Electrons Inside The Atom

1
Electrons Inside The Atom

• Ionization and Excitation
• Franck-Hertz Experiment
• Energy Levels and Spectra
• Photoelectric Effect

2
Ionization and Excitation

• Ionization is the process of creating charged
  atoms.
• Excitation is the process whereby atoms absorb
  energy without ionization. The orbital electrons
  are raised to the next energy level.

3
Franck-Hertz Experiment (1)
http//phys.educ.ksu.edu/vqm/html/FranckHertz.html

• The diagram below shows the apparatus used in the
  experiment.

4
Franck-Hertz Experiment (2)

• The circuit diagram shows the basis of the
  experiment.

e
5
Franck-Hertz Experiment (3)

• The experimental results are shown below.

6
Franck-Hertz Experiment (4)

• From the graph, it can be obtained that
• 1. At the beginning the current increases with
  the accelerating potential difference.
• 2. Up to a critical value (4.9 V for mercury) of
  the accelerating p.d., there was a sudden drop
  in the current.
• 3. Then the current increases again and another
  sudden drop occurred at 9.8 V.
• 4. The current rises and drops again periodically
  as the accelerating p.d increases.
• 5. The peaks of the graph have equal spacing.
• It was also noticed that light was emitted by the
  mercury during the sudden drop in current.

7
Franck-Hertz Experiment (5)

• Interpretation of the experimental results
• Usually, the electrons collide with the mercury
  atom elastically so there is no loss in kinetic
  energy.
• For the critical value the electrons lost all
  their kinetic energy on hitting the mercury atoms
  due to inelastic collision and the mercury atoms
  are then excited. When they do, those electrons
  do not reach the anode and the current drops.
• Further increase in the accelerating p.d. leads
  to an increased current, until another is
  reached.
• Each peak represents an inelastic collision with
  energy exchange between the free electrons and
  the mercury atoms.

8
Discrete Energy Levels

• Some of the energy levels of mercury and the
  wavelengths that can be emitted.

9
Photoelectric Effect

• The photoelectric effect is the emission of
  electrons when light strikes a surface.
• The emitted electrons are called photoelectrons.
• The photoelectrons absorb energy from the
  incident radiation and thus able to overcome the
  attraction of positive charges.

10
A Simple Demonstration of Photoelectric Emission
(1)

• Ultraviolet radiation is directed onto a clean
  zinc plate placed on the cap of a gold-leaf
  electroscope as shown below.

11
A Simple Demonstration of Photoelectric Emission
(2)

• Firstly the electroscope is given a negative
  charge so the leaf rises.
• When ultraviolet radiation is allowed to fall on
  the zinc plate, the leaf gradually falls because
  the electroscope loses charge.
• Free electrons in the zinc plate gain sufficient
  energy to leave the plate.

12
A Simple Demonstration of Photoelectric Emission
(3)

• If the electroscope is made positive to start
  with, then the leaf will not fall because no loss
  of charge takes place.
• The free electrons in the zinc plate need much
  more energy to leave the zinc plate because it is
  charged positively and the radiation cannot
  supply enough energy.

13
Investigations of Photoelectric Effect (1)

• The diagram below shows the arrangement to
  investigate photoelectric effect.

14
Investigation of Photoelectric Effect (2)

• From the investigations it was found that
• When monochromatic light fell on the cathode, no
  photoelectrons were emitted unless the frequency
  of the light was greater than some minimum value
  called threshold frequency.
• When the frequency of light f is greater than
  the threshold frequency, some electrons are
  emitted from the cathode with substantial initial
  speeds.

15
Investigation of Photoelectric Effect (3)

• By reversing the direction of the E-field, it can
  be shown that the highest energy electrons still
  can reach the anode if the E-field is not too
  great.

16
Investigation of Photoelectric Effect (4)
17
Variation of Photocurrent with Voltage for Light
of Constant Frequency (1)

• The diagram below shows graphs of photocurrent as
  a function of potential difference (Accelerating
  voltage) for light of constant frequency and
  different intensities.

18
Variation of Photocurrent with Voltage for Light
of Constant Frequency (2)

• From the graphs, it can be shown that
• When the accelerating voltage is sufficiently
  large and positive, the curves level off, showing
  that all the emitted electrons are being
  collected by the anode.
• If the light intensity is increased while its
  frequency is kept the same, the current levels
  off at a higher value, showing that more
  electrons are being emitted per second.
• The stopping potential is found to be the same.
  That is the maximum kinetic energy of the
  electrons is not proportional to the light
  intensity.

19
Variation of Photocurrent with Voltage for Light
of Different Frequencies (1)

• The graphs below show the variation of
  photocurrent with the accelerating voltage for
  different frequencies, with the same intensity of
  light.

20
Variation of Photocurrent with Voltage for Light
of Different Frequencies (2)

• From the above graphs, we see that
• When the frequency of the light is increased, the
  stopping potential increases.
• The maximum kinetic energy depends on the
  frequency of the incident light since it has been
  shown that

21
Wave Theory predictions for the Photoelectric
Effect

• According to the classical theory,

2. The intensity of an electromagnetic wave such
as light does not depend on frequency, so an
electron should be able to acquire its
needed escape energy from light of any
frequency.
22
Einsteins Theory of Photoelectric Emission (1)

• A beam of light consists of small packages of
  energy
• called photons or quanta.

3. A photon arriving at the surface is absorbed
by an electron. This energy transfer is an
all-or-nothing process.
23
Einsteins Theory of Photoelectric Emission (2)
7. Applying the law of conservation of energy,
24
Relationship between The Stopping Potential and
the Frequency of Light
http//home.a-city.de/walter.fendt/phe/photoeffect
.htm

• The graph below shows how the stopping potential
  varies with the frequency of the incoming light.

25
Uses of Photoelectric Cells

• Photodiode
• Optical sound track on movie film
• Photo-voltaic cells
• Photo-conductive cells (LDR)

26
Types of Spectra (1)

• Continuous Spectra
• Continuous spectra consist of a continuous range
  of colours from deep red to deep blue.
• When an element is heated up, the atoms vibrate
  so much that their energy levels becomes spread
  out. The atoms emits a continuous range of photon
  energies and hence wavelengths.
• A continuous spectrum can be used to determine
  the temperature of the source.

27
Types of Spectra (2)
http//physicsstudio.indstate.edu/java/physlets/ja
va/atomphoton/index.html

• Line Emission Spectra
• Line emission spectrum consists of thin vertical
  lines of different colours, set against a dark
  background. Each line corresponds to one value of
  wavelength.
• The atoms emit photons of certain energies only.
• Each photon is emitted when an electron in an
  atom moves from one energy level to a lower
  energy level.

28
Types of Spectra (3)
http//javalab.uoregon.edu/dcaley/elements/Element
s.html

• Absorption Spectra
• Absorption spectra consist of dark vertical lines
  against a background of continuous spectrum.
• When white light passes through a gas, an
  electron moves from a low energy level to a
  higher level as a result of absorbing a photon of
  energy equal to the difference of the two energy
  levels.
• The absorption spectrum is like the negative of
  the emission spectrum.

29
Solar Spectrum
http//antwrp.gsfc.nasa.gov/apod/ap000815.html

• The solar spectrum consists of a continuum with
  thousands of dark absorption lines superposed.
• The lines are called the Frauenhofer lines, and
  the solar spectrum is sometimes called the
  Frauenhofer spectrum.
• These lines are produced primarily in the
  photosphere.

30
Hydrogen Spectrum (1)

• The hydrogen spectrum can be observed using a
  spectrometer to view light from a hydrogen-filled
  discharged tube.

434 nm
656 nm
486 nm
410 nm
31
Hydrogen Spectrum (2)

• In 1885 Johann Balmer discovered an equation
  which describes the emission-absorption spectrum
  of atomic hydrogen
• 1 / l R (1 / 4 - 1 / n2)        where n 3,
  4, 5, 6, ...
• R1.097?107 m-1, and is called the Rydberg
  constant
• Balmer found this by trial and error, andhad no
  understanding of the physicsunderlying his
  equation.

32
Transition between energy levels in a hydrogen
atom
(Continuum)
33
Energy Levels

• Later experiments on hydrogen showed that

Where n and m are integers.
Light of wavelength ? is emitted as atom jumps
from level m to level n.
34
Energy Equation

• When an atom jumps from level m to level n, a
  single photon of light is emitted whose energy is
  given by

• The nth level of a hydrogen atom is given by

• Ground state energy of hydrogen -13.6 eV

35
Continuum

• If an electron is given enough it can escape from
  the atom.
• The electron is then unbound and the quantization
  of energy levels disappears.
• The hydrogen atom is said to be ionized if the
  electron is promoted into the continuum.
• Ionization energy of hydrogen 13.6 eV.