Franck Hertz Experiment

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Franck Hertz experiment produced exceptional results which were fundamental in quantum theory development. After the presentation of Bohr’s theory of the atom, Franck Hertz’s experiment was undertaken in the year 1914 by James Franck and Gustav Hertz. It showed one of the initial symptoms of atoms having different levels of energy. The electrons in this test are passed and given a boost by mercury vapour. As a result, loss of energy takes place because of inelastic scattering.


Insight to the Franck Hertz Experiment Theory

In this experiment, a thermionic emission made by a filament produces an electron beam. First, electrons are powered, and then they pass through vapour and finally decelerate before anode. All these occur within a tube inside a burner that helps in controlling the temperature of the tube. Plus, it also controls the density of mercury vapour. The following diagram is the setup of the Franck Hertz tube along with electrical connections.

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When the filament is heated, the metal’s conduction electron energy also gets raised. With enhancing temperature, a substantial section of electrons experiences an increase in their kinetic energy more than the cathode material’s work function (here platinum). However, few of them get close to the anode, and a small amount of current flow takes place.

Further, when Va (accelerating voltage) is amplified (4.9 volts), filament electrons are pushed closer to the grid, and a significant amount of current touch the anode. If Vg becomes zero, a few of the electrons that are emitted, reach the anode and generates an electron current. The current path in this set-up propagates across the filament, power supplies, ammeter, and mercury vapour. Note that only some electrons finally reach the anode. With an increase in Va, the current in the anode also increases as the trajectories of electrons stay focused. Plus, less deflection of electron takes place due to scattering.

Next, consider collisions of mercury atoms with electrons. One reason for electron deflection is due to elastic scattering. In this event, an atom “recoils” like a solid sphere, restricting greater loss of energy. Moreover, inelastic scattering can also take place. Electrons of mercury are unable to receive energy unless the same touches the threshold. Note that 4.9 volts is similar to a line in the mercury spectrum at 254 nm. Moreover, drops occur several times in 4.9 volts in the current. The reason being if a raised electron holding energy of 4.9 eV is discarded during a collision, it can be powered again to yield a similar type of collision at 4.9 volts.


Frank and Hertz experiment: Information about the Apparatus

The Franck-Hertz tube is inserted in an oven – a box made of metal consisting of terminals and a thermostatically controllable heater for making connections. Plus, a thermometer can be put through the opening on the upper part of the apparatus.

This system has electrodes placed parallel to each other for generating a uniform electric field. Besides, space between the perforated grip and cathode is significantly high as compared to the average free-electron path across mercury vapour under normal working conditions. To identify a directly heated cathode in this tube, find a ribbon made of platinum alongside a barium-oxide spot. Plus, cathode limits are provided with an electrode that restricts the flow of current and diminishes reflected and secondary electrons. This provides a uniform electric field.

However, to prevent leakage, a ceramic lining is fused inside the glass. Furthermore, evacuation of the tube is done, and the inner part is coated with ‘getter’. This helps to absorb air that leaked out at the time of manufacturing and throughout the apparatus’s lifetime.

Reading of the anode current must be conducted with a picoammeter or Keithley model 610 Electrometer, while the rest of the voltages must be measured with digital voltmeters.


Franck Hertz Experiment Conclusion

  • Franck Hertz Experiment Data for Mercury

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The above image shows that electrons lose 4.9 eV with every collision they make with mercury atoms. One can observe ten sequential bumps after an interval of 4.9 volts.

  • Franck Hertz Experiment Data for Neon

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For neon gas, the powered electrons excite neon electrons to higher states and further decelerate in a way to provide a glow in the gas region. About ten excited levels can be acquired within the range of 18.3 to 19.5 eV, and they decelerate at a range of 16.57 to 16.79 eV. This difference in energy produces light.

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FAQ (Frequently Asked Questions)

1. What is the dissimilarity between Ionization and Excitation?

Ans. The primary difference between ionization and excitation potential is that the former describes an electron removal from an energy level. In contrast, the latter describes the propagation of an electron from low energy to a higher energy level.

2. What is the amount of energy required to excite an Electron?

Ans. Let’s consider an electron is in the ground state, having an energy of -13.6 eV. Its next level of energy is -3.4 eV. Therefore, the amount of energy required to excite the same can be calculated using the following expression:

E2 – E1 = (-3.4 eV) - (-13.6 eV) = 10.2 eV

3. What causes the excitation of an Electron?

Ans. Excitation of electrons can also take place with electrical influence. In this process, an original electron takes up the energy of another electron and the quickest way to do the same is by providing a high amount of temperature, as a result of which collisions occur among the atoms, which eventually excites the electron.