slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Image-potential-state effective mass controlled by light pulses PowerPoint Presentation
Download Presentation
Image-potential-state effective mass controlled by light pulses

Loading in 2 Seconds...

play fullscreen
1 / 21

Image-potential-state effective mass controlled by light pulses - PowerPoint PPT Presentation


  • 85 Views
  • Uploaded on

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)

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Image-potential-state effective mass controlled by light pulses' - long


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

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

slide2

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

slide3

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
slide4

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)

slide5

z

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*

slide6

ToF

Linear vs non-linear photoemission

Angle Resolved LINEAR PHOTOEMISSION (hn>F)

band mapping of OCCUPIED STATES

Angle (and time) RESOLVED MULTI-PHOTONPHOTOEMISSION (hn<F)

band mapping ofUNOCCUPIED STATES and ELECTRON SCATTERING PROCESSES

slide7

Amplified Ti:Sa laser system

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

slide8

sample

detector

PS1

PS2

PS3

PS4

PC

Preamplifier

Discriminator

GPIB

Multiscaler

FAST 7887

stop

start

Laser

Experimental Set-up

  • m-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 @ 5eV

Detector noise:

<10-4 counts/s

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

slide9

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)

slide10

Two-photon photoemission from Cu(111)

Effective mass of n=1 IPS on Cu(111) measured with

angle resolved 2PPE in the literature

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=0 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*/m=0.43+-0.01, binding energy: 434 meV

slide11

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 energy=Ek-hv-Fsp , Fsp= 0.9-1 eV

slide13

Two-photon photoemission from Cu(111)

IPS

m/m*=1.28+-0.07

VL

m/m*=2.2+-0.07

IPS

4.71 eV

SS

FL

slide14

Two-photon photoemission from Cu(111)

IPS

m/m*=1.6+-0.07

VL

IPS

4.28 eV

SS

FL

slide15

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

slide16

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

slide17

Two-color photoemission from Cu(111)

4.71eV+3.14 eV

4.71eV+4.71 eV

slide18

Two-color photoemission from Cu(111)

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)

?

VL

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)]

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

slide19

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 k||=0

-effective mass increases

k||

IPS dispersion

slide20

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.

slide21

People

Fulvio Parmigiani (U Trieste)

Stefania Pagliara (UCSC)

Claudio Giannetti (UCSC)

Gianluca Galimberti (UCSC)

Thank you

Emanuele Pedersoli (ALS-LBNL)