Frank-Herze experiment with Neon

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Frank-Hertz experiment with Neon

  1. Goal

To investigate the first excitation potential of neon and to observe the light emission from the de-excitation of the excited atoms.

  1. Related Topics

Atom structure, electron-molecular interaction.

  1. Introduction

The Franck-Hertz experiment was a physics experiment that provided support for the Bohr model of the atom, a precursor to quantum mechanics. In 1914, the German physicists James Franck and Gustav Ludwig Hertz sought to experimentally probe the energy levels of the atom. The now-famous Franck-Hertz experiment elegantly supported Niels Bohr's model of the atom, with electrons orbiting the nucleus with specific, discrete energies. Franck and Hertz were awarded the Nobel Prize in Physics in 1925 for this work.

In the early 20th century, experiments by Ernest Rutherford established that atoms consisted of a diffuse cloud of negatively charged electrons surrounding a small, dense, positively charged nucleus. Given this experimental data, Rutherford naturally considered a planetary-model atom. The laws of classical mechanics predict that the electron will release electromagnetic radiation while orbiting a nucleus. Because the electron would lose energy, it would gradually spiral inwards, collapsing into the nucleus. This atom model is disastrous, because it predicts that all matter is unstable.

Also, as the electron spirals inward, the emission would gradually increase in frequency as the orbit got smaller and faster. This would produce a continuous smear, in frequency, of electromagnetic radiation. However, late 19th century experiments with electric discharges through various low-pressure gasses in evacuated glass tubes had shown that atoms will only emit light (that is, electromagnetic radiation) at certain discrete frequencies.

To overcome this difficulty, Niels Bohr proposed, in 1913, what is now called the Bohr model of the atom. He suggested that electrons could only have certain classical motions:

  1. The electrons can only travel in special orbits: at a certain discrete set of distances from the nucleus with specific energies.

  2. The electrons do not continuously lose energy as they travel, which energies are constant.

  3. They can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency determined by the energy difference h E of the levels according to Bohr's formula where h is Planck's constant.

In the atoms, the level with the lowest energy is called “the ground state”, the higher energies are called “the first excitation state”, “the second excitation state” and so on in order from the ground state. If the ground state and the first excitation state are described by E0, E1, respectively, the first excitation potential is U1=(E1-E0)/e.

The collisions between electrons and atoms have two types, elastic collisions and inelastic collisions. If the energies of electrons are lower than the required value, when they encountered atoms, they participated in purely elastic collisions. This is due to the prediction of quantum mechanics that an atom can absorb no energy until the collision energy exceeds that required to lift an electron into a higher energy state. That is, a free electron's kinetic energy could be converted into potential energy by raising the energy level of an electron bound to an atom. The excitation state is not stable. The atom will go back to the ground state and releasing the energy with radiation emission. If the wavelength of the radiation lies in the visible range, we will see as light.

  1. Experiment device

An evacuated glass tube is filled with neon. The glass tube contains a planar system of four electrodes (see Fig. 1). The grid-type control electrode G1 is placed in close proximity to the cathode K; the acceleration grid G2 is set up at a somewhat greater distance, and the collector electrode A is set up next to it. The cathode is heated indirectly, in order to prevent a potential differential along K. Electrons are emitted by the hot electrode and form a charge cloud. These electrons are attracted by the driving potential U1 between the cathode and grid G1. A braking voltage U3 is present between grid G2 and the collector A. Only electrons with sufficient kinetic energy can reach the collector electrode and contribute to the collector current.

In this experiment, the acceleration voltage U2 is increased from 0 to 80 V while the driving potential U1 and the braking voltage U3 are held constant, and the corresponding collector current IA is measured. This current initially increases, but reaches a maximum when the kinetic energy of the electrons closely in front of grid G2 is just sufficient to transfer the energy required to excite the neon atoms through collisions. The collector current drops off dramatically, as after collision the electrons can no longer overcome the braking voltage U3. As the acceleration voltage U2 increases, the electrons attain the energy level required for exciting the neon atoms at ever greater distances from grid G2. After collision, they are accelerated once more and, when the acceleration voltage is sufficient, again absorb so much energy from the electrical field that they can excite a neon atom. The result is a second maximum, and at greater voltages U2 further maxima of the collector currents IA.

When the acceleration voltage U2 is high enough, we can observe discrete red luminance layers between grids G1 and G2.

  1. Tasks/Procedure

    • Light emission

Set the operating mode switch to MAN and make yourself familiar to the function knobs of the device. Find out the setting of U1 so that you can see the light emission between the two grids G1 and G2 when you increase the acceleration voltage U2 gradually. Log the detailed experimental phenomenon, especially how does the luminance layers look like and how can they be changed.

  • Measuring the first excitation potential of neon.

In order to obtain an optimized IA-U2 curve, you should select optimal U1 and U3 values. One criterion is that you can see as much luminance layers before the saturation of the collector current IA when you increase the acceleration voltage U2, either in MAN or in Auto mode. Then, set the operating-mode switch to MAN and slowly increase U2 by hand from 0 V to 80 V. Read out the voltage U2 and the related current IA from the display; use the selector switch to toggle between the two quantities for each voltage. Put your data on a drawing paper. Find out the first excitation potential of neon according to the curve you get.

  1. Questions

6.1 Why do you make conclusion that the energy of the electrons in an atom is quantized from this experiment?

6.2 Why is it necessary to apply a deceleration voltage between collector electrode A and anode grid G2?

6.3 Please discuss the reason why the current of the peaks and troughs of the curve increasing with the increasing of the acceleration voltage U2.

6.4 What is the wavelength of the light emission between the two grids? Is it the emission from the de-excitation process of neon atoms from the first excitation state to the ground state?

  1. Reference

  1. 沈元华,陆申龙 基础物理实验 高等教育出版社, 2003


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