Franck hertz principle Franck hertz quantization of energy levels

1. [1]
FRANCK HERTZ QUANTIZATION OF
ENERGY LEVELS
AIM AND OBJECTIVE :
To experimentally demonstrate the concept of quantization of energy levels according to Bohr’s
model of atom.
INTRODUCTION :
In 1914, James Franck and Gustav Hertz performed an experiment which demonstrated the existence of
excited states in mercury atoms, helping to confirm the quantum theory which predicted that electrons
occupied only discrete, quantized energy states. Electrons were accelerated by a voltage toward a
positively charged grid in a glass envelope filled with mercury vapor. Past the grid was a collection plate
held at a small negative voltage with respect to the grid. The values of accelerating voltage where the
current dropped gave a measure of the energy necessary to force an electron to an excited state.
DISCUSSION OF APPARATUS :
 A mercury-filled Franck-Hertz tube,
 A neon-filled Franck-Hertz tube,
 An oven,
 A control unit for power supply,
 A DC current amplifier.
 CRO.
 Voltmeter.
THEORY AND BACKGROUND OF EXPERIMENT :

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James Franck and Gustav Hertz conducted an experiment in 1914, which demonstrated the existence of
excited states in mercury atoms. It confirms the prediction of quantum theory that electrons occupy only
discrete, quantized energy states. This experiment supports Bohr model of atom. For this great invention
they have been awarded Nobel Prize in Physics in the year 1925.
Apparatus used for the experiment consist of a tube containing low pressure gas, fitted with three
electrodes: cathode for electron emission, a mesh grid for the acceleration of electrons and a collecting
plate.
With the help of thermionic emission, electrons are emitted by a heated cathode, and then accelerated
toward a grid which is at a positive potential, relative to the cathode. The collecting plate is at a lower
potential and is negative with respect to mesh grid. If electrons have sufficient energy on reaching the
grid, some will pass through the grid, and reach collecting plate, and it will be measured as current by the
ammeter. Electrons which do not have sufficient energy on reaching the grid will be slowed down, and
will fall back to the grid. The experimental results confirm the existence of discrete energy levels.
As long as the electron collision is elastic, the electrons will not lose energy on colliding with gas
molecules in tube. As the accelerating potential increases, the current also increases. But as the
accelerating potential reaches a particular value, (4.9eV for mercury, 19eV for neon), each electron posses
that much of potential and now the collision become inelastic. As a result, the energy level of electron
bound to the atom is raised. Now the electron almost loses its energy, and measured current drops.
 Franck-Hertz Data for Mercury :
When the accelerating voltage reaches 4.9eV (lowest energy required by the mercury atom for excitation),
the current drops sharply. This drop is due to inelastic collisions between the accelerated electrons and
electrons in the mercury atoms. The sudden onset suggests that the mercury electrons cannot accept
energy until it reaches the threshold to elevate them to an excited state. Collected current drops at
multiples of 4.9eV. At 9.8 V, each electron gets sufficient energy to participate in two inelastic collisions.

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They excite two mercury atoms, and no energy will be left. This process will repeat, for each interval of
4.9eV.
 Franck-Hertz Data for Neon :
For Neon gas, the process of energy absorption from electron collisions is clearly visible. When the
accelerated electrons excite the electrons in neon to upper states, they de-excite in such a way as to
produce a visible glow in the gas region in which the excitation is taking place. There are about ten
excited levels in the range 18.3 to 19.5 eV. They de-excite by dropping to lower states at 16.57 and 16.79
eV. This energy difference gives light in the visible range. If the accelerating voltage is high enough, they
can undergo a series of reactions, by the inelastic collision between electrons and neon gas. Almost
similar pattern is observed in the case of neon gas at intervals of approximately 19 eV.
PROCEDURE :
 The circuit in the figure will already have been set up for you, and the tube will have been
warmed up. Make sure that the oven temperature is about 180deg. C. Record this temperature.

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The ground of the electrometer should be the same as the ground of the metal casing enclosing
the thermionic tube.
 Start with an anode voltage of 0V. Keeping the Keithley Electrometer at its most sensitive scale,
slowly raise the voltage until you get a definite signal on the electrometer. Record this voltage.
 Continue raising the anode voltage gradually. The electrometer responds to these changes slowly,
so make sure to give it time to react. At each local maximum and minimum in current, record the
voltage and its corresponding current.
 Raise the anode voltage to a maximum of 50V (do not exceed this voltage). When you have
reached this level, turn your voltage back down to zero and wait at least five minutes for the
equipment to recover. Check the temperature again and make sure that it is staying relatively
constant.
 Repeat this procedure (2-4) two more times. During your last run, take at least one data point
between the maximum and minimum.
RESULT ANALYSIS :
 Basically, this experiment confirms the existence of quantum theory. The energy states in an atom
seemed to be an abstract concept until this experiment was conducted.
 It confirmed the presence of energy states in the atoms like Mercury and Neon.
 The quantum theory stated that those electrons in an atom exists in the discrete and quantized
states.
 It was observed during the experiment that the current drops suddenly at some specific value of
voltage, indicating the changes in the energy of electrons.
 The overall presentation of this phenomenon on CRO showed several bumps with increasing and
decreasing values of current. This was helpful to interpret the quantized levels of energy.