Why are the peaks in the Franck-Hertz experiment broad?

[tayles123]

As opposed to discrete values, given that the energy levels are discrete.

The gas in mind here is helium.

When studying the peaks a plot of accelerating voltage vs current that match up to the energy levels they should be discrete, right?

 

[marcusl]

Doppler broadening due to atomic motion will smear an otherwise sharp spectrum. Also I think some of the broadening may be due to the stochastic probability of an electron colliding with an atom. At each voltage, there is some variation in the number of collisions that occur, hence some noise in the data.