 9 0 n m

0 2 4 6 8 10 12

-0.04

-0.02

0.00

0.02

0.04

0.06

Ab

so

rba

nce

Ch

an

ge

(a

. u

.)

Delay Time (ps)

0 2-2

0

1

-1

Electron energy

f

Thermal

Nonthermal

0 2-2

0

1

-1

Electron energy

f

Thermal

Nonthermal

Possible when the system exhibits chemical interface damping:Electrons tunnel back and forth between metal and adsorbates Additional damping channel for surface plasmon

References

Transient absorption data for 2.5 nm

Pump fluence dependence for 4.2 nm

J. Chem. Phys. 120, 2004, 9302Chem. Phys. 319, 2005, 409