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1987
Photocurrent generation from Basic metals
Ringler, Thomas Jay.
http://hdl.handle.net/10945/22416
3H00L
Q n rt O
-93943-5002
NAVAL POSTGRADUATE SCHOOL
Monterey, California
THESIS
PHOTOCURRENT GENERATION FROM BASIC METALS,
UTILIZING A SHORT PULSED ArF EXCIMER LASER
by
Thomas J. Ringler
September 1987
Thesis Advisor:
F.
R. Buskirk
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PH0T0CURRENT GENERATION FROM BASIC METALS,
UTILIZING A SHORT PULSED ArF EXCIMER LASER
PERSONAL AuThOR(S)
':
Ringler, Thomas J.
rypj Of REPORT
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1987 September
61
Supplementary notation
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COSAT, COOES
<
ABSTRACT (Continue on
9
'8
SUBJECT TERMS (Continue on reverie
subgroup
GROUP
ElO
reverie
it
it
neceisary and identity by block number)
Photoelectric Effect
ArF Excimer Laser Photoemission
necemry and
identity by block
number)
An excimer
laser used to produce "cold" photoelectrons from common metal
surfaces offers an improvement over a standard heated (thermionic) cathode electron
source.
Photoelectrons accelerated across an anode cathode gap were observed under
both space charge limited and emission limited conditions. Temporal characteristics
of the resulting electron beam showed fast rise times of 3-5 nano seconds (ns) for
space charge limited and 8-1 2ns (full width half maximum) for emission limited cases.
Spatial characteristics of the output pulse shape revealed a classic clipped peak
amplitude in the space charge pulse but identical characteristics in the emission
limited pulse output. Both types of emission produced large current densities up to
Varying the cathode metal used for the photocathode indicates Zinc (Zn)
91 amps/cm2 .
produced the largest current density, and therefore, the highest quantum efficiency
in both space charge and emission limited cases.
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UNCLASSIFIED
Approved for public release; distribution is unlimited,
Photocurrent Generation From Basic Metals,
Utilizing a Short Pulsed ArF Excimer Laser
by
Thomas Jay Ringler
Lieutenant Commander, United States Navy
B.S., North Carolina State University, 1974
Submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN PHYSICS
from the
NAVAL POSTGRADUATE SCHOOL
September 1987
ABSTRACT
produce "cold" photoelectrons from common
An excimer laser used to
metal surfaces offers an improvement over a standard heated (thermionic)
cathode electron source.
cathode
gap
Photoelectrons
accelerated
across
an
anode
were observed under both space charge limited and emission
characteristics of the resulting electron
Temporal
limited conditions.
beam showed fast rise times of 3-5 nano seconds (ns)
for
space
charge
limited and 8-1 2ns (full width half maximum) for emission limited cases.
Spatial
characteristics
of
the
clipped
peak
amplitude
in
the
output pulse shape revealed a classic
space
charge
pulse
characteristics in the emission limited pulse output.
emission
produced
large
identical
but
Both
current densities up to 91 amps/cm
types
.
of
Varying
the cathode metal used for the photocathode indicates Zinc (Zn) produced
the
largest
current
density,
and
therefore,
the
highest
efficiency in both space charge and emission limited cases.
quantum
TABLE OF CONTENTS
I
.
II.
INTRODUCTION
BACKGROUND AND NATURE OF THE PROBLEM
A.
III.
IV.
8
10
ELECTRON SOURCES
10
1.
Heated Cathodes
10
2.
Field Emission Cathodes
11
3.
Cold Photocathodes
12
4.
Elaborate Cathodes
13
B.
SPATIAL CHARACTERISTICS
14
C.
TEMPORAL CHARACTERISTICS
15
D.
VACUUM REQUIREMENTS
16
EXPERIMENTAL PROCEDURE
17
A.
LASER/OPTICAL DESCRIPTION
17
B.
VACUUM CHAMBER AND TARGET CATHODE DESCRIPTION
20
C.
DATA ACQUISITION
22
THEORETICAL CONSIDERATIONS
24
A.
GENERAL
B.
BACKGROUND
C.
THREE-STEP PHOTOEMISSION CONCEPT
25
D.
OTHER FACTORS INFLUENCING PHOTOEMISSION
28
24
•
24
1
Photon Polarization
2.
Surface Contamination
29
3.
Metal Crystal Orientation
30
4.
Surface Roughness
30
r
28
V.
VI.
VII.
E.
THE SCHOTTKY EFFECT
31
F.
PLASMA FORMATION
32
EXPERIMENTAL RESULTS AND DISCUSSION
33
A.
CATHODE MATERIALS AND WORK FUNCTIONS
33
B.
LASER PULSE/PHOTOCATHODE OUTPUT CORRELATION
36
C.
CATHODE REFLECTIVITIES
38
D.
LASER POLARIZATION
42
E.
CHILD-LANGMUIR FLOW
43
F.
CURRENT DENSITIES ACHIEVED
46
G.
RELATIVE QUANTUM EFFICIENCY DETERMINATION
47
CONCLUSIONS
51
SUGGESTED FOLLOW UP EXPERIMENTATION
53
APPENDIX A:
TABLE OF SYMBOLS AND ABBREVIATIONS
54
APPENDIX B:
LIST OF EQUIPMENT
55
APPENDIX C:
IMPROVED ANODE/COLLECTOR APPARATUS
56
LIST OF REFERENCES
57
BIBLIOGRAPHY
59
INITIAL DISTRIBUTION LIST
60
LIST OF FIGURES
Typical Pulse Shapes of:
(a) Laser Output; (b) Photocurrent
Output for Emission Limited Flow; (c) Photocurrent Output for
Space Charge Limited Flow. Horizontal Scale Shows a Time Base
of 5ns/div, with the Vertical Scale Representing a Variable
Output Voltage.
14
2.
Overall Schematic of Excimer Photocathode Experiment.
18
3.
Initial Experiment Vacuum Chamber and Cathode Target.
21
4.
Three-Step Process of Photoelectron Emission:
(a) Photon Energyis Totally Absorbed by Single Electron Within Metal; (b) Excited
Electron May Undergo Multiple Collisions; (c) Electron Will
Escape if Perperdicular Component of Momentum at Surface is
Greater Than or Equal to Metal Work Function.
27
5.
Optimal Geometry for Most Efficient Coupling of Polarized
Laser Beam into Metal Cathode Surface.
29
Nine Cathode Materials Plotted at 20KV, Showing Correlation
of Photodiode Output to Computed Current Density Generated
at the Collector.
37
Nine Cathode Materials Plotted at 10KV, Showing a Linear Slope
for Emission Limited Flow Changing to a Vertical Slope as
Space Charge Limit is Reached.
39
Selected Cathode Materials Plotted at 15KV, Showing a Linear
Slope for Emission Limited Flow Changing to a Vertical Slope
as Space Charge Limit is Reached.
40
Comparison of Current Density vs Anode-Cathode Accelerating
Potential for Theoretical Space Charge Limited Flow and Two
Sets of Experimental Space Charge Data.
44
Relative Quantum Efficiency Comparison at 20KV Accelerating
Voltage
49
Relative Quantum Efficiency Comparison at 15KV Accelerating
Voltage
50
Improved Experimental Anode and Collector Geometry.
56
1
6.
7.
8.
9.
10.
11.
12.
ACKNOWLEDGEMENT
With great appreciation to Dr. D. C. Moir and Dr. S. W.
the
Los
Alamos
National
Downey
of
Laboratory, for their guidance, patience and
long hours assisting with the Poloroid pictures.
To my wife Leslie for the many hours assisting on the computer.
such
devices,
Modern
I.
INTRODUCTION
as
accelerators
require enormous numbers of
beams.
electrons
and
for
free electron lasers,
their high current electron
The quality of the electron source is important
injection
into
currently
consist
general
three
of
emitters
emitters, field
Sources
accelerator.
an
and
used
types.
when
thermionic
are
emitters,
offers different spatial and temporal characteristics.
the
for
to produce electrons
These
photoelectron
used
of which
each
In
addition
to
general types of emitters, materials used to construct cathodes may
further
increase
quantum
the
efficiency
enhance
and
desirable
characteristics of the electron source.
observations
From the initial
increased
current
flow
observed
was
incident ultraviolet light),
Rudolph
of
through
across
Albert
Hertz
in
(where
1887
spark gap exposed to
a
Einstein's formulation of
the photoelectric equation in 1905, and up through our present
day
use
of excimer lasers, we have continued to make significant improvements in
Utilizing Max Plank's energy
our knowledge of the photoelectric effect.
effect
allowed
concept
quantization
which,
prior
was
generally
to
explain
could
time,
not
the photoelectric
be
Photoelectric research in the
electromagnetic theory.
1940's
that
to
Einstein
explained
1930*s
and
with
the
accomplished with electromagnetic wavelengths in
o
the range of 4000 to 7600
visible
light
from
Angstroms (A), corresponding to the region of
extreme
ultraviolet (UV)
o
to
the
far
infrared.
o
Wavelengths in the lower ranges of 1900 A to 2480 A have only been fully
8
Both synchotron radiation made
investigated fairly recently.
the
in
1960' s
available
and development of the excimer laser, which can produce
consistent high intensity, monochromatic light in the ultraviolet region
have permitted further research
this region of shorter wavelengths.
in
Recent advances in pulsed excimer lasers make
high
photon
fluxes
about
of
10
16
2
advances
generation
using
have
sparked
possible
to
achieve
photons per square centimeter per
second (photons/cm *sec), for unfocused beams
recent
it
renewed
in
the UV region.
interest
These
photoelectron
in
metals, even though metals in general produce
common
very low quantum efficiencies.
This thesis investigates the
relative
quantum efficiencies of nine
common metals for the production of photoelectrons.
(ArF),
excimer
results are to be
laser
is
used
in
Radiographic
Argon
Florine
used as the illuminating photon source.
determining
the
construction of an improved electron injection
High-Energy
An
The
best cathode material for
source
for
the
Pulsed
Machine Emitting X-Rays (PHERMEX) accelerator
currently in use at the Los Alamos National Laboratory.
BACKGROUND AND NATURE OF THE PROBLEM
II.
A.
ELECTRON SOURCES
generate
The method used to
electrons is of significant importance
because it influences the electron energy distribution, radial
components
momentum
and
electron beam.
distribution
properties
of
the
velocity
resulting
These properties are important when the electron beam is
used as an injector for an
electron accelerator.
There are three basic
types of phenomena that permit electrons to escape from metal
field emission (from large applied electric
thermionic,
include
These
photoelectric
fields) and the
effect.
are
thermionic
photoelectric
(photon
excitation
structure).
These
electron
sources
cathodes.
major
The
(heated
most
important of these
cathode
emission)
and
the cold cathode atomic electron
of
sources
have
intrinsic
advantages
and
disadvantages which are discussed below.
1
.
Heated Cathodes
The most utilized electron source is the heated filament,
electrons
are "boiled" off the surface of a cathode material.
the single advantage of producing
numbers.
excitation
However, since
of
material,
the
high temperature.
the
High
not
This has
a continuous electron source of large
electrons
are
emitted
through
thermal
the cathode must be maintained at a very
temperatures
accelerate unwanted chemical reactions.
are
where
associated with filaments usually
Metals with low melting
points
desirable since they cannot be heated to an efficient electron
emission point
prior
to
Usually
melting.
10
low
melting
points are
associated
with
that
metals
lower
and
energies,
large spectrum of kinetic
surface
work
more importantly, show a wide
variance in the radial components of momentum that are not
direction
the
beam
of
electron
enhancement methods
beam
such
to
Efficient utilization
be constrained unless further beam
may
focusing
as
parallel
This means the electrons tend to
propagation.
spread out in all directions as they are emitted.
of this type of
functions.
from thermionic cathodes demonstrate a
emitted
electrons
Furthermore,
have
aperture
or
restrictions
are
to remove the undesired divergence effects of the electron
incorporated
beam.
Field Emission Cathodes
2.
utilizes
This process
strengths on the
[Ref.
1 ]
of
1
.0 X 10
6
electric
fields with threshold
5.0 X 10 7
to
Volts/Meter (V/M)
to extract electrons, normally from a tungsten cathode surface.
material
cathode
The
order
intense
further reduce the
is
usually
work
surface
unheated, but can also be heated to
function.
In
general, the intense
electric fields shift the surface potential energy barrier
level
valence
where
energy to tunnel out
electrons
and
to
a
lower
in the bulk metal have enough internal
An
escape.
electron beam produced by this
method has a large radial momentum component associated with the emitted
electrons and usually needs to be focused or restricted by a
apertures to remove the divergence effects.
of
apertures
intensity.
may
This
result
system
in
is
a
several
series
Use of a restrictive series
fold
inefficient,
energy to generate the electric fields needed
reduction
requiring
for
in the beam
large amounts of
electron
extraction
and is complicated to construct as compared to thermionic emitters.
11
of
Cold Photocathodea
3.
Utilization
workable
theory,
provides
with
light
allows pulsed photon illumination.
production
Although
targets.
continuous
effect and application of a
electron
for
metal
irradiation of simple
accomplished
photoelectric
the
of
as
in
both
early
research
requiring
continuous
structure
length
length of the
of
of the target, allowing electrons
The
the emitted electrons is directly proportional to the
incident
laser
pulse.
intensity of the incident
The
photon radiation can be used to limited extent for control
numbers.
high
Incident photons
to escape the cathode surface, producing a "cold" electron source.
pulse
was
The production of electrons by this
the thermionic and field emitter.
interact with the electron
photon
sources, excimer laser technology
method offers the distinct advantage of not
power
through
of
electron
The maximum kinetic energy of emitted electrons is a function
of the incident photon
Electrons
energy.
escape with maximum excess
kinetic energy that is the difference between the incident photon energy
and the cathode material's work function and is usually on the order
a few electron volts.
of
Because of energy loss within the material due to
electron-electron scattering, there is an energy distribution associated
with
the
emitted
Photoelectrons
electrons.
and
transverse momentum components,
divergence of the electron beam.
source,
low.
when
Quantum
using
The
therefore,
relatively small
have
do
disadvantage
not
this
of
electron
metal cathodes, is that quantum efficiency is very
efficiency
is
defined
as
the
ratio
of the number of
photoelectrons produced to the incident number of photons.
with
cause a rapid
For
metals,
incident photon energies below 10 e.v., there is general agreement
12
exceed 10
that quantum efficiencies will not
efficiencies are usually in the 10
energies
quantum
e.v.),
(6.4
-4
_2
region.
At
efficiency
Metal quantum
[Ref. 2],
incident
low
photon
calculated assuming one
is
incident photon will generate only one photoelectron, and therefore, its
At higher incident photon energies, where
value will not exceed unity.
may
production
secondary electron
within
occur
metal,
the
quantum
efficiencies higher than unity may be obtained.
Elaborate Cathodes
4.
addition to simple metal cathodes there are a number of much
In
more elaborate cathodes [Refs. 2, 3] that have been constructed over the
using
last 10 years
exotic
combinations
several elements.
of
usually entail the use of an active alkali metal layer
combined
high
one
with,
quantum
two other metals.
efficiencies
characteristics.
quantum
or
and
Current densities up to
top
200
emitted
amps/cm
electron momentum
2
[Ref. 4]
efficiencies as high as 0.2 to 0.3 have been achieved.
one order of
cathodes.
magnitude
better
than
can
or
of,
These cathodes provide very
good
have
on
These
be
with
This is
achieved by simple metal
However, these systems pay a price in their complexity.
generally require surgically clean cathode surfaces and
suffer
They
intense
degradation due to surface "poisoning" from vacuum chamber contaminants.
Ultra-high
vacuums, on the order of 10
additional effort
required
to
-10
maintain
torr must be maintained.
these
systems
The
is an extreme
disadvantage
Of course, there are hybrid combinations of these three types of
emitters that may
thermionic
heating
provide
improved
efficiencies.
The
addition
of
in conjunction with field emission or photoelectron
13
activation can produce more electrons and higher
quantum
efficiencies,
however, it may also degrade the cathode causing untimely replacement.
B.
SPATIAL CHARACTERISTICS
advantage
of
laser induced photocurrent generation is
that the electron beam can
be
pulsed
A
distinct
emitted electrons can be controlled.
the
two
distinct
shown in Fig.
1
.
output
The
and
that the pulse shape of the
The input laser pulse, along
pulse shapes observed in this experiment are
input
pulse
is generally Gaussian in shape when
the laser gas had been "broken in" and is the case for most of the
acquisition.
with
Output pulses consist of the Gaussian
data
shaped photocurrent
3-5 ns
(FWHM)
(a)
Figure 1.
(b)
(c)
(a) Laser Output; (b) Photocurrent
Typical Pulse Shapes of:
Output for Emission Limited Flow; (c) Photocurrent Output
for Space Charge Limited Flow. Horizontal Scale Shows a
Time Base of 5 ns/div, with the Vertical Scale Representing
a Variable Output Voltage.
14
pulse
photocurrent
emission
with
associated
When emission limited,
photoelectrons
extent with each other
as
controlled
by
they
electrons already
to
emitted
pulse
The
build
either
increasing or decreasing the
power conditions,
a
to
is
Space charge on the
enough to cause those
great
that
those
are
being
beam
laser
Gaussian
bunches
these
of
shaped
where they prevent other
level
a
Electron
with
fashion
shape
in space charge is clipped when electrons
emerging.
from
to any great
pulse
The
with
interfere
shape
outside the metal surface
repeatable
interfere
the shape of the input laser pulse.
is
"clipped"
the
space charge limited flow.
emitted.
production
electrons
and
not
do
are
other hand, means electron
emitted.
with
associated
shape
pulse
flow
limited
can
specific
power
output
generated
be
pulse
output.
can
shapes by
For low laser
obtained.
be
in
At
relatively large power outputs, where the cathode is driven into a space
charge limit, a clipped pulse shape is observed.
C.
TEMPORAL CHARACTERISTICS
The
major
"instantaneous"
advantage
production
photocathode material.
produced
laser
of
of
electrons
electrons
the
after sub-pico second time delays.
metal
the laser input pulse.
rates to be achieved when
showed
by
changing
the
It also allows high pulse repetition
excimer laser pulse technology achieves laser
repetition rates in the kilo Hertz region.
research
near
from photon irradiation of a
This allows direct control of the output pulse length
of
the
Following photon incidence, electrons are raised
to excited levels within
length
is
Pulses
observed
in
this
rise times of 8-12 nano seconds (ns) when operating in
15
the emission limited region.
operating
when
in
Faster rise times of 3-5 ns
charge region.
space
the
measured to the point where the
in Fig.
D.
space
were
typical
The latter rise time is
charge limit is reached as shown
1
VACUUM REQUIREMENTS
Another major restriction for present photoelectron sources
requirement
range
of
for
10
— fi
a moderately high to
—1
to
current
technology allows
ultra-high vacuums to be achieved, simple maintenance and
efforts
vacuum
are
required
systems
to
sustain
generally
require
establish a pure surface
intervals
to
remain
required to maintain
and
may
peak
at
cathode
have
to
preparation
be
efficiency.
compound
5 X 10
was
to 2 X 10"' torr.
performed
This
level
with
frequent
alkali metal surface free
which
make
up
torr are maintained.
only
moderate vacuums of
was readily obtained within 10
minutes using a Cryogenics, Cryo-Torr-8 vacuum pump.
16
at
"situ" to
sophistication is
Great
the vacuum chamber structure when vacuums of 10
investigation
Ultra-high
in
repeated
from mono-layer absorption of gases and other materials
This
extraordinary
the high performance.
elaborate
an
the
ultra-high vacuum, defined in the
Although
torr.
10
is
EXPERIMENTAL PROCEDURE
III.
The experiment was designed with simplicity in mind so that
research
or
maintain
the
a
larger
scale-up
simplicity
same
of
the
photocathode
construction,
in
further
assembly could
reliability, cathode
material availability and remain within low monetary constraints.
experimental
The
apparatus for the generation of photoelectrons is shown in
Fig. 2.
A.
LASER/OPTICAL DESCRIPTION
The excimer laser used as
Florine
photon
source
was an "off the shelf"
This gas filled laser can be used
LAMBDA-PHYSIK EMG-150T.
Argon
a
tuned
to
respectively.
producing
photon.
either
nanometers
193.4
(nm)
or
Specifically, it can
248.0 nm wavelengths
For this experiment, the laser was used in the ArF
photons
with
energy
an
The laser was pulsed
of
6.4
mode,
electron volts (e.v.) per
at 2-3 Hertz (Hz) while determining data,
but was capable of repetition rates of several tens of Hz.
was
either
(ArF) or Krypton Florine (KrF) gas to produce an emitted
laser beam in the ultraviolet end of the spectrum.
be
with
The
laser
mounted, along with most other equipment, on a large optical table.
The vacuum chamber, with
a
volume
of
target cathode and collector assembly.
about
10 liters, contained the
It was mounted separately on the
floor at the end of the optical table to allow a Cryogenic,
Cryo-Torr-8
vacuum pump to be attached as an integral part of the target chamber.
17
Figure 2.
Overall Schematic of Excimer Photocathode Experiment.
18
As the laser beam exited the excimer laser case it was translated 18
lower than the exit height, via two quartz plates adjusted at 45
inches
degrees to the beam path
create
to
parallel beam path.
a
The quartz
plates were 98$ reflective for the tuned laser wavelength of
path
the
Translating
produced
photon beam
permitted
within
apertures
internal
by
the
beam
photon
the
vacuum
excimer
the
into the vacuum
Additional fine adjustments of
surface.
position
alignment
beam
final
cathode
chamber and onto the
the target chamber
allowed
nm.
193.4
chamber.
was
traverse
to
One drawback of the
non-circular
the
and the
non-homogeneous intensity composition over the beam's roughly elliptical
To obtain a more uniform beam,
cross section.
aperture
an
adjustable
circular
positioned just after the 45 degree reflecting plates and
was
prior to the laser attenuator
allow selection of a circular portion
to
of the beam with fairly uniform intensity characteristics.
This
also
narrowed
the beam, allowing it to pass through a NRC Model 935-16 Laser
Variable
Attenuator,
reflections.
reducing
scattering
internal
A variable attenuator was
used
to
limited electron production region.
emission
data, intensity reductions on the order
required.
The beam was then passed
plate
produce
to
a
split
One
FND-100Q Ultra Fast silicon photodiode
data
a
30%
reflecting
second
standard
silicon
8%
photocell
to monitor the laser pulse
Suprasil
The Suprasil quartz window
transmission due to reflection.
loss
in
was
used
19
quartz
portion was passed to an EGG
used
entrance window and into the vacuum chamber.
the
the
in
For most emission limited
shape while the other portion of the beam was passed through a
creates
unwanted
of 100 in laser beam power were
through
beam.
obtain
and
to
monitor
(from
laser
A
beam
pulse
scattering) the
reflecting
future
plate.
correlation
shape
of
the
at
the
first
98%
Beam monitoring was necessary to provide a means of
between
individual
laser
resulting current pulse produced from the
the
beam
laser
collecting apparatus.
pulse
extracted
power
and
photoelectrons
the
at
Redundant photocell monitoring was used with
the laser in the ArF configuration,
in the excimer output and also as
to track pulse to pulse instability
master
a
trigger
control
for
the
monitoring oscilloscopes.
B.
VACUUM CHAMBER AND TARGET CATHODE DESCRIPTION
Two
separate
geometries were used for photoelectron collection and
the cathode arrangement in the
target
chamber.
described below and is shown in Fig. 3.
appeared
and
After
The first geometry is
alignment
difficulties
concern arose over the reliability of current collection,
geometry improvements for more accurate current collection were effected
and are diagramed in Appendix C.
Two major improvements to the geometry
and optimizing the distance from the
were increasing the collector area
anode face to the collector surface such that it was 2.5 times the anode
aperture diameter.
electron
beam
as
This minimizes any focusing
it
transits
the
distance
or
divergence
between
of
the
anode face and
collector surface.
Upon entering the vacuum chamber, the laser beam was restricted by a
circular aperture to project a spot
size of 0.0742 cm
a 2.54 cm diameter cathode disk surface.
(HVDC)
potential
was
maintained
across
2
at the center of
A high voltage direct
the
current
anode-cathode gap.
The
vacuum chamber, anode and all other parts of the support structure were
20
•COAXIAL CHARGE COLLECTOR
LASER
r-
LASER APERTURE
HIGH
A
\
"
,
\ \ \ \ \
".
"*
"£Z3
'\
\ s s gsc
VOLTAGE
miZ
4
—
A-£F\~kCATHODE
gWT
SIGNAL
PINHOLE APERTURE
VACUUM CHAMBER
£W\1
Figure 3.
Initial Experiment Vacuum Chamber and Cathode Target,
21
maintained at ground potential.
up to 29 kilo-Volts
apparatus.
before
across
photoelectron
This
reduce
the
arrival
at
to
prior to
voltage
arcing occurred in the
metal
the
photocathode
2
,
final
the
beam
then
was
restricted by a second
which was an integral part of
surface
of
collector
the
coaxial cable with an impedance of 50 Ohms
the
collector
anode
the
cross sectional area of the electron beam
cable.
photoelectron beam was collected on the center conductor of
at
were
a 0.332 cm gap towards the anode and the collection
aperture of size 0.003167 cm
face,
high
Photoelectrons escaping
cable material.
accelerated
(KV)
In most cases, voltages were adjustable
(
fl
) .
a
The
standard
Photocurrent generated
was routed from the vacuum chamber and measured as a
voltage pulse on a Techtronics
R7103 oscilloscope with a one giga-Hertz
response capability.
C.
DATA ACQUISITION
Experimental data for the nine metals utilized the improved geometry
of Appendix C, and were obtained
calibration
reference.
With
chamber, the laser was pulsed
was monitored.
reduced
with
by
the
using
gold
the
gold
the
then
the
cathode set up in the vacuum
and the photocurrent (output) pulse shape
was
variable attenuator until an emission limited output
This should
have
insured an emission limit at all
higher accelerating potentials but experimentally this
was
as
Using an accelerating potential of 4KV, laser power
pulse was obtained.
Voltage
cathode
increased
concurrently of both the laser
did
not
occur.
to 5KV where a set of 16 photos was taken
pulse and the photocurrent output pulse.
Data sets were repeated at 10KV and 20KV.
22
The cathode
metal
was
then
substituted
with another and data sets repeated at 5KV, 10KV, and 20KV.
When some of the higher efficiency
For
space charge limited flow at 5KV.
points
were
Pulses
for
pulse
shape
made
were
half maximum
(FWHM),
materials,
emission
or
space
charge
pulses.
This
limit).
of the peak amplitude, amplitude at full width
time
base
for
the
input
laser
pulse and the
photocurrent output, and time base at the FWHM point for the
output
data
the photos were subjectively evaluated
determine
(to
Measurements
on
cathode
these
Photos were needed to "freeze" pulses
also taken at 15KV.
for later analysis.
cathodes were used, they showed only
information
input
and
was then manipulated by a computer
program written to reduce the data, and determine current density, power
per pulse and quantum efficiencies.
23
THEORETICAL CONSIDERATIONS
IV.
A.
GENERAL
It
is
not
the
intent of this section to offer an all encompasing
of
the
photoelectric
explanation
quantitatively verify prior models
has not yet been developed.
follows
What
concepts and a few specific factors
conduct of this research.
investigate
to
new model to replace
for
total
a
simple
a
quantitatively
that
a review of some basic
is
considered
were
effective
but
all
evaluate
these
the
method of electron production, a
accepted "as is" without an attempt
are
degree
the
of
influence
factors
is
made,
then
provided
each
If an attempt to evaluate and then minimize
individually.
during
As the entire purpose of this research was to
great many influencing factors
of
a
intended
it
development is simply too large and an all inclusive theory
theoretical
effects
provide
or
is
The number of variables required
those currently in use.
to
nor
process,
the
adverse
the simplicity of the
research is compromised.
B.
BACKGROUND
During
the
early
development
photoelectric effect was studied as
introduction
of
quantum
theory
of
an
electromagnetic
observed
anomoly.
theory,
the
With
the
and the dual wave/particle concept of
light consisting of quantum bundles of photons, more complex theories on
surface and bulk material of matter
the interaction of photons with the
emerged.
Whether true photoemission is strictly a "surface"
24
effect
or
predominately
has been argued for many years.
effect
volume
a
indicate
earlier investigations tended to
a volume effect, more recent
investigations [Ref. 5] and the current trend
indicates
Where
information
relavent
in
that the surface effect is the main source of photoelectrons.
One of the most
general
accepted
widely
and
early 1970' s has been the so
called
"three
theories up through the
process
step"
originally
in the 1960' s by C. N. Bergland and W. E. Spicer [Refs. 6, 7].
proposed
on whether the cathode material being
This basic model varies depending
considered is a metal or a semiconductor.
aspects
discusses
that
relate
In
simple
to
this
general,
metals
unless
section
otherwise
specified.
C.
THREE-STEP PH0T0EMISSI0N CONCEPT
The three-step process considers a photon of energy hu
with,
and
momentum
transfer
to,
a
single
electron
,
in
According to basic theories generated in the early 1900' s
by
to interact
the metal.
P. Drude,
metals are considered to contain a free flowing "sea of electrons" which
exist
thermal
in
equilibrium
Incident photons excite this sea
with
the
atoms of the metal [Ref. 8].
of electrons by momentum transfer that
imparts enough energy to allow some to escape from the metal.
In step one of the three-step process (Fig. 4a),
totally
transferred
ranging up to
usually
at the
several
comparable
associated
to
to
a
energy
is
single electron at a depth (within a metal)
hundred
the
angstroms.
The
electromagnetic
[Ref. 9] with the illuminating photon.
frequencies
photon
associated
with
25
ultraviolet
penetration depth is
radiation
wavelength
Certainly, in metals
light, "photons have
depths of penetration.
penetration
with
which
dimensional collision process.
vector
the
100 A"
[Ref. 10],
being a function of incident photon energy and
depth
electron
single
a
.which always are larger than
Photon energy is assumed to be transferred as momentum to
the material.
only
.
sum
responds
components.
interaction transfers momentum that
will
metal to an excited level, with energy
ground
raise one electron within the
above its ground state.
noj,
undergo
may
it
elastic
and
inelastic electron-electron
collisions until its energy is dissipated throughout the metal
until
collision
the
process
results
enough to the metal surface for
it
escape as a photoelectron.
to
greatest
collisions
represent
electrons.
The mean free path
bulk
(proportional
the
should
to
be
atomic
the
transferred to the individual
By
far,
[Ref. 9]
of the excited (higher energy) electrons
atomic
the
of
and
number)
depending
synchrotron radiation in
the
For
electron-electron
total
the
electrons.
electron
initial
on
UV
region
Synchrotron (DESY) [Ref. 11] indicates
an
at
tioj
,
Typically, in metals
photon energy.
the
density
energy,
one finds an escape depth of from several angstroms to several
angstroms
or
loss mechanism for excited
energy
function
a
bulk
the electron arriving close
in
metals, escape depths are generally short.
in the
above
liu,
may undergo no collisions before escaping the metal
state,
surface, or
of
Each incident photon
In step two, the excited electron (Fig. 4b), with energy
the
a three
in
That is, total momentum will consist
momentum
the
of
typically
then
tens
of
Work done using
the Deutsches Elecktronen
effective
escape
depth
of
o
about 30 A for metals.
this
escape
depth,
If electrons are excited in the bulk metal below
they
will
not
26
travel
far enough in a direction
towards the metal surface to
enable them to "escape" as photoelectrons.
electron
excited
In addition, even though an
may
enough
have
total
when it arrives within range of the surface, the critical aspect
energy
of its momentum is the component perpendicular to the surface.
In step three
of
three-step
the
perpendicular momentum component
required
to
process
greater
is
(Fig. 4c), if the final
than
or
equal
to
break through the surface potential barrier (i.e. the work
function of the metal)
then
the
excited
electron
will escape and be
emitted as a photoelectron into the adjoining medium (a vacuum for
experiment).
that
This emitted electron may have any
this
kinetic energy (K.E.),
Perpendicular
Component of
Momentum
Surface
a
\\
va
(a)
Figure 4»
w
^ a
w w / w w
(b)
\\)
<\\
w < t w
\\)
(c)
Three-Step Process of Photoelectron Emission:
(a) Photon
Energy is Totally Absorbed by Single Electron Within Metal;
(b) Excited Electron May Undergo Multiple Collisions; (c)
Electron Will Escape if Perpendicular Component of Momentum
at Surface is Greater Than or Equal to Metal Work Function.
27
outside the metal surface, up to a maximum energy of
function
(
$
)
of
metal.
the
tiu
,
minus the work
This last relationship is exactly what
Einstein theorized in his photoelectric equation in 1905.
K. E.
= nu — $
max
The maximum kinetic energy
for
subsequently
is
ejected
.
a photoelectron would correspond to
an electron excited at the first atomic
which
That is:
layer
with
perpendicular to the metal surface.
its
the
of
metal
momentum
total
surface,
component
only energy loss in this case
The
would be due to the work function of the metal.
D.
OTHER FACTORS INFLUENCING PH0T0EMISSI0N
1 .
Photon Polarization
The
three-step
simplified
process
can
lead
to
reasonable
interpretations and calculations for photolectron production when either
monochromatic
simplified
broad
or
However, as with any simplified
to be considered.
resolved
ejected
band
photon
sources
are
used.
process, other influencing factors need
Research conducted
J. Barton
by
et al.,
on
angle
electron energy curves for copper [Ref. 12], confirms
that the polarization direction of the incoming photon energy can have a
significant
influence
on
the
efficiency
of
the
photon absorption.
Generally, higher quantum efficiencies (more photoelectrons produced for
each photon) are
incoming
obtained
electromagnetic
when
the
electric
field
vector
for
the
wave (photon) lies perpendicular to the plane
formed by the incoming photon direction
28
and a unit vector normal to the
metal surface as shown in
Fig. 5.
Although not fully understood,
this
increased absorption is thought to be due to surface phonon coupling.
Incident polarized
hoton wave
Normal
Cathode surface
Figure 5.
Optimal Geometry for Most Efficient Coupling of Polarized
Laser Beam into Metal Cathode Surface.
Surface Contamination
2.
In
the
three-step
practice, emission is
we assume a pure metal surface.
theory
affected
by
the
absorbate
surface due to chemisorbtion (chemical interaction
This
chemisorbtion
subtracting from the
layer
with
on the metal
the
surface).
may significantly alter photoemission, by adding or
basic
work
function
of
the
pure
metal.
produces a larger or smaller observed work function which in
decrease
In
or increase the number of emitted photoelectrons.
turn
This
will
A monolayer
of adsorbed gases will form rapidly in all but the very highest vacuums.
29
-5
5 X 10 ^
For vacuums of
2X10 -7
to
used
torr,
in this experiment,
expected monolayer formation occurs in 0.1 to 10 seconds on a pure metal
o
cathode [Ref. Z]
If we consider an electron escape depth of 10 A and a
.
o
metal lattice constant for the surface contamination of just
the
topmost
then
4 A,
layer of contaminating atoms would contribute about 30% to
the photoemission [Ref. 10].
Therefore,
surface layer purity may
the
significantly effect the photoemission.
3.
Metal Crystal Orientation
function
A metal's work
crystal
photons
plane
expressed as
Miller
a
may
is
indexes,
function
a
on
incident upon [Refs. 2, 13]
are
the
produce better photon coupling than
production
depend
also
the
of
plane.
Photoelectron
densities
associated with
(100)
planer
For example,
•
crystalographic plane may
(111)
the
particular
the
different crystal planes because of the change in planer atomic electron
Additionally,
density.
body-centered
cubic
type
the
of
crystal
structures,
hexagonal-close-packed,
or
also
such
determines
as
how
photons transfer their energy to electrons in the metal cathodes.
4.
Surface Roughness
Photoelectron production may
vary
considerably
enhanced
when
large
This effect is
roughness.
due to surface
electric
fields
are
applied.
The apparent work function may vary as a result of macroscopic
features
such
microscopic
faults
Irregularities
electric
fields
grain
larger
as
in
the
in
on
the
a
boundaries, dislocation boundaries or
localized
surface
structure
area
such
create
as
intense
etch
pits.
localized
surface protuberances that can initiate field
emission or which could cause
explosive ionization and plasma formation
30
Additionally,
above the metal.
electron
beam
photoelectrons.
proposed
been
It has
roughness
affect
will
better beam quality (by a
that
factor of five) [Ref. 14] might be obtained by operating a
in
photocathode
limited region vice the emission limited region.
charge
space
the
the
producing greater divergence of the emitted
by
quality
surface
This is due to the greater divergence of emitted electrons from emission
limited protuberances,
whereas
charge
space
the
in
strong electric fields created by the space charge
limit case, the
reduces
the
radial
components of momentum that cause divergence.
E.
THE SCHOTTKY EFFECT
Schottky
The
effect
[Refs. 10, 15]
defined as an increase in
is
electron emission (i.e. current) caused by the increasing electric field
strength applied between the anode and
an applied electric field, photoelectrons
directly
above
the
surface
metal
Under the influence of
cathode.
removed
are
immediately.
from
the
area
This allows higher
photocurrents to be obtained because electrons are not being retarded by
the electron cloud which builds up just above the surface.
effect creates an apparent
decrease
in
The Schottky
the metal cathode surface work
function and an apparent increase in the quantum efficiency.
The
work
function is decreased by an amount:
A*
where
E,
e
and
C
are
kilo-Volts, the charge on
-C(eE)
=
'
5
respectively
the
electric field strength in
electron
and
a constant.
an
31
This apparent
change in the work function is very small and can usually
intense electric fields are used.
unless
be
neglected
Generally fields on the order
V/M will decrease the work function by only 0.4 volts.
of 5.0 X 10
For
this experiment the effect was deemed negligible.
F.
PLASMA FORMATION
Due to the intensity
of
pulsed
the
laser
power delivered to the
cathode, it might be expected that the metal would be heated locally
surface,
the
and possibly in the bulk, by inelastic collisions and the
creation of surface phonons.
A
certain amount of thermal activity will
occur but its contribution to enhance electron emission
production
electron
thermionic
via
from the laser "heated" cathode should be minimal.
With the laser pulse duration on
the
order of 30ns, a laser repetition
rate of 3 Hz and about 0.016 mj per pulse delivered to the surface,
total power delivered would be about 9.9 X 10^
portion
of
at
the
incident
Watts.
the
However, since a
energy is reflected, the actual power on the
surface would certainly be less by
a
minimum
of 20% for all the metal
cathodes.
In general, it is unlikely that 9.9 X 10
5
Watts would be high enough
energy delivery to vaporize and ionize the surface layers on the cathode
material.
field
Higher local concentrations
strengths
increased
plasma formation and
the
but
may
assumed
were
resultant
32
the
occured
negligible.
production
created from positive ion bombardment of
neglected.
have
electric
as
Therefore,
of secondary electrons
cathode
surface
can
be
EXPERIMENTAL RESULTS AND DISCUSSION
V.
The
current
experiment
preliminary
densities
was
objective
greater than 20-30 amps/cm
laser driven cold photocathode
whether
could be achieved from a
was chosen as a threshold
value
This
.
determine
to
because it is the present capability of the thermionic electron
gun
in
o
operation at the Los Alamos PHERMEX accelerator.
was
to allow a 100% improvement in the capacity of the present
desired
PHERMEX thermionic electron source.
cold
electron
cathode
simplicity
A goal of 50 amps/cm
source
maintenance.
and
In addition, a simple laser driven
would
vast
improvement
50-70
of
amps/cm^
achieved
emission and space charge limited regions for a copper cathode.
investigation,
and
the
in
feasibility of this improvement
the
densities
current
a
Results of the initial investigation in
April 1986 [Ref. 16], demonstrated
with experimental
offer
in
Further
primary objective of this thesis, was directed
towards comparing electron production
from
several metals to determine
which cathode material would produce the highest quantum efficiency.
A.
CATHODE MATERIALS AND WORK FUNCTIONS
Nine different metal cathode surfaces were used for this experiment.
Eight samples were stock metals that had
2.54 cm
diameter
disk
been
that was 0.40 cm thick.
copper cathode base that was
plated
with
metals were of high purities exceeding 99% •
attempt was
made
to
highly
polish
33
the
diamond
machined
to
a
The ninth sample was a
1000 A of gold.
In general,
After machining, no further
surface
or
remove
surface
Prior
irregularities.
several minutes
testing,
paper
lens
each
cathode
clear
to
was "cleaned" for
away
any obvious surface
The cathode surfaces were not microscopically examined
residues.
to
with
to
microscope
Electron
testing.
aluminum and copper
cathodes
examination
of the surface of the
after extensive experimentation, revealed
some deep machining marks of about 7 microns in width
fairly
smooth
protuberances
featureless
were
surface.
larger
no
than
photocathodes in this experiment
ordered
from
highest
prior
are
was
It
listed
a
estimated that surface
microns.
10
otherwise
but
in
Metals
Table I.
used
They
as
are
to lowest in their relative quantum efficiencies
(Q.E.) and were evaluated at three anode-cathode accelerating voltage of
10KV, 15KV and 20KV.
It
was
expected
that
quantum
relative
the
efficiencies
might
correspond directly to the relative ordering of the metal work functions
which are also listed
in
Table I.
general, a trend was observed
In
which can be interpreted as indicating that the lower the
function
the higher the experimental quantum efficiency.
quantum efficiency
and
work
function
ordering
can
metal s
'
work
Variations in
explained in
be
several ways.
First of all, values for work
"pure"
functions
are
generally
metals, which the experimental cathodes were not.
accuracy of any given value for
a
an
individual metal's work function.
found, since there are
many
factors
value
When
referenced
Usually a range of values is
(as delineated in the theoretical
considerations section) that influence photoemission and ultimately
34
for
Secondly, the
work function is questionable.
checking multiple references, there is never a single
for
given
the
determination of work functions.
experimental
listed
Table I,
in
were
determined
include both photoemission and
established
vacuum,
for
thermionic
temperature,
effect the ultimate assignment of a work
values
guidelines
are
comparisons unless
they
are
all
surface
techniques, which
The
conditions
contamination,
metal
or bulk emission, ect., all
function
should
and
several
emission.
surface
purity, metal crystal orientation,
these
from
Accepted values [Ref. 2]
not
determined
value.
Therefore,
used
for absolute
be
under
exactly
the same
conditions by the same experimental procedure.
Thirdly, the metal work functions spanned
nine
the
metals
between them.
Most
tested.
It
was
not
only
1
.23 volts
between
varied only several tenths of a volt
possible
under
established in this experiment to determine
if
the
accuracy conditions
work
functions
had
direct effect on the relative quantum efficiencies.
TABLE I
RELATIVE QUANTUM EFFICIENCY DETERMINATIONS
Q.E.
ORDER
CATHODE
METAL
(10 KV)
1
2
3
4
5
6
7
8
9
*
WORK
FUNCTIi
(e.v.
—
—
—
—
—
—
—
—
Zinc (Zn)
Tin (Sn)
Aluminum (Al)
Copper (Cu)
Gold (Au on Cu)
Magnesium (Mg)
Nickel (Ni)
Stainless Steel (s- -s)~
Tungsten (w)
4.24
4.50
4.20
4.54
4.92
3.68
5.01
4.77
4.69
CATHODE
METAL
(15 KV)*
CATHODE
METAL
(20 KV)
Sn
Zn
Cu
Zn
Al
Cu
Al
Au
Au
Ni
Sn
Mg
s-s
W
Incomplete data obtained for comparison, on metals not listed.
35
a
B.
LASER PULSE/PHOTOCATHODE OUTPUT CORRELATION
Much attention was directed towards perfecting an adequate method to
the power of a specific laser pulse and its resulting current
correlate
density calculated
individual
cathode.
the
photocathode
monitored
pulses,
laser
correlation)
from
at
output.
repeated
at
iterations
Four
with
laser
were
pulse
to
evaluated
performed
power
by
outside
Values
computer
a
smooth
to
outputs
deviations eliminated after each iteration.
used
power
(for
three accelerating potentials.
obtained from photographs were statistically
calculations,
photodiode
the
and the collector output, were photographed for each metal
This was
program.
Approximately 16
power
the
two standard
The remaining
points
were
calculate the average power, and from that the cathode quantum
efficiencies.
The
relationship
between
photodiode
output (which is
proportional to laser power) and the photocurrent densities was expected
to be linear for
a
obtained
Results
potential
are
particular
for
plotted
metal
emission
in
at
outputs
limited
Fig. 6,
accelerating
each
and
voltage.
at 20KV accelerating
show
the
expected
linear
relationships
As the anode-cathode accelerating voltage was decreased,
evident
from
emission
limited
electron
flow
from
limited flow.
The distinctive change to a vertical
power
photocurrent
input
to
the
as
photocathode
surface
to
slope
and
space charge
(of
the
plotted
resulting
output), reaches a maximum value and then remains constant
regardless of the input laser
Here,
became
plotted data that the high quantum efficiency metals were
transitioning
laser
it
anode-cathode
power.
accelerating
36
This is the space charge limit.
voltage
is
decreased,
maximum
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electron flow to the collector is prevented due to an
intense
build-up outside the surface of the cathode.
"cloud"
evident in the
plots
of
and
Fig. 7
plotted data suddenly becomes vertical.
This condition is
where
Fig. 8,
electron
the slope of the
When this limit is reached,
no
increase in electron collection can be expected as accelerating
further
voltage is decreased or as
laser intensity is increased.
the
if the anode-cathode accelerating potential is increased, the
electrons
can
However,
cloud
of
prevented from accumulating since photoelectrons are
be
removed across the gap immediately as they escape the surface.
Although most of the high
space charge region, to
produce
would
emission
operate
in
the
interfere
bunch and
surfaces.
with
addition,
In
photoelectrons
in
the
density data was obtained in the
current
each
photoelectrons
after
other
power
emission
is
emission limited region.
this particular LAMBDA-PHYSIK
efficiently
one
limited region where electrons do not
laser
less
most
could
required
produce
to
It was estimated that
attenuated
be
from the metal
100 times in power
output and still achieve the maximum number of electrons from the
metal
cathode at 10KV accelerating voltage.
C.
CATHODE REFLECTIVITIES
The
main
goal
of
this
nine
relative ranking for the
not a
factor
importance
this
in
unless
experiment
determination,
laser
by
a
determine an "as is"
to
metal cathodes.
nor
is
it
of
Reflectivity is
any
practical
power is of very low order and the goal is to
maximize the quantum efficiency.
energy absorbed
tested
was
However, the absolute amount of photon
particular
metal
38
cathode
will
depend
on the
ARF LASER MATERIALS DATA 10KV
CO
o
o
A
X
3
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X
X
X
T-
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6
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
CURRENT DENSITY (A/SQ CM)
Figure 7.
Nine Cathode Materials Plotted at 10KV, Showing a Linear
Slope for Emission Limited Flow Changing to a Vertical
Slope as Space Charge Limit is Reached.
39
ARF LASER MATERIALS DATA 15KV
A
A
A
CM
+
+
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CURRENT DENSITY (A/SQ CM)
Figure 8.
Selected Cathode Materials Plotted at 15KV, Showing a Linear
Slope for Emission Limited Flow Changing to a Vertical
Slope as Space Charge Limit is Reached.
40
reflectivity of the surface at the particular incident photon wavelength
was
attempt
An
involved.
made
absolute quantum
the
experimentally
using
efficiencies of the metal cathodes by
reflectivities.
determine
to
determined
Cathode reflectivity was experimentally determined from
ratio of the measured incident energy normal to the cathode surface
the
as compared to the measured energy reflected when the cathode was placed
at 45 degrees to the incident
This energy ratio was accomplished
path.
with a Gentec-200 calorimeter and had an accuracy
The
ArF
an average power of 100 KWatts/cm
was
developed at the cathode surface when determining the reflectivity.
The
laser
was
attenuated
that
so
of
±4$.
lower laser power was used to minimize pulse to pulse laser fluctuations
inherent in this LAMBDA-PHYSIK laser at
high power settings and also to
reduce the possibility of plasma formation on the
affected
have
the
reflectivity calculations.
surface
which
could
Values for experimental
reflectivity are listed in Table II.
As
a
quick
reflectivity
comparison,
theoretical
relationships
[Ref. 17] are also listed.
polarization
of
the
and
values
constants
optical
These comparison
incident
laser
beam
incidence that was normal to the surface.
calculated
values
and
using
for
assumed
known
the
metals
a
random
a laser beam angle of
Although this represents a 45
degree difference in incidence angle between the two values, the angular
difference is deemed to have
a
small effect at this photon wavelength.
Values not listed are due to lack of available
optical
parameters
for
calculations.
The
experimental
values
for reflectivity show that in all but one
case (aluminum) the reflectivities
vary
41
by
just
±6% about an average
value of 26$.
Only in the case of aluminum, which indicated a very high
reflectivity, would an "absolute" quantum efficiency ordering change its
Q.E. ranking in Table II.
This deviation is a
significant
amount
and
would make aluminum the best metal cathode tested on an absolute scale.
TABLE II
CATHODE REFLECTIVITIES
CATHODE
METAL
EXPERIMENTAL
REFLECTIVITY
CALCULATED
REFLECTIVITY
(percent)
(percent)
Zinc (Zn)
Tin (Sn)
—
27
60
Aluminum (Al)
Copper (C u)
Gold (Au on Cu)
Magnesium (Mg)
Nickel (Ni)
Stainless Steel (s-s)
Tungsten (w)
D.
—
25
92
34
25
20
25
32
25
27
21
—
35
—
65
LASER POLARIZATION
For
this
experiment
we
consider
monochromatic plane wave of frequency
v
the
,
laser
and energy
radiation
"n"),
to
incident onto
a bulk cathode metal.
Because the source of incident radiation
ArF
the
excimer
laser,
incoming
wave
assumptions
be a
are
was
an
plausible.
Normally we might expect the laser photons to be highly polarized due to
multiple internal reflections from
laser cavity.
partial
mirrors
For this particular LAMBDA-PHYSIK
,
on the ends of the
however, there are few
multiple light passes between the laser mirrors prior to the laser pulse
discharge.
Estimates
of
beam
polarization
42
indicate
80$
random
and a 20$ horizontal polarization.
polarization
beam
laser
fully polarize the
horizontally,
No attempt was made to
or vertically, to assess
It is probable that an increase in absolute quantum efficiency
effects.
with
could be obtained
laser
a
beam
that
totally
is
horizontally
polarized.
E.
CHILD- LANGMUIR FLOW
was
observed
accelerating
potential
It
production
electron
that
applied
a
anode-cathode
the
to
as
function
geometry
of
was
consistent with the theoretical Child-Langmuir flow [Ref. 13] calculated
for a planer diode arrangement.
predicted
A
charge
space
limit
on
current density was calculated using the relationship:
=
J
V
K
d
where J,
V and
K,
accelerating
2.334 X 10
—6
—1.5
"
2
2
and
volts,
in
The assumed planer diode
AV
,
are the current in A/cm
d
potential
centimeters.
1,3
,
perveance in
anode-cathode
AV
—15
separation
in
geometry uses a perveance (K) of
Perveance is identified as a measure
[Ref. 18].
,
of
the effectiveness in confining the space charge of the electron beam.
Experimental
for
data
two
cathode
copper
arrangements
theoretical space charge limit are plotted in Fig. 9»
experimental
output
was
slightly
less
predicted from the space charge limit.
a
copper
cathode
representative
of
with
the
the
first
response
in
all
and the
It shows that the
cases
over
what is
This plot is based on the use of
experimental
geometry
but
is
observed for each of the nine cathode
metals used with the second geometry.
43
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considered
factors
Additional
current densities in
experimental
the
could
that
setup
produced
have
were
higher
ion production and
secondary electron production due to electrons impacting the edge of the
These contributions were
anode restrictive aperture.
not
individually-
assessed in this experiment.
The fact that positive metal ions might be increasing current due to
secondary
electron generation after impact with the cathode surface was
considered.
observed
However, the
shapes of the output current
pulse
did not show any distortion nor any "tails" compared to the input
The
shape.
pulse
being
ions
heavier and slower would have added a
delayed pulse shape distortion if present.
secondary
In addition,
with
increased the
little
alteration
observed
aperture
from primary electron
would
increased
cause
output pulse shape and would have
in
difference
None was observed.
generated
electrons
collision with the edges of the anode
current
laser
the
in
experimental
data and the
theoretical space charge limit plotted in Fig. 9.
experimental
Excellent correlation between theoretical and
data
plotted
photoelectrons
in
occurs
Fig. 9»
impacting
the
when
electron
collector
derived
backscattering
surface
considered.
is
collector surface (center of the coax cable) was made of aluminum.
backscattering data run by a
computer
backscatter.
Lower
energy,
10KV
electrons
would
backscatter.
These
backscattered
electrons
are
to
the
output
voltage
calculated photocurrent lower than
pulse,
its
45
The
From
program [Ref. 19] at Los Alamos,
about
we know that 20KV electrons impacting aluminum would produce
contribute
from
making
actual
^5%
produce about 8%
lost
and
do
not
the experimentally
value.
If
just
the
backscattering
percentages
are
incorporated, then the theoretical and
the experimental values agree within 2%
20KV and 1% at 10KV.
at
Known
experimental errors were determined to be -2% due to measurements of the
physical geometry and ±2% in measurement of the accelerating
potential,
producing a maximum error of ±7%*
F.
CURRENT DENSITIES ACHIEVED
experimental
The
electron current density, J, was calculated using
the illuminated collector area
and
output
the
voltage pulse from the
relation:
J = Z/R (1/Area),
where J, Z, R
photocathode
and
Area
are
photocurrent
the
output in volts, 50
fl
density
amps/cm
in
2
,
impedance of the collector cable and
the final illuminated area of the collector surface respectively.
As there were two different
geometries
used in the experiment, the
magnitude of the measured current densities varied.
91
A/cm
with
2
the
of
first
This
geometry.
than
less
2
A/ cm
of
first
geometry,
field
anode-cathode gap.
to
a
89
small
KV/cm.
were obtained with the second geometry.
to 13 KV/cm, and lower laser power due to
electric
up
with
the lower electric field strengths of up
These lower values were due to
lower
values
for both emission and space charge limited data were obtained
anode-cathode gap, produced electric field strengths
Values
Maximum
required
attenuation.
The
strengths were due to a larger separation in the
However,
much larger collector area,
because
the
total
46
current
current
density is spread over a
is
comparable.
The
objective
with
the second geometry was to obtain a relative comparison
of quantum efficiency for the metal cathodes and not to maximize current
densities.
G.
RELATIVE QUANTUM EFFICIENCY DETERMINATIONS
Relative
quantum
efficiencies
related
are
to
results only when emission limited data is considered.
efficiencies experimentally determined at
experimental
the
Relative quantum
20KV accelerating voltage are
displayed in Fig. 10.
Relative quantum efficiencies determined at
accelerating
are
voltage
further listed in Table I,
ordering
the
was
shown
in
consistent
accelerating voltages tested.
pulse
in
These nine metals are
Fig. 11.
of decreasing Q. E..
order
between
metal
15KV
cathodes
In general,
at
the
Deviations were thought to be due to
three
the
to pulse power changes in the laser which led to inaccurate power
determinations.
Quantum
efficiencies
(
n
were
)
calculated using the
relationship:
6.41
where J and P are the photoelectron
power
in
Watts/cm
J
P
,
current
respectively.
The
density
in
amps/cm
and
constant 6.41 is the quantum
energy (in electron volts) for a photon with a wavelength of 193 «4 nm»
From Fig. 10, we note that the highest and lowest relative Q. E. are
separated by about
samples.
a
one
order
magnitude
of
between
the nine cathode
The experimental accuracy in determining power allows at
qualitative
validity to these orderings.
of metals as having a
higher
Q.
E.
47
best
Comparisons between groups
can be recognized, but an absolute
comparison between individual Q. E. or an absolute scaling of the
can
not
be
made.
demonstrates a better
It
Q.
would
E.
than
fairly
be
stainless
accurate
Q.
E.
to say that zinc
steel but not necessarily
correct that aluminum is much better than copper in quantum efficiency.
48
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09
CONCLUSIONS
VI.
laser
excimer
Using an ArF
generate "cold" photoelectrons, an
to
experimental investigation was conducted on nine metal
primary
The
of the experiment was to perform a relative evaluation of
goal
efficiencies
quantum
the
cathodes.
for
(Q.E.)
metal
the
cathode
materials.
Results showed, with an anode-cathode accelerating voltage of 20KV, that
for
tested,
produced
zinc
higher
photoelectron
as
stainless steel and tungsten as
emitters
than
as
quantum
group,
a
nickel,
and
magnesium,
It must be remembered that
group.
a
highest
the
Tin, aluminum, copper and gold followed
efficiency.
ranked
metals
nine
the
these are relative rankings and that the actual quantum efficiencies
metals
generally
are
efficiencies
produced
2X10 -4
to
maximum
value
X 10
1
-5
of
.
Attempts
poor.
values
for
to calculate relative quantum
nine
the
metals
that
range from
These values are lower than the generally
-2
of
agreed
obtainable with 10 e.v. photons and pure metals
10
but do closely approach the general
Q.E.
value of 10
-4
for metals.
As
calibration of the laser pulse power was not very reliable, experimental
calculation inaccuracies could account for
these
efficiencies.
Furthermore,
account, then Q.E. may be
10~2 to 10
the
apparent
reflectivities
if
disparity
are taken into
favorably to accepted values in the
compared
region for metals.
Additional results demonstrated a current density of 91 A/cm
be
achieved
Reflectivity
in
with
a
copper
determinations
cathode
indicated
51
at
2
could
29KV accelerating potential.
that
only
aluminum
had
a
substantial reflectance.
If an absolute comparison of the
nine
metals
considered, this would translate to aluminum producing the highest
were
quantum efficiency.
It would appear
that,
even
utilizing simple metals, photoelectron
sources can be manufactured that would offer large current densities and
tailored pulse shapes.
These photocathode sources
would
also
provide
the added advantage of being easy to construct and maintain.
Presently,
incorporate
accelerator.
which
a
initial
practical
of this research are being utilized to
working
electron
The first test cathode is
being
gun
into
the
constructed
PHERMEX
from
lead,
was evaluated as a more efficient photoelectron source than zinc,
as suggested by a
appear
results
that
even
parallel
denser
investigation
thesis
metals
may
present
[Ref. 20].
still
better
It would
quantum
efficiencies.
The full theory of photon interaction with metal cathode surfaces is
in general not fully understood.
on
Although extensive research
continues
individual facets of the overall processes involved in photoelectron
production, a complete theory
encompasing
all the variables associated
with the photoelectric effect has not been promulgated.
52
SUGGESTED FOLLOW UP EXPERIMENTATION
VII.
efficient
Investigations into the more
pulse
could
be
utilization
laser
the
of
Determining the minimum power requirements for
made.
electron production would
also
valuable
be
information.
entail an experimental setup to determine the laser pulse
This would
power
on
an
absolute scale.
The
two
excitation
photon
coupling of photon energy
total reflection (ATR)
at
90
degrees
efficiency may be
delivered
to
the cathode.
Using attenuated
[Ref. 21], which uses two cathode
each
to
concept may be a method to improve the
other,
possible.
an
plates
placed
increase in photoelectron quantum
However,
because
this
would involve a
change in the planer geometry arrangment, it may detrimentally alter the
electric fields between anode
and
cathode
adversely
and
affect
the
electron beam quality.
Since
photoemission
depends
on
the
photon
energy,
TI40
,
and the
electronic structure of the cathode material, an investigation to locate
a resonance in one of the more efficient metal cathodes might be made to
increase the quantum
electronic
shell
Resonances
efficiency.
structure
of
the
cathode
are associated with the
material.
frequency for the incident photon energy could be found by
incident photon frequency with a tuned dye laser.
53
A
resonant
varying
the
APPENDIX A
TABLE OF SYMBOLS AND ABBREVIATIONS
—10
A
Angstroms (10
E
Anode-cathode electric field (V/M)
V
Anode-cathode potential (Volts)
ArF
Argon Florine
1
Current (amps)
J
Current density (amps/cm
DESY
Deutsches Elektronen Synchrotron
e.v.
Electron Volt
v
Frequency of photon (Hertz)
Hz
Hertz (sec
KrF
Krypton Florine
$
Metal work function (electron Volts)
M
Meter
K
Perveance (amps/Volt"
Z
Photocathode output (Volts)
4m>
Photon energy (electron Volts)
h
Planks constant (joule-sec)
P
Power per pulse (Watts/cm
PHERMEX
Pulsed High-Energy Radiographic Machine Emitting X-Rays
n
Quantum efficiency (percent)
R
Resistance (Ohms)
M)
)
)
)
)
54
APPENDIX B
LIST OF EQUIPMENT
Osciloscopes:
Techtronics R-7103, 1GHZ
Vertical Base 7A29
Horizontal Time Base 7B15
DC Power Supply:
Laser:
Hipotronics Power
LAMBDA PHYSIK EMG 150T
Laser Variable Attenuator:
High Vacuum Pump:
Photodiode
:
Calorimeter:
Photodiode:
NRC Model 935-10
CTI Cryogenics, Cryo-Torr-8
Hamamatsutu R1193U-04
Gentec-200
EG and G Electronics FND-100Q
Photodiode Power Supply:
Boxcar Averager:
BNC Portanim Model AP-3
EG and G Princeton Applied Electronics Model 162
55
APPENDIX C
IMPROVED ANODE/COLLECTOR APPARATUS
Anode-Cathode Gap:
1
.859 cm.
Cathode Spot Size:
1
.797 cm
2
.
Restriction Aperture Area on Anode Face:
Total Collector Area on the 50a
2
0.744 cm
Cable Face:
.
1.552 cm'
Incident
Photons
Cathode
Spot
Anode Restrictive
Aperture
50n
Collector Cable
Anode-Cathode Gap.
Figure 12.
Improved Experimental Anode and Collector Geometry.
56
LIST OF REFERENCES
1.
Reifenberger J. C,
Lee, M.
and
Photocurrent from a Tungsten Field
70, pp. 114-130, 25 April 1977.
,
"Periodic Field Dependent
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,
2.
Sommer, A. H., Photoemissive Materials pp.
15
57-174, Robert E. Krieger Publishing Co., 1980.
3.
Burroughs, E. G.,
"External
Field Enhanced Photoemission in
Silver-Cesium Oxygen Photocathodes" Applied Optics
v. 8, no. 2,
February
261-265,
1969.
pp.
,
,
4.
»
19,
21-26,
33,
,
Oettinger, P. E., Pugh, E. R.,
Lee, C. H.,
Klinkowsstein, R.,
Jacob, J. H.,
Fraser, J. S.,
and
Sheffield, R. L.,
"Electron
A/cm^
from a Pulsed-Laser Irradiated
Emission of over 200
Photocathode" , IEEE Transcripts on Nuclear Science
v.
32,
pp.
October
1985.
3045-3047,
,
5.
Walldeim, L.,
"Surface
Photoelectric Effect for Thin Metal
Overlays", Physics Letters , v. 54, pp. 943-946, 4 March 1985.
6.
Bergland, C. N. and Spicer, W. E., "Photoemission Studies of Copper
and Silver: Theory", Physics Review
v.
136, pp. 1 030-1 044, 16
November 1964.
,
7.
Bergland, C. N. and Spicer, W. E., "Photoemission Studies of Copper
and Silver: Experiment", Physics Review , v. 136, pp. 1044-1064, 16
November 1964.
8.
Seitz, F., The Modern Theory of Solids , p. 139, McGraw
Co., 1940.
9.
Feuerbacher, B., Fitton, B., and Willis, R. F., Photoemission and
the Electronic Properties of Surfaces ,
12, John Wiley and
pp. 3,
Sons, Inc., 1978.
10.
Hill
Book
Cardona, M. and Ley, L., Topics in Applied Physics:
Photoemission
in Solids I
v. 26, pp. 2, 21, 57, Springer-Verlag Co., 1978.
,
11.
Gudat, W. and Kunz, C,
"Close Similarity between Photoelectric
Yield and Photoabsorption Spectra
in the Soft-X-Ray Range",
Physical Review Letters , v. 29, p. 170, 17 July 1972.
12.
Barton, J. J.,
with
"Direct Surface Structure Determination
Photoelectric Diffraction", Physics Letters , v. 51, pp. 272-275, 25
July 1983.
57
13.
Tredgold, R. H., Space Charge
Elsevier Publishing Co., 1966.
14.
Naval Research Laboratory Memorandum Report 5853, Effects
Cathode Surface Roughness on the Quality of Electron Beams
Y. Y. Lau, p. 4, 12 September 1986.
Conduction
in
Solids
,
pp. 70-71,
of
by
,
15.
Hasted, J. B. and Phil, D., Physics of Atomic Collisions
p. 171, Elsevier Publishing Co., 1972.
16.
Downey, S. W., Builta, L. A., Moir, D. C,
Ringler, T. J., and
Saunders, J. D., "Simple Laser-Driven, Metal-Photocathode Electron
Source", Applied Physics Letters , v. 49, pp. 91 1-91 3
13 October
1986.
,
2nd ed.,
>
17.
Academic Press Handbook Series; Handbook of Optical
Palik, E. D.,
Constants of Solids pp. 31-365, Academic Press, Inc., 1985.
,
18.
Lawson, J. D., The Physics of Charged
Academic Press, Inc., 1958.
19.
Sandia Laboratory Report SAND 84-0573* IIS; The Integrated TIGER
Series Coupled Electron/Photon Monte Carlo Transport Codes by
J. A. Halbeib and T. A. Mehlhorn, 1984.
Particle Beams , pp. 172-177,
,
20.
Saunders, J. D., Simple Laser-Driven, Metal Photocathodes as High
Current Electron Sources Master of Science Thesis, p. 29, Naval
Postgraduate School, Monterey, California, December 1986.
,
21.
Rudolf, H. W. and Steinmann, W., "Two Photon Photoelectric Effect
in the Surface Plasma Resonance of Aluminum", Physics Letters v.
61, pp. 471-472, 27 June 1977.
,
58
BIBLIOGRAPHY
Kittel, C, Introduction to Solid State Physics
Sons, 1956.
Koller, L. R., The Physics of Electron Tubes
Co., 1937.
Miller, R. B.,
Particle Beams
,
An Introduction to
Plenum Press, 1982.
the
,
,
2nd ed., John Wiley and
2nd ed., McGraw-Hill
Physics
Intense
of
Book
Charged
Nagy, G. A. and Szilagyi, M., Introduction to the Theory of Space Charge
Wiley Publishing, 1974.
,
Optics
Pierce, J. R., Theory
Nostrand Co.
1954.
and
Design
of
Electron Beams
,
2nd ed., D. Van
,
and Shreider, E. Y.,
Zaidel, A. N.
Humphrey Science Publishers, 1970.
59
Vacuum
Ultraviolet
Spectroscopy
,
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2.
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Naval Postgraduate School
Monterey, California 93943-5002
2
3.
Physics Library, Code 61
Department of Physics
Naval Postgraduate School
Monterey, California 93943
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4.
Professor F.R. Buskirk, Code 61 Bs
Department of Physics
Naval Postgraduate School
Monterey, California 93943
2
5.
Professor J.R. Neighbours, Code 61 Nb
Department of Physics
Naval Postgraduate School
Monterey, California 93943
2
6.
Dr. D. C. Moir
2
1
Hydrodynamics Group (M-4), P940
Los Alamos National Laboratory
Los Alamos, New Mexico 87545
7.
LCDR Thomas J. Ringler, USN
337 Tuxford Place
Fayetteville, North Carolina 28303
3
60
18
7 6
7-1
ft
?HOOL
MONTEREY, CALIFORNIA 9S943-B002
me
from Basic
pulsed
ehnrt
„
zing a snoru v
excimer laser.
ju-e
Thesis
R567
c.l
Ririgler
Photocurrent generation
from Basic metals, utilizing a short pulsed ArF
excimer laser.