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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.