A student learns that neutral gaseous vanadium has the ground-state electron configuration \([\text{Ar}]\,4s^2 3d^3\), but the \(\text{V}^{2+}\) ion has the electron configuration \([\text{Ar}]\,3d^3\). The student claims that in neutral \(\text{V}\) atoms, the \(3d\) electrons experience a greater effective nuclear charge and are more tightly bound to the nucleus than the \(4s\) electrons, which causes the \(4s\) electrons to be removed first during ionization. Which of the following pieces of experimental evidence provides the most direct support for the student's claim?