INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

EFFECTIVE MASS AND MOMENTUM RESOLVED INTRINSIC LINEWIDTH OF IMAGE-POTENTIAL

STATES ON Ag(100)

INFM and Università Cattolica del Sacro Cuore

Dipartimento di Matematica e Fisica, Via Musei 41 Brescia.

Claudio Giannetti

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

•Femtosecond Laser and tunability of photon energy

→ Non-Linear Photoemission

•Study of image-potential states on Ag(100) at different photon energies: effective masses and lifetimes.

INTRODUCTION

•WHY IMAGE POTENTIAL STATES?

Relaxation of electrons into bulk states can be studied directly in the time-

domain with pump-probe techniques.U. Höfer et al., Science 277, 1480 (1997).W. Berthold et al., Phys. Rev. Lett. 88, 056805 (2002).

It is a many body interesting problem.A.García-Lekue et al., Phys. Rev. Lett. 89, 096401 (2002).J. Kliewer et al., Science 288, 1399 (2000).

Interplay between image-potential states and dynamics of molecules at surfaces.A. D. Miller et al., Science 297, 1163 (2002).

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

LASER APPARATUS

Ti:Sapphire laser system:

Tunability:1150-

1500nm(0.8-1.1 eV)

4th (3.2-4.4 eV)

Average power 30mW

Travelling-wave optical parametric generation (TOPG):

Amplified Ti:Sapphire oscillator

Tunability: 750-850nm Pulse width: 150fs

Rep. rate: 1kHz

Average Power: 0.5W

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

ULTRA-HIGH-VACUUM SYSTEM

-metal UHV chamber

B<10 mG

Base pressure <2x10-10 mbar

photoemitted electrons detector:

time of flight spectrometer (TOF)

PCGPIB

Multiscaler FAST 7887

PS1 PS2 PS3 PS4

start stop

PreamplifierDiscriminator

Laser

TOF: ToF length = 432 mmTemporal resolution = 0.5 nsAcceptance angle = 2.6°Energy resolution about 30 meV @ 2eV

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

IMAGE-POTENTIAL STATES ON METALS

Coulomb potential due to electronic image-charge:

V(z)1/z

Image-charge of an electronat a metal surface.

U. Hofer, I.L. Shumay, Ch. Reuß,U. Thomann, W. Wallauer, Th. Fauster, Science 277, 1480 (1997).

forbidden gap in bulk states

2-D electron gas

Ag(100) n=1n=2

Rydbergseries:

2

850

a)(n

eV.En

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

DISPERSION OF IS

*

2//

2

// 2)(

m

kEkE n

Band structure of Ag(100)

En: binding energym*: effective mass

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

IS SPECTRA

Ekin = h-Ebin

2-photon photoemissionEbin=

0.5eV

RADIATION:Polarization: PIncident angle: 30°

h=4.32 eVAg(100)

n=1FWHM:62meV

n=2FWHM:52meV

Efermi

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

DIPOLE SELECTION RULES:

J=0 in S-polarization

J≠0 in P-polarization

J is the current density associated to image-potential states

n

P

S

Ag(100)

SELECTION RULES

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

IS DISPERSION

*

2//

2

// 2)(

m

kEkE n

2-Dimensionalelectron gas: DISPERSION

k||

2mEKin sin

Ag(100)

G.Ferrini, C.Giannetti, D.Fausti, G.Galimberti, M.Peloi, G.P.Banfi, F.Parmigiani, PRB 67, 235407 (2003).

n=1

n=2

Inte

nsit

y (l

og s

cale

)

Ekin (eV)

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

m*

2

d2E / dk||2

EFFECTIVE MASS

Measured values Calculated values

n=1 n=2 n=1 n=2

0.99±0.02 1.06±0.090.97±0.02 1.03±0.061.15±0.1*

0.95*

1.03**1.03**

* K.Giesen et al., PRB 35 975 (1987).** Z.Li and S.Gao , PRB 50 15349 (1994).

n=1n=1

n=2 n=2

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

Gaussian: experimental resolution FWHM45meV

Lorentzian: intrinsic linewidth 14meV @ k//=0

/

IS LIFETIMES

FITTING: Gaussian-Lorentzian convolution

LIFETIME

Linewidth dependance on k//

Measured values (fs) Calculated values (fs)

n=1 n=2 n=1 n=2

47±7 ≥5555±5# 160±10#

55## 132###

# I.L.Shumay et al., PRB 58 13974 (1998).## A.García-Lekue et al., Phys. Rev. Lett. 89, 096401 (2002).### E.V Chulkov et al., Surf. Sci 391 L1217 (1997).

fs658meV

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

heVE

FIRST OBSERVATION OF IMAGE STATES OUT OF RESONANCE

EEV

EF

n=1

e-

E=3.8 eV

0.5 eV

Ekin= h-Ebin= (3.14-0.5)eV 2.7eV

e-

LINSCALE

Fermi edge

inte

nsit

y

Ekin (eV)

LOGSCALE

Fermi edge

inte

nsit

y

Ekin (eV)

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

DISPERSION AND SELECTION RULES

m*/m0.95

n=1

Fermi edge

Ekin= h-Ebin

h=0.39eV

h=3.15eV h=3.54eVEV

EF

n=1e-

Ebin

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

SELECTION RULES

DIPOLE SELECTION RULES:

J=0 in S-polarizationJ≠0 in P-polarization

n

P

S

Ag(100)

WHICH IS THE POPULATION AND PHOTOEMISSION MECHANISM?

We can exclude:

Role of surface roughness

Ponderomotive and tunnel effects at the surface,

because the radiation intensity is too low. (I~0.1GW/cm2)

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

CONCLUSIONS

•Non-linear photoemission on image-potential states in Ag(100).

• Improvement of the precision in the measurement of the effective masses and lifetimes.

• Observation of image-potential states also when hE1-Efermi. In this case the image-potential states can be populated and photoemitted also in S-polarization.

INFM DMF

INFMeeting, Genova 23-25 Giugno 2003

Responsibles:

F. Parmigiani, G. Ferrini.

Co-workers:

F. Banfi, D. Fausti, G. Galimberti, S. Pagliara, M. Peloi.