Liberation of electron using a photon photoelectric effect/photoemission Science By The Slice Xiaoshan Xu July 22, 2016
Liberation of electron using a photon
photoelectric effect/photoemission
Science By The Slice
Xiaoshan Xu
July 22, 2016
PHOTOELECTRIC EFFECT
The complete absorption of a photon by a solid with the emission of an
electron. (Handbook of chemistry and physics, David Lide, 87th edition,
Boca Raton, FL : CRC Press, 2006)
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Light (πΌ, π ) Electron (πΈπ , πΌπΈ)
Metal: Li, Na, K
Photoelectric effect
https://phet.colorado.edu/en/simulation/photoelectric
πΌ: light intensity
π: light frequency
πΈπ: kinetic energy of
emitted electron
πΌπΈ: photoelectric
current
Light (πΌ, π )
Electron (πΈπ , πΌπΈ)
Properties of the photoelectric effect
πΌπΈ β πΌ (The intensity of light is
proportional to the induced photo
electric current.)
There is an threshold for the light
frequency to generate photocurrent.
The maximum kinetic energy
increases with the light frequencyhttp://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html
Na
Quantization of light: photon
β’ The energy of the light propagates in discrete wave packets (photons):
πΈπ = βπ, β is the Plank constant
β’ Conservation of energy:
πΈππππ₯ = πΈπ β π = βπ β π
Vacuum level
Metal
π:π€πππ ππ’πππ‘πππ
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Light (πΌ, π )
Measurement of Plank constant
Robert Millikan
β = 6.6 Γ 10β34 Js
http://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html
πΈππππ₯ = πΈπ β π = βπ β π
Work functions of metals (eV)Ag 4.26 β 4.74 Al 4.06 β 4.26 As 3.75
Au 5.1 β 5.47 B ~4.45 Ba 2.52 β 2.7
Be 4.98 Bi 4.31 C ~5
Ca 2.87 Cd 4.08 Ce 2.9
Co 5 Cr 4.5 Cs 2.1
Cu 4.53 β 5.10 Eu 2.5 Fe: 4.67 β 4.81
Ga 4.32 Gd 2.90 Hf 3.9
Hg 4.475 In 4.09 Ir 5.00 β 5.67
K 2.29 La 3.5 Li 2.9
Lu ~3.3 Mg 3.66 Mn 4.1
Mo 4.36 β 4.95 Na 2.36 Nb 3.95 β 4.87
Nd 3.2 Ni 5.04 β 5.35 Os 5.93
Pb 4.25 Pd 5.22 β 5.6 Pt 5.12 β 5.93
Rb 2.261 Re 4.72 Rh 4.98
Ru 4.71 Sb 4.55 β 4.7 Sc 3.5
Se 5.9 Si 4.60 β 4.85 Sm 2.7
Sn 4.42 Sr ~2.59 Ta 4.00 β 4.80
Tb 3.00 Te 4.95 Th 3.4
Ti 4.33 Tl ~3.84 U 3.63 β 3.90
V 4.3 W 4.32 β 5.22 Y 3.1
Yb 2.60 [13] Zn 3.63 β 4.9 Zr 4.05
Visible light:
1.6-3.1 eV
Light: Wave and particle
β’ KleinβGordon equation (Relativistic)
ββ2π2
ππ‘2π = ββ2π2π»2 +π2π4 π = πΈ2
m=0 for photon:
ββ2π2
ππ‘2π = ββ2π2π»2π = πΈ2
π = Aeπ(2πππ₯βππ‘)
, πΈ = βπ = βπ
π β‘ 2ππ, β β‘β
2π
Light Induced Quantum Transitions
β’ Transition matrix
π»12 = π1 πΈ0πβπππ‘ π2
= π10 πΈ0 π2
0 πβπ(πΈ2βπΈ1
ββπ)π‘
Transition probability
π π‘ β ΰΆ±0
π‘
π»12ππ‘
2
β ΰΆ±0
π‘
πβπ
πΈ2βπΈ1β
βπ π‘ππ‘
2
β π10 πΈ0 π2
0 2πΏ(πΈ2 β πΈ1
ββ π)
Light eβπππ‘
π1 = π10πβπ
πΈ1βπ‘
π2 = π20πβπ
πΈ2βπ‘
Application of photoelectric effect
β’ Photoelectric cell for light sensing
β’ Photomultiplier tube for single-photon
detection
Excitation in an insulator (semiconductor)
Vacuum level
Metal
π:π€πππ ππ’πππ‘πππ
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Light (πΌ, π )
Vacuum level
Insulator
πΈπ: ππππ πππ
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Light (πΌ, π )
πππππππ ππππ
πΆππππ’ππ‘πππ ππππ Si
Si
Si
Si
Si
Si
Si
Si
Si
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Light induced electron-hole pair
Photovotaic effect
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---
π₯
πππππ
Electric potential
Depletion zone
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+-
Photodetector: charge coupled device (CCD)
Gate
SiO2
p-Si
ππΊ
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PHOTOEMISSION
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Light (πΌ, π ) Electron (πΈπ , πΌπΈ)
Metal and insulators
Photoemission
Binding energy: πΈπ΅ = βπ β πΈπBinding energy of valence electron
< 100 eV ultraviolet light
Binding energy of core electron:
> keV x-ray
K, n=1
L, n=2M, n=3N, n=4Valence
Vacuum
https://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy
Ultraviolet photoelectron spectroscopy (UPS)
http://en.wikipedia.org/wiki/File:ARPESgeneral.png
Fix photon energy
Measure kinetic energy
β’ Surface sensitive due to small kinetic energy
β’ Must be in UHV (typically <10-9 Torr)
X-ray photoelectron spectroscopy (XPS)
J. Phys.: Condens. Matter 27 (2015) 175004 .
h-LuFeO3
Auger effect (Secondary photoemission)
β’ 1st , x-ray excites electron to
conduction band and
generate a core hole
β’ 2nd , electron recombine
with the hole; the emitted
energy expels another
electronK, n=1
L, n=2M, n=3N, n=4Valence
Vacuum
K, n=1
L, n=2M, n=3N, n=4Valence
Vacuum
X-ray absorption Electron-hole recombination and
emission of another electron
ABSORPTION SPECTROSCOPY
π1 = π10πβπ
πΈ1βπ‘
π2 = π20πβπ
πΈ2βπ‘ π π‘ β π1
0 πΈ0 π20
2πΏ(πΈ2 β πΈ1
ββ π)
Optical absorption spectroscopy
1.2 1.4 1.6 1.8 2.0 2.20
100
200
300
400
500
600
700
(
cm-1
)
Energy (eV)
300 K
4 K
6A
1g
4T
1g
6A
1g
4T
2g
PHYSICAL REVIEW B 79, 134425 2009
BiFeO3
t2g
eg
Fe3+ 3d5
Spin Parity
Initial 5/2 Even
Final 3/2 Even6A1g
4T1g
Color of BiFeO3 comes from the absorption
X-ray absorption spectroscopy (XAS)
Transmission
Incident x-ray
Secondary
photoemission
(surface sensitive,
a few nm)Fluorescence
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β’ Transmission is normally
difficult to measure,
especially for thin films
β’ Fluorescence spectra is
often distorted by self
absorption
β’ Photoemission is often
used for its surface
sensitivity, especially for
thin films.
XAS using synchrotron x-ray
β’ Unlike XPS, UPS, the x-ray
energy is canned, which
requires synchrotron source
β’ UHV is also necessary
X-ray
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XAS at Canadian Light Source
Electronic structures by XAS
LuFeO3
hexagonal
orthorhombic
hexagonal
orthorhombic
X-ray magnetic circular dichroism (XMCD)
Stohr, Siegmann, Magnetism From Fundamentals to Nanoscale Dynamics, Springer, 2006.
Photoemission electron microscopy (PEEM)
Fe magnetic domains
Magnetic domains
hexagonal
orthorhombic
hexagonal
orthorhombic
Structural
domains
Thank you for your attention !