How deep into a solid sample does XPS (ESCA) probe?
Incident x-rays
Emitted electrons
How far can the x-ray beam probe?
From what depth can electrons escape
without loss of energy?
Several microns (mm)
A few nanometres (nm)
Photoelectron spectroscopy requires ultrahigh vacuum
Escape Depth
I= I0 e -ax
x
a is the inelastic
scattering cross-section
(m-1)
a = 1/l
l is the escape depth
I= I0 e -x/l
When x = l,
Increasing a
(or decreasing l)
(e = 2.718)
The escape depth, l, is that depth at
which the probability of escape is
reduced to 1/e (~36.8%) compared to
emission from an atom at the surface
(x=0)
l
I = I0 e -1
I
x
I = I0 e - l . 1
I = I0
e
15Å
10Å
5Å
0Å
1
10
100
1000 eV
Kinetic energy of e
I = I0 exp(-x/l)
For the clean metal surface
x
q
Imet = I0 0∫∞exp(-x/l) dx = l I0
d/cos q
d
For the metal signal in the presence of an
overlayer of thickness d
I’met = I0 d/cos q∫∞exp(-x/l) dx = l I0 exp(-d/lcos q)
I’met /Imet = exp(-d/lcos q)
ln(Imet /I’met) = d/lcos q
Be XPS
Angle-dependent XPS of a Silicon Wafer
with a Native Oxide
=0o
=80o
cos
80o
0o
http://www.phi.com/publications/
9603.htm
XPS Imaging
Cr
O
Al
A scanned X-ray image of 100 mm x 100 mm
Cr pads on Al2O3
Application Note Physical
Electronics.
Auger
Auger
Analysis of a Multi-Layer
Paint Cross-Section
Line scan showing diffusion of Cl
into surrounding layers.
NB All elements can be
detected except H
Atomic Concentration (%) (not based on line scan)
Analysis Area
C
O
N
Cl
Si
Al
Substrate
100.0 ----------Adhesion Layer 90.0
----10.0
----Base Coat
72.0
16.4 3.5
3.3
2.6
2.2
Clear Coat
70.6
22.2 7.2 -------
X-ray spectroscopy is primarily concerned with the behaviour of CORE level electrons
Vacuum
level
LUMO
Valence levels
Energy
HOMO
hn (x-ray)
Emission spectroscopy
Core levels
“Core hole”
Isolated molecule
(ionised)
“Fluorescence”
Created by emission of electron
following bombardment ofsample by
X-rays (XPS), high energy electrons,
protons, ions etc
http://www.amptek.com
Conservation of angular momentum
Photon has one
unit of angular
momentum
Electron orbital
angular momentum
Allowed
transitions
l
s
p
d
f
etc
0
1
2
3
s ---> p
p ---> s
d ---> p
f ---> d
etc
p ---> d
d ---> f
f ---> g
Dl = ±1
XRF analysis of St Gaudens US $20 gold coin with 90% Au and 10% Cu
http://www.amptek.com/5x5x500.html
Energy dispersive
X-ray fluorescence
4
8
keV
12
http://www.thebritishmuseum.ac.uk/science/techniques/sr-tech-xrf.html#
XRF mapping can reveal hidden paintings
Portrait of a Woman (1876-1880)
Edgar Degas
Scientific Reports 6, 29594 (2016)
High-definition 31.6 megapixel X-ray fluorescence
elemental maps of Portrait of a Woman.
False colour reconstruction
of Degas’ hidden portrait
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.