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How to explore a system?
Photons
Electrons
Atoms
Electrons
Photons
Atoms
How to explore a system?
Photons
Electrons Electrons
With differentkinetic energies
Detect the charged particles fora given energy range with good energy (and space) resolution
Electron Energy Analyzers
Retarding Field Analyzer (RFA)Cylindrical Mirror Analyzer (CMA)Hemispherical Analyzer (HA)
Separate the electrons with a defined energy band
Electron Energy Analyzers
Electron Energy (eV)
N(E)
E0E0 + ΔE
Energy distribution curve: response of the system
Retarding Field Analyzer (RFA)
V0
Retarding Field Analyzer (RFA)
N(E)
V0 = Retarding potential
E0 = eV0 = Pass energy
Current to screen
0
)()()( 0 EEdENEI
pE
EEdENEI
0
)()()( 0
To obtain N(E) one has to differentiate
V0
Electron Energy (eV)
N(E)
E0E0 + ΔE
V V V k sin t
...!4!3
!243
2
VVI
VVI
VVI
VVIVIVVI
Retarding Field Analyzer (RFA)
Modulation
VV 25.0
tkVIkVI 2cos...
484
42
tk
VIkVI sin...
83
VIVVI 0
First harmonic
Second harmonic
N(E)
dN(E)/dE
RFA: poor sensitivity and energy resolutionNo angular resolution
Electrostatic deflection analyzers
Energy band pass
Cylindrical Mirror Analyzer (CMA)Hemispherical Analyzer (HA)
Dispersing field
Deflection is functionof electron energy
v1
v1
v2
-
+
Electrons have angular spread around the entrance direction
Electrons with same v willbe deflected by different amounts
Degradation of energy resolution
Concept
Cylindrical Mirror Analyzer (CMA)
202
1mveVE e
1
20 log
r
r
V
VK e
V
rr
eKE
1
2
0
log
V=deflecting voltage between cylinders
e- energy
consider an e- arriving at an angle 0 with
e- energy inside cylinder
)(reVU work of the e.m. field on e-
10
ln2
)(r
r
L
QrV
V
rr
LCVQ
1
2
0
ln
2
V
rr
L
r
r
L
ereVU
1
2
0
10 ln
2ln
2)(
10
1
1
2
loglog
logr
rU
r
r
rr
eVU
00
1
2
1
2
0
loglogK
E
K
eV
V
V
rr
eV
rr
eVU e
e
e
e- cross the inner cylinder through a slitexperience the field –V of the outer cylindergo to second slit and and arrive in F
Cylindrical Mirror Analyzer (CMA)
Electrons with same E willbe deflected by different amountsdepending on the entrance angle
The trajectory for which the e- is focussedis a solution of the equation of motion (cyl. coord.)
The maximum deflection depends on the entrance angle,
00 r
Urm
02
0 sin21
Km err
and shows that K0 depends also on
focussing condition
V
rr
eKE
1
2
0
log
1
20 log
r
r
V
VK e
00 K
EU
A single focussing length L correspondto different acceptance angles (see curve (c))
High sensitivity with one pass energy
The numerical solution shows that for a single K0 there are two values of entrance angle
This means that in general there are two focussing distances
For K0 = 1.31 the twofocal distances merge into one
Cylindrical Mirror Analyzer (CMA)
L = 6.130 r1
Cylindrical Mirror Analyzer (CMA)
The emission angle determines three main factors
(a) source-image distance on the common cylinders axis (L)
(b) the deflecting voltage for particles with energy E
(c) the required ratio between the cylinders radii
For 0= 42°18.5' the first spherical aberration term = 0
L
For small and small E, the shift in the axis crossing point is
(Taylor series)
)(3.10)(4.156.5 13
11
EE
rrEE
rL
One looks for L 0
3)(75.2 EE
Base resolution
3)(5.5
EE
E
EB
Cylindrical Mirror Analyzer (CMA)
r1 = inner cylinder
Neglecting the product
0.0873rad 5
EE
xE
E
B
B
0037,0
0037,0.087305.5 3
Transmission = fraction of space in front of the sample intercepted by the analyser
analyser transmission
346.1sin2T
12.0T 0037.0 5
E
Efor B
3255.2 TE
EB
Cylindrical Mirror Analyzer (CMA)
For a fixed slit, T does not depend on energy
3)(5.5 E
EB
For a given slit, T does not depend on energy while EB E
peak area (T x EB)
3
1
)(45.518.0
W
rE
EB
Electron Energy (eV)
N(E)
E0E0 + ΔEBPeak area N(E)xE
Cylindrical Mirror Analyzer (CMA)
So the energy resolution is not constant with E
The spectrum contains the intensity-energy response function of the analyser
but (T x EB) E
The image of the source can be reduced by inserting a slit before the focus that reduces the coefficientsof 3 by a factor of 4
Finite source + ring slit of width W and radius r1
006.0.08730100
318.0
mm 3 w mm; 100r0.0873rad; 5
3
1
xEE
Two spherical electrodes
Concentric Hemispherical Analyzer (CHA)
1/r electrostatic potential
Electrons are injected with energy eV0 at slit S
in the point corresponding to radius R0
0
22
0
110 22 R
RV
RR
VV
The condition to allow e- to describe the central orbit is (point source)
2
002
1
001
23
23
RR
VV
RR
VV
2
1
1
2012 R
RRR
VVV 120 VVkV
Focussing condition in F
For R1=115 R2=185 mm K = 1.013
221
0
RRR
e- forming angle with tangential direction
Concentric Hemispherical Analyzer (CHA)
200 22 R
EE
RR
The resolution is mainly determined bythe central hemispherical radius
R0=150 mm W1 = W2 = 3 mm
e- with energy E with respect to E0
Considering two slits of width W1 and W2
Base resolution
rad096.05
2
0
21 )(2
RWW
EEB
Shift in the radial position
R0=150 mm W1 = W2 = 1 mm 015.0EE
028.0EE
Worse than CMA (lower transmission and resolution)???
Transmission of the analyser
02.0E
EB
0
22RW
T
Concentric Hemispherical Analyzer (CHA)
If the sample is at the position of the slit W1, we assume W1 = 0 and neglect so the angular acceptance in the plane depends on the slit W2
We also have to consider the angular acceptance in the plane perpendicular to the screen ()
2
22
0
21
222
T
RWW
EEB
Transmission
150mmR
) 1(57
0 rad
In analogy to the CMA
0
22
2RW
1.00.0421
T
mm 3W2 mm 1W2
06.00.0421
T
007.0E
EB
2sin2 T
Concentric Hemispherical Analyzer (CHA)
Problem: sample cannot be at position of slit 1
solutionsReduce the analyzer
angle to 150°Use lenses to focus
beam at slit 1
r = source radius = cone semiangle of sourceE = e- energy at the sourcerp = source image at entrance slit W1 = cone seminagle of imageEp = e- energy at the image
pp ErEr Helmoltz-Lagrange equation
Lens magnificationr
rM p
pEE
R Retarding ratio
21
MR = cone semiangle of sourcedefined by the lens
Concentric Hemispherical Analyzer (CHA)What is defining the transmission of the analyser?
Consider the cone with semiangle
1. The lens defines the transmission
2. The lens defines the transmission in and the spectrometer in
3. The spectrometer defines the transmission
21
21
;
MRMR
RMRM ;
RMRM ;
%027.0EE
The CHA is designedto accept of about 4-5° (similar to = 5)
21
MR
0
22RW
T
Electrostatic lenses
Optical ray refraction Electron refraction
e speed
Refractionindex
electrostaticpotential
The potential changes abruptly at the interface:only the perpendicular component of the momentum changes
2221
21 2
121
eVmveVmv Snell’s law: n1 sin1 = n2 sin2
2211 sinsin mvmv
2211 sinsin VV
Electrostatic lenses
For real lenses there are no abrupt changes in the potential, as shown in the figure
But one can assume the asymptotic behavior of the electron trajectories to make use of the lens equations
Equipotential lines
1
2
1
2
M
rr
M
e path
Transverse magnification
Angular magnification
2
1
2
1
2
1
ff
rr
M
fq
pf
M
2
1
2
1
2
1
VV
nn
ff
MM Helmoltz-Lagrangeequation
Conservation of brightness
Electron lenses formed using metallic apertures.
Electrostatic lenses
Lenses has the effect to change the kinetic energy of the beam
Focussing Defocussing Focussing
Electrostatic deflection analyzers
CHA
Energy resolution
nnB CBAsE
E
0
E0 = pass energyΔEB = Emin-Emax transmitted
s = slit width, angular apertures
A B C n
Cylindrical mirror 2.2/l 5.55 0 3
Cylindrical deflector 127°
2/r 4/3 1 2
Spherical deflector 180° 1/r 1 0 2
CMA
Electrostatic deflection analyzersGeometry of the acceptance slit is very different
CHA CMA5°6°
42.3°
Small signalCompatible with simple electrostatic
aperture and tube lensesLong focal distance
Radius 100 - 150 mmRes. Power about 1000-5000
Working distance about 25-50 mm
Large signalNon compatible with simple electrostatic
aperture and tube lensesShort focal distance
Cyl diam 100 - 150 mmRes. Power about 200
Working distance about 5 mm
meV 2 5000
0 E
EB eV 0.1 200
0 E
EB
Electrostatic deflection analyzers
Detection mode
Single
Scan over voltages acquiringcounts at each energy
Scan over voltages acquiringthe position and therefore the energy
of the electrons with different trajectories
Multi
E1 E2 E3E(V)
Electrostatic deflection analyzers
Detection mode
Scan over voltages acquiringthe position and therefore the energy
of the e- with different trajectories
Multi ChanneltronChannelplates+ ccd camera
Electrostatic deflection analyzers
Mode of operation
No pre-retarding potential
Vary E0 with Vscan
EB is increasing with energy
Pre-retarding potential
Vary the pre-retarding potentialand not E0
EB is constant
nn CBAsEE
0
nn CBAsEE
0
0EEE Rk
Hemispherical Analyzer ofElectron Kinetic Energy
Lay-out of an Electron Spectroscopy Experiment Based onto a Double-Pass Cylindrical Mirror Analyzer
Hemispherical Analyzer of Electron Kinetic Energy with Entrance Optics Designed for Lateral Resolution
Electron Source and Energy MonochromatorandElectron Kinetic Energy Analyzer
HREELS Apparatus
Electron Multiplier
Detection efficiency 80 %
Gain 108
Microchannel plates
Thin Si-Pb oxide glass wafers
Channel walls act as electron multipliers
Channel density 105 cm-1
Detection efficiency 80 %
Gain 105 - 108