Binding energy vs. ionization energy I am studying binding energy in the context of photoelectric spectroscopy, and I came across a graph of relative numbers of electrons vs binding energy (in units of megajoules per mole). The element in question is lithium. My chemistry resource (Khan academy, to be precise) noted that the binding energy for the furthest right peak corresponds to the binding energy for the electron in the 2s subshell, and that this is also equal to the first ionization energy. However, they go on to say that the the binding energy of the second peak is not equal to the second ionization energy, since the electrons are held more tightly by the nucleus once the outermost electron is removed. I actually have two questions about this. First, why are they more tightly held once the outermost electron is removed? Is the electric field of the protons somehow getting weakened by the presence of the outer electron? I don't think this would be the case, based on my understanding of electric fields. So what is the explanation?
My other question is why the binding energy is different from the ionization energy? Is it because the electrons are removed all at once versus them being removed one-by-one? Or is there some nuance to this situation I am not seeing?
I found a question on this website that seems to be asking a similar thing, but no one answered it. Here's the link to it anyway.
Chemistry: What&#39;s the difference between &quot;binding energy&quot; and &quot;ionization energy&quot;?
Thanks in advance for the help.
First some basics....
Lithium has three electrons.

$\ce{Li}$ electronic configuration: $1s^22s^1$
$\ce{Li -&gt; Li+ + e-\quad\quad\mathrm{Ionization\ Energy= 0.520\ MJ/mol}}$
$\ce{Li+}$ electronic configuration: $1s^2$
$\ce{Li+ -&gt; Li^2+ + e-\quad\quad\mathrm{Ionization\ Energy= 7.298\ MJ/mol}}$
$\ce{Li^2+}$ electronic configuration: $1s^1$
$\ce{Li^2+ -&gt; Li^3+ + e-\quad\quad\mathrm{Ionization\ Energy=11.815\ MJ/mol}}$
$\ce{Li^3+}$ electronic configuration: $1s^0$, i.e. no electrons
Ionization energies from Wikipedia

Thus the different ionization reactions remove the electrons from the outer most to the inner most electrons.
Below is an another image of the idealized spectra of lithium that has the binding energies stated. (from https://www.andrew.cmu.edu/course/09-105/S98Slides9.html)

$\mathrm{Binding\ Energy= 0.52\ MJ/mol}$ corresponds to $\ce{Li -&gt; Li+ + e-}$
or $1s^2 2s^1\ce{-&gt;} 1s^2$, hence the first ionization energy of a Li atom.
However $\mathrm{Binding\ Energy= 6.26\ MJ/mol}$ corresponds to $\ce{Li -&gt; Li+ + e-}$
or $1s^2 2s^1\ce{-&gt;} 1s^12s^1$, which is not the first or second ionization energy of a Li atom. Rather it is the binding energy of the 1s orbital in a neutral lithium atom.
Hence the photoelectron spectra is called the binding energy spectra because that is what it measures. Note that the sweep of the spectra would not have to cover the ionization energy. For example the spectra could have been taken from 5 MJ/mol to 100 MJ/mol.