Experimental Physics EP3
Atoms and Molecules
– Photoelectric effect –
energy quantization, photons
http://uni-leipzig.de/~valiu/
Experimental Physics III - Concept of atoms
1
Electric charges
British natural philosopher Richard Laming first hypothesized
the concept of an indivisible quantity of electric charge to
explain the chemical properties of atoms in 1838.
Irish physicist George Johnstone Stoney named this charge
'electron' in 1891.
J. J. Thomson and his team of British physicists identified it as
a particle in 1897.
The oil drop experiment was an experiment
performed by Robert A. Millikan and Harvey
Fletcher in 1909 to measure the elementary
electric charge (the charge of the electron).
Experimental Physics III - Concept of atoms
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Millikan’s experiment
r
Fg = (m − ρ airV p )g
r
r
F f = −6πηRv
vt
r
Ff
r
Fg
(
m − ρ V )g
=
l
p
6πηR
4 3
V p = πR
3
9ηvt
R =
2 g (ρ − ρ air )
2
FE = qE
4 2

=  m − ρ air πR  g
3


FE − Fg = F f
Experimental Physics III - Concept of atoms
3
Light-matter interaction
Light absorption and radiation is not
a continuous process, but is quantized,
i.e. occurs in some finite portions or quanta.
E = hν = hω
Planck: Light propagation is described by the classical
Maxwell equations, the quantization occurs only during
the interaction events with matter.
Einstein: Light may considered as a collection of particles,
Energies of which are given by the Planck’s equation.
Relativistic mechanics:
r2
(E / c ) − p = (m0c )2
2
mphoton=0
• Number of photons is not conserved
• Energy and momentum conservation
E = pc
Free electron can scatter photon, but
cannot absorb or emit photon.
Experimental Physics III - Concept of atoms
m=
m0
1 − (v / c )
2
Planck's constant =
6.62606957 × 10-34 m2 kg / s
4
Photoelectric effect
Hertz helped establish the photoelectric effect (which was later explained by Albert
Einstein) when he noticed that a charged object loses its charge more readily when
illuminated by ultraviolet light. In 1887, he made observations of the photoelectric
effect and of the production and reception of electromagnetic (EM) waves, published in
the journal Annalen der Physik.
Upon irradiating negatively charged metallic objects by
an ultraviolet light the object looses its negative electric charge.
Neutral objects can be charged to a potential of a few volts.
From 1888 to 1891: Stoletov invented a new experimental setup
which was more suitable for a quantitative analysis of photoeffect.
Using this setup, he discovered the direct proportionality between
the intensity of light and the induced photo electric current (the
first law of photoeffect or Stoletov's law). Based on this effect, he
built the first solar cell.
Soon after the discovery of electron in 1897, it has been shown that the ratio e/m for the particles forming
photoelectric current is the same as for those forming “cathode rays”, i.e. they are electrons..
Experimental Physics III - Concept of atoms
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Photoelectric effect
-
-
- -
-
-
-
-
+
+
+
+
+
+
+
+
ϕ
-
A = eϕ
The work function required to
remove an electron to a close
vicinity above the surface.
Experimental Physics III - Concept of atoms
Element
Work Function (eV)
Calcium
2.9
Iron
4.5
Platinum
6.35
Zinc
4.3
6
Response time
L=1m
Emax
P = 100 W
P
=
σt
2
4πL
Maximal energy obtained by an atom.
Ee < Emax
To estimate assume that
To remove an electron from the metal
 M 

σ ≈ d ≈ 
 ρN A 
2
Emax > A
2/3
≈ 6 × 10−20 m 2
A 4πL2
t>
σ P
Ee = Emax
≈ 1.7 s
Zinc:
M = 65 g/mol;
ρ = 7 g/cm3;
But the photoelectric effect is non-inertial!
Experimental Physics III - Concept of atoms
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Photons
Photons are non-elastically colliding with electrons and get absorbed.
Free electrons in metals are not free (more precisely this is not a two-body problem)!
Photon may transfer its energy to an electron and its momentum to the whole material.
The maximal energy acquired by an emitted electron:
Emax =
1
2
mυ max
= hν − A
2
• Emax is a linear function of the incident light frequency and does not depend on its intensity.
• Light intensity determines only a number of the emitted electrons.
• The exist a lower bond for frequency, below which no photo-effect is observed.
A
A
B
B
1
2
mυ max
= −e(VA + Vc )
2
0 = − e(VB + Vc )
1
2
mυ max
= e(VB − VA )
2
Experimental Physics III - Concept of atoms
8
To remember!
Light has properties of both particles and electromagnetic
waves – wave-particle duality.
These particles are called photons and their energy,
according to Planck, is E = hv. Photons are massless, but they
have a momentum which is related to their energy E = pc.
In photoelectric effect, photons are absorbed
by electrons increasing thus their kinetic energy.
If the kinetic energy is higher than the work
function, photoelectric current is observed.
Classical electrodynamics cannot explain
the occurrence of the photoelectric effect.
Experimental Physics III - Concept of atoms
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