Ultrafast Dynamics of Electrons at Interfaces [thesis]

190 Pages·1999·8.95 MB·English

LBNL-43186
ERNEST CIRLANDCI LAWRENCE
BERKELEY NATIONAL ILABORATORY
UltrafastDynamicsof
Electronsat Interfaces
JasonD.McNeill
Chemical Sciences Division F@EIVED
JlJLt2$99
1999
May ~sT\
Ph.D. Thesis
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LBNL-43186
Ultrafast Dynamics of Electrons at Interfaces
by
Jason Douglas McNeil]
Ph.D. Thesis
Department of Chemistry
University of California, Berkeley
and
Chemical Sciences Division
Lawrence Berkeley National Laboratory
University of California
Berkeley, California 94720
May 1999
This work was supported by the Director, Office of Energy Research, Office of Basic
Energy Sciences, Chemical Sciences Division, of the U.S. Department of Energy
under Contract No. DE-AC03-76SFOO098.
Ecydei pap.]
@
Ultrafast Dynamics of Electronsat Interfaces
Copyright @ 1999
by
Jason Douglas McNeill
TheU.S.DepartmentofEnergyhastherighttousethisdocument
foranypurposewhatsoeverincludingtherighttoreproduce
alloranypartthereof.
Abstract
Ultrafast Dynamics of Electrons at Interfaces
by
Jason Douglas McNeill
Doctor of Philosophy in Chemistry
University of California at Berkeley
Professor Charles B. Harris, Chair
Electronic states of a thin layer of material on a surface possess unique physical and
chemical properties. Some of these properties arise from the reduced dimensionality
of the thin layer with respect to the bulk or the properties of the electric field where
two materials of differing dielectric constants meet at an interface. Other properties
are related to the nature of the surface chemical bond. Here, the properties of excess
electrons in thin layers of Xenon, Krypton, and alkali metals are investigated, and
the bound state energies and effective masses of the excess electrons are determined
using two-photon photoemission. For Xenon, the dependence of bound state energy,
effective mass, and lifetime on layer thickness from one to nine layers is examined.
Not all quantities were measured at each coverage.
The two photon photoemission spectra of thin layers of Xenon on a Ag(lll)
substrate exhibit a number of sharp, well-defined peaks. The binding energy of
the excess electronic states of Xenon layers exhibited a pronounced dependence on
coverage. A discrete energy shift was observed for each additional atomic layer. At
low coverage, a series of states resembling a Rydberg series is observed. This series
is similar to the image state series observed on clean metal surfaces. Deviations
from image state energies can be described in terms of the dielectric constant of the
overlayer material and its effect on the image potential.
For thicker layers of Xe (beyond the first few atomic layers), the coverage de-
pendence of the features begins to resemble that of quantum well states. Quantum
2
well states are related to bulk band states. However, the finite thickness of the layer
restricts the perpendicular wavevector to a discrete set of values. Therefore, the
spectrum of quantum well states contains a series of peaks which correspond to the
various allowed values of the perpendicular wavevector. Analysis of the quantum well
spectrum yields electronic band structure information. In this case, the quantum well
states examined are derived from the Xenon conduction band. Measurements of the
energies as a function of coverage yield the dispersion along the axis perpendicular
to the surface while angle-resolved two-photon photoemission measurements yield
information about dispersion along the surface parallel.
The relative importance of the image potential and the overlayer band structure
also depends on the quantum number and energy of the state. Some members of
the image series may have an energy which is in an energy gap of the layer material,
therefore such states may tend to remain physically outside the layer and retain
much of their image character even at higher coverages. This is the case for the
n = 1 image state of the Xe/Ag(l 11) system. The energies of image states which
are excluded from the layer have a complex dependence on the thickness of the layer
and its dielectric constant.
The population decay kinetics of excited electronic states of the layer were also
determined. Lifetimes are reported for the first three excited states for 1–6 atomic
layers of Xe on Ag(lll). As the image states evolve into quantum well states with
increasing coverage, the lifetimes undergo an oscillation which marks a change in the
spatial extent of the state. For example, the n = 2 quantum well state decreases
substantially at 3–5 layers as the electron probability density in the layer increases.
The lifetime data are modeled by extending the two-band nearly-free-electron ap-
proximation to account for the insulating Xe layer.
...
111
Tomy dear wife, Carolina, our wonderful son, Andr6s, and Mom and Dad
iv
Contents
List of Figures vi
List of Tables vii
1 Introduction 1
2 Background 5
2.1 History of Surface States... . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 The Image Potential State . . . . . . . . . . . . . . . . . . . . 6
2.2 Spectroscopy of Surface Electronic States . . . . . . . . . . . . . . . . 9
2.2.1 Photoemission Spectroscopy . . . . . . . . . . . . . . . . . . . 10
2.2.2 Inverse Photoemission . . . . . . . . . . . . . . . . . . . . . . 10
2.2.3 Two Photon Photoemission Spectroscopy . . . . . . . . . . . . 12
2.2.4 Angle resolved Two Photon Photoemission . . . . . . . . . . . 15
2.3 Theoretical Models of Surface States . . . . . . . . . . . . . . . . . . 16
2.3.1 The Image Potential or Surface Barrier . . . . . . . . . . . . . 17
2.3.2 The Two Band Nearly-Free-Electron Model . . . . . . . . . . 18
2.3.3 Multiple Reflection Theory . . . . . . . . . . . . . . . . . . . . 22
2.3.4 Effective Mass of Image States . . . . . . . . . . . . . . . . . . 27
2.3.5 Lifetime of Image States . . . . . . . . . . . . . . . . . . . . . 30
2.4 Quantum Well States . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.5 The Photoemission Process . . . . . . . . . . . . . . . . . . . . . . . 36
3 Experimental 40
3.1 The Ultra-High Vacuum Chamber . . . . . . . . . . . . . . . . . . . . 42
3.1.1 Sample Manipulator . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.2 Vacuum Pumping System . . . . . . . . . . . . . . . . . . . . 43
3.1.3 LEED and Auger Spectrometer . . . . . . . . . . . . . . . . . 44
3.2 Laser System and Optics... . . . . . . . . . . . . . . . . . . . . . . 44
3.3 Data Acquisition System.. . . . . . . . . . . . . . . . . . . . . . . . 47
3.3.1 Amplification, Discrimination, and Timing . . . . . . . . . . . 48
3.3.2 Time to Digital Conversion . . . . . . . . . . . . . . . . . . 50