Experiment: Franck-Hertz Experiment for Neon
(115 V, 50/60 Hz)

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Description
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Thematic focus:_..

• Record and evaluate the Franck-Hertz curve for neon and observe emission of light.
• Compare the distribution of current maxima with the excitation energies of neon atoms.
• Observe the light emitted by the excited neon atoms.
• Determine the number of light-emitting levels for various accelerating voltages.

Experiment description:_..

The Franck-Hertz experiment for neon involves observing how energy is transferred from electrons as a result of inelastic collision while passing through neon gas. The transfer of energy occurs in discrete steps corresponding to the excitement by such collision of distinct energy level transitions in the neon atoms. The excited atoms then emit visible light.

Observation and Interpretation:_..

An evacuated glass tube that has been filled with neon gas to a pressure of 10 hPa contains a heated cathode C, a control grid S, a grid G and a target electrode A arranged in that sequence. Electrons are emitted from the cathode and are accelerated by a voltage U towards the grid. Having passed through the grid they reach the target and thus contribute to a target current I if their kinetic energy is sufficient to overcome a decelerating voltage U_GA between the grid and the target.
The I(U)-characteristic has a similar pattern to the original Franck-Hertz experiment using mercury gas but this time the intervals between minima where the current falls to almost zero for a specific voltage U = U₁ corresponding to the electrons reaching sufficient kinetic energy to excite a neon atom by inelastic collision just before reaching the grid are about 19 V. Simultaneously it is possible to observe a faint orange light close to the grid since the energy transition to the base state of a neon atom results in the emission of such light. The zone of illumination moves towards the cathode as the voltage U increases and the target current I rises once more.
For a higher voltage U = U₂ the target current also drops drastically and it is possible to see two zones of illumination. The electrons can in this case retain enough energy after an initial collision to excite a second neon atom. As the voltages are further increased, other minima in the target current along with further zones of illumination can be observed.