The Franck-Hertz Experiment*

In 1914, James Franck and Gustav Hertz performed an experiment which demonstrated the existence of excited states in mercury atoms, helping to confirm the quantum theory which predicted that electrons occupied only discrete, quantized energy states. Electrons were accelerated by a voltage toward a positively charged grid in a glass envelope filled with mercury vapor. Past the grid was a collection plate held at a small negative voltage with respect to the grid. The values of accelerating voltage where the current dropped gave a measure of the energy necessary to force an electron to an excited state.

*Nobel Prize in physics, 1925

Further discussion 进一步讨论

Sketch of apparatus 装置示意图

Data for mercury 水银的数据

Data for neon 氖的数据

The Franck-Hertz Experiment

Electrons are accelerated in the Franck-Hertz apparatus and the collected current rises with accelerated voltage. As the Franck-Hertz data shows, when the accelerating voltage reaches 4.9 volts, the current sharply drops, indicating the sharp onset of a new phenomenon which takes enough energy away from the electrons that they cannot reach the collector. This drop is attributed to inelastic collisions between the accelerated electrons and atomic electrons in the mercury atoms. The sudden onset suggests that the mercury electrons cannot accept energy until it reaches the threshold for elevating them to an excited state. This 4.9 volt excited state corresponds to a strong line in the ultraviolet emission spectrum of mercury at 254 nm (a 4.9eV photon). Drops in the collected current occur at multiples of 4.9 volts since an accelerated electron which has 4.9 eV of energy removed in a collision can be re-accelerated to produce other such collisions at multiples of 4.9 volts. This experiment was strong confirmation of the idea of quantized atomic energy levels.

Sketch of Franck-Hertz Apparatus

Franck-Hertz Data for Mercury

This original Franck-Hertz data shows electrons losing 4.9 eV per collision with mercury atoms. It is possible to observe ten sequential bumps at intervals of 4.9 volts. 这些原始的弗兰克-赫兹实验数据表明，电子每次与汞原子碰撞时会损失4.9 eV的能量。可以观察到十个连续的峰，每隔4.9伏特出现一次。 Sketch of apparatus Further discussion 装置示意图 进一步讨论

Data by Jason Lee and Bruce Rountree Jason Lee 和 Bruce Rountree 提供的数据

The Franck-Hertz display for mercury shown at left was formed by sweeping the accelerating voltage and recording current vs voltage on an oscilloscope in x-y mode. The measured separation of the peaks corresponds to the excitation energy of the involved mercury transition. 左边所示的弗兰克-赫兹示意图是通过扫频加速电压，并在X-Y模式下将电流与电压记录在示波器上形成的。测量到的峰间分离对应于所涉及的汞跃迁的激发能。

Franck-Hertz Data for Neon

For Neon gas, the process of absorbing energy from electron collisions produces visible evidence. When the accelerated electrons excite the electrons in neon to upper states, they de-excite in such a way as to produce a visible glow in the gas region in which the excitation is taking place. There are about ten excited levels in the range 18.3 to 19.5 eV. They de-excite by dropping to lower states at 16.57 and 16.79 eV. This energy difference gives light in the visible range. Since the accelerated electrons undergo inelastic collisions with the neon and are then accelerated again, they can undergo a series of such collisions if the accelerating voltage is high enough. The accelerating voltage from the Franck-Hertz apparatus used to produce the picture was capable of producing and accelerating voltage of about 80 volts, so you could get up to four collisions. This can be seen under proper conditions as four bands of light from the de-excitation in the collison regions. 对于氖气，吸收电子碰撞中的能量会产生可见的证据。当加速的电子激发氖气中的电子到较高能级时，它们会以这种方式退激发，产生可见的光 glow 在激发发生的空间区域。在18.3到19.5 eV的范围内大约有十个激发能级。它们通过退激发到16.57和16.79 eV的较低能级来退激发。这种能量差会产生可见光范围内的光。由于加速电子与氖气发生非弹性碰撞并再次被加速，如果加速电压足够高，它们可以发生一系列这样的碰撞。弗兰克-赫兹装置用于产生图像时的加速电压大约为80伏，因此可以得到最多四次碰撞。在适当的条件下，这可以观察到四条来自碰撞区域退激发的光带。 The Franck-Hertz display for neon shown at left was formed by sweeping the accelerating voltage and recording current vs voltage on an x-y plot. The measured separation of the peaks corresponds to about the midpoint of the range of excitation energies of the involved neon transitions. 左边所示的弗兰克-赫兹示意图是通过扫频加速电压，并在x-y图上记录电流与电压的关系而形成的。测得的峰间分离对应于所涉及的氖跃迁激发能范围的中点。