# B Question in Franck and Hertz Experiment

Tags:
1. Jun 21, 2017

### Wrichik Basu

My book gives the following graph for current vs accelerating potential for Franck and Hertz experiment (used to prove existence of discrete energy levels in atoms) using Mercury vapours in the tube:

The book then writes:

"Actually, atoms have more than one excitation potential and also an ionisation potential. For example, the second excitation potential if Mercury is 6.67 volt and the ionisation potential is 10.4 volt. Hence, the second and third peaks in the graph become complicated."

Now, is this Ionisation Potential the same as "amount of energy required to remove the most loosely bound electron from the outermost shell of an isolated, neutral, gaseous atom to form a unicharged ion"? If so, then how is that measured? If not, then what is the proper definition?

Secondly, how will the graph change?

2. Jun 21, 2017

Your second question is more easily answered=the gas will become very conductive and the current will get very large as the ionization voltage is reached, corresponding to free electrons being able to acquire enough energy from the voltage to create a chain reaction that generates more free electrons. (Basically, you can sustain an arc in the lamp at that voltage.). $\\$ Additional question you ask is how is it measured? The Franck Hertz type apparatus will give you an approximate answer for that voltage, but a spectroscopic apparatus that observes the wavelength $\lambda$ of the emitted light as a free electron gets captured by an ion and goes back into the ground state might give a more precise answer for the energy required to ionize the atom. Note energy $E=\frac{hc}{\lambda}$.

Last edited: Jun 21, 2017