image-potential-state effective mass controlled by light pulses

VUV XV, Berlin, 29 July -03 August 2007

Image-potential-state effective mass controlled by light pulses

ELPHOS LabUCSC (Università Cattolica del Sacro Cuore-

Brescia)Dipartimento di Matematica e Fisica (Brescia, Italy)

Gabriele Ferrini, Stefania Pagliara, Gianluca Galimberti,

Emanuele Pedersoli, Claudio Giannetti, Fulvio Parmigiani

DMF


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

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

Introduction


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)


Two dimensional electron gas

z

E k/ / EV En

h2k/ /2

2me

Image Potential States

Bound solution in the z direction

Electrons are quasi-free in the surface plane

Interactions may result in a modifiedelectron mass m*

m*


Linear vs non-linear photoemission

ToF

E kin h EB

k // 2mE kin/2 sin

Angle Resolved LINEAR PHOTOEMISSION (h > ) band mapping of OCCUPIED STATES

Angle (and time) RESOLVED MULTI-PHOTON PHOTOEMISSION (h < ) band mapping of UNOCCUPIED STATES and ELECTRON SCATTERING PROCESSES


Pulse width: 100-150 fs

Repetition rate: 1 kHz

Average Power: 0.6 W

Tunability: 750-850 nm

Second harmonic: h = 3.14 eV

Third harmonic: h = 4.71 eV

Fourth harmonic: h = 6.28 eV

Amplified Ti:Sa laser system

Traveling-wave optical parametric generation (TOPG)Average power: 30 mWTunability: 1150-1500 nm (0.8-1.1 eV)Fourth harmonic: h = 3.2-4.4 eV

Non-collinear optical parametric amplifier (NOPA)Pulse width: 20 fsTunability: 500-1000 nm (1.2-2.5 eV)Second harmonic: h = 2.5-5 eV

Experimental Set-up


Experimental Set-up

-metal UHV chamber

base pressure < 2·10-10 mbar

residual magnetic field < 10 mG

electron energy analyzer: Time of Flight (ToF) spectrometer

Acceptance angle: 0.83°Energy resolution:

30 meV @ 5eVDetector noise:

<10-4 counts/sPCGPIB

Multiscaler FAST 7887

PS1 PS2 PS3 PS4

start stop

PreamplifierDiscriminator

Laser

sample detector

G. Paolicelli et al. Surf. Rev. and Lett. 9, 541 (2002)


projected band structure of Cu(111) with the non-linear photoemission spectrum collected with photon energy = 4.71 eV.

Two-photon photoemission from Cu(111)

VL

FL

Light grey spectrum: R. Matzdorf, Surf. Sci. reports,30 153 (1998)


G. D. Kubiak, Surf. Sci. 201, L475 (1988), m*/m=1.0+-0.1, hv=4.38eV

M. Weinelt, Appl. Phys. A 71, 493 (2000) on clean Cu(111) @ hv=4.5eV+1.5eV, m*/m=1.3+-0.1

Hotzel, M. Wolf, J. P. Gauyacq, J. Phys. Chem. B 104, 8438 (2000) on 1ML N2 / 1ML Xe/ Cu(111) @ hv=4.28eV+2.14eV, m*/m=1.3+-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) @ hv=3.14eV m*/m = 1.28+-0.07

Effective mass of n=1 IPS on Cu(111) measured withangle resolved 2PPE in the literature


F. Forster, G. Nicolay, F. Reinert, D. Ehm, S. Schmidt, S. Hufner, Surf. Sci. 160, 532 (2003), SS m*/m=0.43+-0.01, binding energy: 434 meV

Effective mass of n=0 SS on Cu(111) measured withhigh resolution angle resolved photoemission in the recent literature


S. Pagliara, G. Ferrini, G. Galimberti, E. Pedersoli,C. Giannetti, F. Parmigiani, Surf. Sci. 600, 4290 (2006)


IPS Binding energy=Ek-hv-sp , sp= 0.9-1 eV

ss

ips

k||

IPS and SS dispersion on the same data set


IPS photoemission from Cu(111)


SS

IPS

4.7

1 e

V

VL

FL

IPS


m/m*=1.28+-0.07

m/m*=2.2+-0.07


SS

IPS

4.2

8 e

V

VL

FL

IPS

m/m*=1.6+-0.07




SS

IPS4.2

8 e

V

4.7

1 e

V

VL

FL

control point: one-photonphotoemission SS

3.1

4 e

V


2.0

1.5

1.0

0.5 E

ffec

tive

mas

s

642 Pump photons per pulse (x 10

10)

Two-color photoemission from Cu(111)

pump

probe

static limit

SS

IPS

SS

IPS

1.3 1010ph/pulse= 10 nJ/pulse at 4.71 eVfluence 10 J/cm2

4.71 eV

3.14 eVVL

FL


4.71eV+3.14 eV 4.71eV+4.71 eV



?

pump

How many electrons do we pump into the bulk bands?

From band structure: 4·1018 cm−3 states available in |k|<0.2 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)]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

IPS

VL

FL


T. Fauster, W. Steinmann, “Two Photon Photoemission Spectroscopy of Image States”

Phase shift model: Cu(111)

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 k||

=0-effective mass increases

k||

IPS dispersion


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.


People

Thank you

Fulvio Parmigiani (U Trieste) Stefania Pagliara (UCSC)

Claudio Giannetti (UCSC) Gianluca Galimberti (UCSC)

Emanuele Pedersoli (ALS-LBNL)

image-potential-state effective mass controlled by light pulses

Documents