Diapositiva 1

1
Image-potential-state effective mass controlled
by light pulses
Gabriele Ferrini, Stefania Pagliara, Gianluca
Galimberti, Emanuele Pedersoli, Claudio
Giannetti, Fulvio Parmigiani
ELPHOS Lab UCSC (Università Cattolica del Sacro
Cuore-Brescia) Dipartimento di Matematica e
Fisica (Brescia, Italy)
DMF
2
Introduction
The study of the electron dynamics at surfaces is
an important topic of current research in surface
science. Experimental techniques that combine
surface and band-structure specificity are
essential tools to investigate these
dynamics. Angle-resolved non-linear photoemission
using short laser pulses is particularly suited
for such experiments. In typical experiments a
short laser pulse, with pulse widths of 10-100
femtoseconds, is used to photoemit the electrons
using multiple photon absorpion. Electrons are
first excited into empty states below the vacuum
level and then emitted by subsequent photon
absorption
3
Introduction

• A rather interesting system to study the electron
  dynamics at the metal surfaces is represented by
  Image Potential States (IPS) and Shockley States
  (SS).
• IPS are a 2-D electronic gas suitable to study
• band dispersion
• direct versus indirect population mechanisms
• polarization selection rules
• effective mass (in the plane of the surface)
• electron scattering processes and lifetime

4
Image Potential States
In most metals exists a gap in the bulk bands
projection on the surface. When an electron is
taken outside the solid it could be trapped
between the Coulomb-like potential induced by the
image charge into the solid, and the high
reflectivity barrier due the band gap at the
surface.
P. M. Echenique, J. Osma, V. M. Silkin, E. V.
Chulkov, J. M. Pitarke, Appl. Phys. A 71, 503
(2000)
5
Image Potential States
Two dimensional electron gas
Bound solution in the z direction Electrons are
quasi-free in the surface plane Interactions may
result in a modified electron mass m
m
6
Linear vs non-linear photoemission
Angle Resolved LINEAR PHOTOEMISSION (hn gt F)
band mapping of OCCUPIED STATES
Angle (and time) RESOLVED MULTI-PHOTON
PHOTOEMISSION (hn lt F) band mapping of
UNOCCUPIED STATES and ELECTRON SCATTERING
PROCESSES
7
Experimental Set-up
Pulse width 100-150 fs Repetition rate 1
kHz Average Power 0.6 W Tunability 750-850
nm Second harmonic hn 3.14 eV Third harmonic
hn 4.71 eV Fourth harmonic hn 6.28 eV
Traveling-wave optical parametric generation
(TOPG) Average power 30 mW Tunability 1150-1500
nm (0.8-1.1 eV) Fourth harmonic hn 3.2-4.4 eV
Non-collinear optical parametric amplifier
(NOPA) Pulse width 20 fs Tunability 500-1000 nm
(1.2-2.5 eV) Second harmonic hn 2.5-5 eV
8
Experimental Set-up

• m-metal UHV chamber
• base pressure lt 210-10 mbar
• residual magnetic field lt 10 mG
• electron energy analyzer Time of Flight (ToF)
• spectrometer

Acceptance angle ? 0.83 Energy
resolution 30 meV _at_ 5eV Detector noise lt10-4
counts/s
G. Paolicelli et al. Surf. Rev. and Lett. 9, 541
(2002)
9
Two-photon photoemission from Cu(111)
VL
FL
projected band structure of Cu(111) with the
non-linear photoemission spectrum collected with
photon energy 4.71 eV.
Light grey spectrum R. Matzdorf, Surf. Sci.
reports,30 153 (1998)
10
Two-photon photoemission from Cu(111)
Effective mass of n1 IPS on Cu(111) measured
with angle resolved 2PPE in the literature
G. D. Kubiak, Surf. Sci. 201, L475 (1988),
m/m1.0-0.1, hv4.38eV M. Weinelt, Appl. Phys.
A 71, 493 (2000) on clean Cu(111) _at_
hv4.5eV1.5eV, m/m1.3-0.1 Hotzel, M. Wolf,
J. P. Gauyacq, J. Phys. Chem. B 104, 8438 (2000)
on 1ML N2 / 1ML Xe/ Cu(111) _at_ hv4.28eV2.14eV,
m/m1.3-0.3 S. Caravati , G. Butti , G.P.
Brivio , M.I. Trioni , S. Pagliara , G.
Ferrini, G. Galimberti, E. Pedersoli, C.
Giannetti, F. Parmigiani, Surf. Sci. 600, 3901
(2006), theory m/m 1.1, exp. on clean Cu(111)
_at_ hv3.14eV m/m 1.28-0.07
Effective mass of n0 SS on Cu(111) measured
with high resolution angle resolved photoemission
in the recent literature
F. Forster, G. Nicolay, F. Reinert, D. Ehm, S.
Schmidt, S. Hufner, Surf. Sci. 160, 532 (2003),
SS m/m0.43-0.01, binding energy 434 meV
11
Two-photon photoemission from Cu(111)
IPS and SS dispersion on the same data set
ips
k
ss
S. Pagliara, G. Ferrini, G. Galimberti, E.
Pedersoli, C. Giannetti, F. Parmigiani, Surf.
Sci. 600, 4290 (2006)
IPS Binding energyEk-hv-Fsp , Fsp 0.9-1 eV
12
IPS photoemission from Cu(111)
13
Two-photon photoemission from Cu(111)
IPS
m/m1.28-0.07
VL
m/m2.2-0.07
IPS
4.71 eV
SS
FL
14
Two-photon photoemission from Cu(111)
IPS
m/m1.6-0.07
VL
IPS
4.28 eV
SS
FL
15
Two-photon photoemission from Cu(111)
VL
IPS
4.28 eV
4.71 eV
3.14 eV
SS
FL
control point one-photon photoemission SS
16
Two-color photoemission from Cu(111)
IPS
probe
3.14 eV
static limit
VL
IPS
pump
SS
4.71 eV
SS
FL
1.3 1010ph/pulse 10 nJ/pulse at 4.71 eV fluence
10 mJ/cm2
17
Two-color photoemission from Cu(111)
4.71eV3.14 eV
4.71eV4.71 eV
18
Two-color photoemission from Cu(111)
How many electrons do we pump into the bulk bands?
From band structure 41018 cm-3 states available
in klt0.2 A-1, and in an energy interval of 300
meV from the upper edge of gap. (calculations
courtesy of C.A. Rozzi, S3 INFM-CNR and UniMoRe)
?
From scanning tunnel microscopy SS constitute
about 60 of the total surface electron density
on (111) surfaces of noble metals. L. Burgi, N.
Knorr, H. Brune, M.A. Schneider, K. Kern, Appl.
Phys. A 75, 141 (2002)
VL
IPS
pump
Assuming that the photons in the pump pulse are
absorbed in the surface layer in proportion to
the surface density of states and that the
totality of the SS excited electrons are promoted
to the empty bulk states at the bottom of the
gap, we estimate an upper limit for the
hot-electron gas density in the bulk bands of the
order of 1018 cm-3, a substantial fraction of the
sp-bulk unoccupied states
SS
FL
19
Phase shift model Cu(111)
T. Fauster, W. Steinmann, Two Photon
Photoemission Spectroscopy of Image States
qualitative explanation -Cu(111) IPS
penetrates into the bulk because it is at the gap
edge. -Excited e- density interacts with IPS
wavefunction increasing dephasing processes
and/or decreasing lifetime - Excited e- density
push IPS wavefunction outside, decreasing binding
energy preferentially at k0 -effective mass
increases
k
IPS dispersion
20
Conclusions
The effective mass of the Cu(111) IPS depends on
the excited electron density generated by the
laser pulses in the unoccupied sp-band. A
qualitative explanation based on the phase shift
model is given. Interest in these processes for
controlling band structure and chemical reaction
at surfaces.
21
People
Fulvio Parmigiani (U Trieste)
Stefania Pagliara (UCSC)
Claudio Giannetti (UCSC)
Gianluca Galimberti (UCSC)
Thank you
Emanuele Pedersoli (ALS-LBNL)