FRANCK-HERTZ EXPERIMENT

Nurhidayah*), Nurul Ilmi Lukman, Zainal Bakri.

Modern Physics Laboratory of Physics Departement of Mathematics and Science State University of Makassar

Abstract. Has been done Franck-Hertz experiment that aims to measure the excitation energy of argon atoms Ar. This lab is done by providing a voltage source so that the electron can be fired into the grid and mashing cathode with argon atoms are capable of absorbing energy so as to reach kinetic energy electron excitation that can excite electrons. Based on the experimental results obtained by the amount of argon atom excitation energy of 2 eV excitation energy while the theory is based on the value of 2 eV.

Key words: Franck-Hertz experiment, excitation energy, Argon

INTRODUCTION

In 1913, Niels Bohr proposed the Bohr model of the atom which assumes that atoms can exists only in certain bound energy states. This idea was given a powerful boost in 1914 when James Franck and Gustav Hertz performed an experiment that demonstrated the existence of quantizised energy levels in mercury.

Both are specifically interested in the ionization event. To be able to measure the ionization energy, Franck and Hertz make a tool that they can use to learn the ionization produced in the atoms of a gas or vapor by electrons emitted from a hot wire through thermionic emission process. These electrons are then accelerated in an electric field so that energy can be well understood. For an electron with energy less than the ionization energy, Franck and Hertz expect no energy transfer between electrons and atoms. Conversely, for greater energy, they expect the electron energy loss in the amount equal to the energy of the ionization.

Results of experiments carried out in line with expectations. At first current rise with a growth potential U value to reach a Uo

potential. Once potential Uo is reached, the current flows down drastically but this was soon increased again at a voltage U = 2Uo, and so on. Uo values were calculated by Franck and Hertz is at 4.9 V.

Franck and Hertz explain these results as follows . When the electron energy is smaller than Eo = eUo , electrons can not experience a loss of energy in collisions with the atoms of mercury . When the electrons reach the grid, its energy is large enough to fight the terrain that

arise between grid with external electrodes. At a voltage slightly greater than Uo , electron energy value Eo reach before reaching the grid. In this condition, the electron will lose energy during an impact , and the electrons can no longer obtain sufficient energy from the field to fight the terrain that is rejected from the outside grid . Therefore flow down . By the time the voltage is raised constantly , collision events will occur earlier , namely in the area near the origin of the loss of an electron wire . Thus , after collide , the electrons can still gain enough energy to reach the outer electrode . As a result, the current will rise again and will go back down when the voltage reaches 2Uo , and so on . Based on these results , Franck and Hertz believes that the value of Eo is the value of the ionization energy of mercury atoms .

From these experiments, Franck and Hertz can also indicate that the energy Eo can be connected with vodengan frequency using the equation Eo = hvo, where h is Planck's constant. Thus, they not only managed to show that the electron kinetic energy is lost due to collisions with mercury atoms occur in the form of quanta of energy Eo, but they also managed to show that the energy quanta of light is equal to the energy emitted by atoms of the same if interpretation of Einstein's light quantum hypothesis is accepted. In the second experiment conducted by this collaboration, they can even show that they can excite transmitting a spectrum with a single line of frequency vo using fewer electrons have energies above Eo.

At the time they wrote their paper about it in 1914, Franck and Hertz do not know the new theory Bohr in 1913. Bohr was the one who showed in 1915 that Franck and Hertz

experiment is likely consistent with the assumption that the ionization energy Eo but not only the energy required to produce an excited state of the mercury atoms. Light transmission through a quantum frequency vo, the atom can return to its ground state. On this basis, Franck and Hertz experiment further considered as a new experiment and give support not only to the quantum theory of light quantum hypothesis of Planck and Einstein, but also to the Bohr atomic theory which said stationary states with discrete energy.

On this remarkable achievement, in 1926, Franck and Hertz was awarded the Nobel prize for physics in 1925.[1]

In this practicum we will prove their research, but with different atom, that is Argon with excitation energy is 11.5 eV.[3]

THEORY

An atom can excite to the energy levels above the level of basic energy that causes the atoms emit radiation in two ways. One is through collisions with other particles. A series of experiments conducted the collision by Franck and Hertz had begun in 1914. These experiments demonstrate directly that the atomic energy levels do exist and these levels similar to the levels found in the line spectrum.

Franck and Hertz shoot out steam with variety of elements with electrons whose energy is determined by a series of experiments in figure 6.1. Difference of small potential Vo installed between grid and collector chips, so that each electron has energy greater than a certain minimum price to contribute on the current I through the ammeter. The ability of the electrons to pass through and reach the anode grid is influenced by three factors, namely: accelerating potential, barrier potential and potential collisions between the gas molecules in the tube.

If the kinetic energy is conserved in a collision between an electron and an atomic vapor, electrons only bouncing in different directions with the direction it came from. In this process, atoms almost lose no energy. After critical energy is reached, the chips current dropped suddenly. Interpretation of this effect is that the electrons collide with atoms provide part or all of its kinetic energy to excite the atoms to the energy levels above the

ground level. Such collisions are called not elastic, as opposed to elastic collisions with kinetic energy that conserved.

Then, when the accelerator potential V increases, chips current increases again. Finally, the dropping current of chips I very acute and excitation energy level same to another atom. As seen in Figure 6.2, a series of critical potential to a particular atom obtained in that way. Thus , the highest potential is obtained from the number of times the collision and is a multiple of the lowest.

FIGURE 1 Basic scheme of Franck-Hertz Experiment

Franck and Hertz observed emission vapor spectrum when electrons fired. In the case of mercury vapor, they found that the minimum electron energy 4.9 eV is required to excite mercury spectral line 253.6 nm - 253.6 nm light photon energy 4.9 eV right. Because it is not easy to experiment with the use of hydrogen, the experiments carried out using argon gas (Ar). This is done so that the results of the experiment can be more easily interpreted. Hydrogen is naturally appear in the form of molecules not atoms. Graph the results of experiments using the argon atoms are shown in Figure 6.2 as follows.[2]

FIGURE 2 Example chart Ar atom excitation in Franck-Hertz experiment

V Vo

AA

EXPERIMENTAL METHOD

In this lab, we will measure the excitation energy of argon atoms Ar by using the Franck-Hertz alloy with oscilloscope and it’s probe. The series of experiments can be seen in the following figure:

FIGURE 3. The design of Franck-Hertz experiment

The first step before we take the data is heat up the argon gas with a filament voltage (V) at 4.5 V, for 20-30 minutes. Then Adjust the position of manual scanning and adjust the plate collectors or current flow multiple at position 10-8 A. after that set VG1 (Voltage Grid 1) at 2.5 V position, VG2 (Voltage Grid 2) at 7.5 V position, and VG3 (Voltage Grid 3) at 70 V position. Connect the oscilloscope with the Franck-Hertz Apparatus with channel 1 on the oscilloscope to the X- output and Channel 2 to the Y-Output, then set Channel 1 of 5 V and Channel 2 by 10 mV on the oscilloscope, slide scanning position towards auto and don’t forget to set scanning to show a good image.

EXPERIMENTAL RESULTS AND ANALYSIS DATA

Graphic image obtained excitation of argon atoms Ar on Franck-Hertz experiment is as follows:

FIGURE 4 Energy excitation graph of Ar

TABLE 6.1 Relation between Voltage (V) and Current (I)No V (V) I (A)1 2 82 4 203 6 32

Calculation AnalysisU1 = 2 eVU2 = 4 eVU3 = 6 eV

Based on the value of the electron kinetic energy, the value of the atomic excitation energy can be determined by using the equation:

U = U3 – U2 = U2 – U1

U = (6 – 4) eV = (4 – 2) eV = 2 eV

With the value of uncertainty measuring devices (in this case the oscilloscope grid value) by:

ΔU = 0,5 eV

So that the calculation result can be written by:

U = U ± ΔUU = (2 ± 0,5) eV

DiscussionA can excite the atoms to the energy

levels above the level of basic energy that causes the atoms emit radiation in two ways.One of them is a collision with another particle.At the time of the collision, some of the kinetic energy of the particle will be absorbed by the atom . Atoms are excited in this way will be returned to baseline levels within an average of 10-8 second by emitting one or more photons. Another way is with an electric discharge in a low pressure gas causing an electric field that accelerates electrons and atomic ions up enough kinetic energy to atoms megeksitasi when the collision occurred.

In lab experiments Franck-Hertz, the apprentice is expected to determine the excitation energy of the atom Argon (Ar) Based franck-Hertz experiment. From these experiments, apprentice obtain the results as shown in figure 4, which graphs the relationship between the potential accelerator and current I (Energy Exsitasion of Argon atoms). From experimental accelerator voltage

of 2 V starts up to 6 V. When the accelerating voltage is greater then the current I will go up, and after accelerating voltage reaches 2 volts then the current will go down, then the current will rise again , and when the voltage reaches 4 volts accelerator will flow back down. This is because when a potential accelerator (V) increases, the more free electrons from the cathode to the anode so that the current is detected by the ammeter will go up, as long as the electrons move from the cathode to the anode electrons will pound atomic Argon (Ar),but during electron energy is smaller than the energy to excite atoms Ar is a collision that occurs collisions so that no elastic energy is released. Then when the electron energy has reached the Ar atomic excitation energies, collisions are inelastic collisions that occur so that the energy will be absorbed by the excitation energy of Ne atoms so that the electron energy will be reduced. Because energy electrons can not be reduced until the chip so that the anode current will go down. When a voltage V is increased electron energy continues to rise again. But after returning electron energy reaches the atomic excitation energy, resilience and no collision energy absorption occurs, eventually decreasing the current occurs.

CONCLUSION

Based on the experiment result, we can conclude that the excitation energy of argon atoms Ar is 2 eV. This value accordance to the excitation energy of atoms based on theory.

BIBLIOGRAPHY

[1]Beiser, Arthur. 2003. Concepts of Modern Physics – Sixth Edition. New York: McGraw-Hill.

[2]Subaer, dkk. 2013. Manual Practicum of Physics Experiment 1, Physics Department Mathematics and Science Faculty of UNM.

[3]Pels, Christian. 2004. Ferromagnetic Electrodes for Spin Polarized Transport – Technology and Experiments. Hamburg: Cuvillier Verlag Göttingen