Photoelectron spectroscopy is one of the more unusual topics in Unit 1, mostly because it's rarely covered outside AP Chemistry specifically, which means most students are reading this kind of graph for the very first time.
What PES actually measures
High energy photons are aimed at a sample, knocking electrons out of their orbitals. The kinetic energy of the ejected electrons is measured, and since the photon's energy is known, the electron's original binding energy can be calculated as the difference between the two. A more tightly held electron leaves with less kinetic energy, which means a higher calculated binding energy.
Reading the graph
The x-axis shows binding energy, and it's commonly plotted decreasing from left to right, which catches a lot of students off guard the first time. The y-axis shows relative number of electrons, meaning peak height reflects how many electrons occupy that subshell, not an absolute count on its own.
Matching peaks to subshells
Each peak on the graph corresponds to one subshell. Peaks farther to the side representing higher binding energy correspond to electrons held closer to the nucleus, typically lower energy, more penetrating subshells. The relative heights of the peaks should appear in simple whole number ratios that match subshell capacity, for example a 1:3 ratio between an s subshell holding 2 electrons and a p subshell holding 6.
Where students actually lose points here
Misreading which axis is which
Peak position tells you binding energy, how tightly an electron is held. Peak height tells you the relative number of electrons. Swapping these two leads to a completely backward interpretation of the graph.
Not noticing the x-axis often decreases left to right
Most graphs increase left to right, but PES graphs are conventionally plotted with binding energy decreasing in that direction, which is an easy detail to miss under time pressure.
Treating peak height as an absolute electron count
Peak height only gives a relative ratio between peaks on the same graph, not a direct absolute number of electrons without more context.
Practice makes this graph familiar fast
PES is unusual specifically because it's uncommon outside AP Chemistry, which means a few practiced examples make a bigger difference here than in almost any other Unit 1 topic.
Common questions
What does each peak on a PES graph represent?
Each peak represents one subshell of electrons. Its position on the x-axis shows the binding energy of electrons in that subshell, and its height shows the relative number of electrons present in it.
Which direction does the binding energy axis go on a PES graph?
It's conventionally plotted decreasing from left to right, which is the opposite of most graphs and a common source of misreading if it isn't checked first.
Can peak height tell you the exact number of electrons in a subshell?
Only in relative terms compared to other peaks on the same graph. The heights should appear in simple whole number ratios matching subshell capacities, like 1 to 3 for an s and a p subshell.