Franck-Hertz experiment

In 1914, 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.

The classic experiment involved a tube containing low pressure gas and bounded at each end by an electrode, and containing a mesh accelerating grid near the ground electrode. (This 'ground' was actually held very slightly negative, so that electrons had to have a small amount of kinetic energy to reach it.) Instruments were fitted to measure the current passing between the electrodes, and to adjust the potential difference (the voltage) between the cathode (negative electrode) and the accelerating grid.

  • At low voltages--up to 4.9 volts when the tube contained mercury vapour--the current through the tube increased steadily with increasing potential difference. The higher voltage increased the electric field in the tube and electrons were drawn more forcefully towards and through the accelerating grid.
  • At 4.9 volts the current drops sharply, almost back to zero.
  • The current increases steadily once again if the voltage is increased further, until 9.8 volts is reached (exactly twice 4.9 volts).
  • At 9.8 volts a similar sharp drop is observed.
  • This series of dips in current at 4.9 volt increments will visibly continue to potentials of at least 100 volts.

Franck and Hertz were able to explain their experiment in terms of elastic and inelastic collisions. At low potentials, electrons acquired only a modest amount of kinetic energy. When they encountered mercury atoms in the tube, they participated in elastic collisions. The total amount of kinetic energy in the system remained the same. (Since electrons are significantly less massive than mercury atoms, this meant that the electrons held on to the vast majority of that energy, too.) Higher potentials served to drive more electrons to the ground and increase the observed current.

The lowest energy electronic excitation a mercury atom can participate in requires 4.9 electron volts (eV). When the accelerating potential reached 4.9 volts, each electron possessed exactly that amount of energy when it reached the anode grid. Consequently, a collision between a mercury atom and an electron at that point could be inelastic. Its kinetic energy could be converted into potential energy, and used to excite the mercury atom. With the loss of all of its kinetic energy, the electron can no longer overcome the slight negative potential at the ground electrode, and the measured current drops sharply.

As the voltage is increased, electrons will participate in one inelastic collision, lose their 4.9 eV, but then continue to be accelerated. In this manner, the current rises again after the accelerating potential exceeds 4.9 V. At 9.8 V, the situation changes again. There, each electron now has just enough energy to participate in two inelastic collisions, excite two mercury atoms, and then be left with no kinetic energy. Once again, the observed current drops. At intervals of 4.9 volts this process will repeat; each time the electrons will undergo one additional inelastic collision.

A similar pattern is observed with neon gas, but at intervals of approximately 19 volts. The process is identical, just with a much different threshold. One additional difference is that a glow will appear near the accelerating grid at 19 volts--one of the transitions of relaxing neon atoms emits red-orange light. This glow will move closer to the cathode with increasing accelerating potential, to whatever point in the tube the electrons acquire the 19 eV required to excite a neon atom. At 38 volts two distinct glows will be visible: one between the cathode and grid, and one right at the accelerating grid. Higher potentials will result in additional glowing regions in the tube, spaced at 19 volt intervals.

The Franck-Hertz experiment confirmed Bohr's quantized model of the atom by demonstrating that atoms could indeed only absorb (and be excited by) specific amounts of energy (quanta).

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