Ionisation and excitation energy In these notes (Interaction with matter, Katharina Müller, 2014), discussing the Bethe-Bloch formula, on slide 12, it is written as Mean excitation energy $>$ Ionization energy. How is this possible?
 A: The interaction of an incoming particle and the target atom has to consider the interaction between the incoming particle and all the electrons which make up a target atom.
You can consider the mean excitation energy as a weighted average of all the possible energy transfers from the incoming particle to the electrons of the target atom which will include excitations as well as ionisations.
It is perhaps therefore not surprising that the formula given on the slide for the mean excitation energy $I = 10 \,{\rm eV}  \cdot Z$ depends on the number of electrons in an atom $Z$ as more electrons results in more candidates to accept energy from the incoming particle.  

Update in response to a comment.
In a collision treated non-relativistically if the incident particle travelling at speed $v$ has a mass which is much larger than that of the target electron mass $m_{\rm e}$ then the maximum amount of energy which can be transferred to the electron is $2 m_{\rm e} v^2$.  
This means that the lower limit of the impact parameter has to be set to reflect this maximum possible amount of energy transfer from the incoming particle to an electron.  
The relativistic version of that upper limit of energy transfer is what you see towards the bottom of slide 6.
