A student analyzes the photoelectron spectra (PES) of gaseous magnesium, \(\text{Mg}\), and aluminum, \(\text{Al}\). The student observes that the peak corresponding to the lowest binding energy for \(\text{Al}\) occurs at a lower energy than the peak corresponding to the lowest binding energy for \(\text{Mg}\). To support the claim that this difference is caused by the shielding and higher energy of the \(3p\) subshell in \(\text{Al}\), rather than a weaker effective nuclear charge experienced by the \(n = 3\) valence shell overall, which of the following pieces of evidence from the PES data is most decisive?