PES measures electron binding energies to experimentally determine a sample’s electronic structure. Analyzing the energy required to remove electrons provides deep insight into how they are arranged in distinct energy levels. This technique offers direct evidence that electrons occupy specific, quantized orbitals.
The Physical Basis of Photoelectron Spectroscopy
Photoelectron spectroscopy relies on the photoelectric effect. A sample is irradiated with high-energy photons (\(E_{photon}\)) from a UV or X-ray source. When a photon strikes an electron, the energy transfer causes the electron to be ejected (photoionization). The instrument measures the kinetic energy (\(E_{kinetic}\)) of the emitted electron.
This measured kinetic energy is used to calculate the electron’s binding energy (\(E_{binding}\)), the energy that originally held the electron to the nucleus. The relationship is defined by the energy conservation equation: \(E_{binding} = E_{photon} – E_{kinetic}\). Since \(E_{photon}\) is constant and \(E_{kinetic}\) is measured, the binding energy is precisely determined for every detected electron.
The resulting photoelectron spectrum plots the number of detected electrons against their calculated binding energy. By convention, the binding energy axis often increases from right to left, positioning the highest binding energies on the far left. This arrangement places electrons held most tightly to the nucleus, which require the most energy to remove, closer to the left side of the plot.
Interpreting Peak Position (Binding Energy)
The horizontal position of a peak corresponds directly to the electron’s binding energy, measured in units like electron volts (eV). Each distinct peak signifies a different energy level or subshell from which electrons were ejected. Thus, the number of peaks indicates the number of unique electronic environments in the atom.
Peaks with high binding energy, found toward the left, signify electrons strongly attracted to the nucleus. These are typically core electrons, like those in the 1s orbital, which are closest to the nucleus and experience minimal shielding. Conversely, peaks with low binding energy, located toward the right, represent valence electrons in the outermost shells that are easier to remove.
The specific value of the binding energy helps identify the exact orbital the electron originated from, such as 1s, 2s, or 2p. The effective nuclear charge greatly influences binding energy; as it increases across a period, electrons are pulled closer, resulting in higher binding energy values for corresponding orbitals. For example, 1s electrons in carbon have a higher binding energy than 1s electrons in boron because carbon has one more proton.
Interpreting Peak Area (Electron Count)
The area of a peak provides information about the number of electrons occupying that energy level. The area is directly proportional to the total count of electrons ejected from that specific subshell. This relationship allows determination of the occupancy of each orbital represented by a peak.
Comparing the relative areas of different peaks determines the electron population ratios. For instance, an area ratio of 1:3 indicates the smaller peak’s subshell holds one-third the number of electrons as the larger peak’s subshell. This is useful because the maximum electron capacities of s, p, and d subshells are known to be 2, 6, and 10, respectively.
An atom like Neon (\(1s^2 2s^2 2p^6\)) would show three peaks with an idealized area ratio of 1:1:3, reflecting the \(1s\), \(2s\), and \(2p\) subshells. The largest peak corresponds to the \(2p\) subshell (six electrons). The two smaller, equal-sized peaks correspond to the \(1s\) and \(2s\) subshells (two electrons each). This quantification is essential for determining the full electronic structure.
Determining Electron Configuration from the Spectrum
The process of determining an atom’s full electron configuration involves synthesizing the information from both the peak position and the peak area. First, the peaks are ordered from the highest binding energy (far left) to the lowest binding energy (far right). This sequence establishes the order of the subshells, starting with the most tightly bound core electrons and moving outward to the valence electrons.
Next, the relative areas of the peaks are used to assign the number of electrons to each sequential subshell. The peak with the highest binding energy is always assigned as the \(1s\) orbital, which holds a maximum of two electrons. Subsequent peaks are assigned as \(2s\), \(2p\), \(3s\), and so on, based on the established order of orbital filling and the electron count ratio.
This combination of energy level identification and electron count allows for the construction of the complete electron configuration, such as \(1s^2 2s^2 2p^6\). This ability to translate experimental data into a fundamental chemical property is invaluable for identifying unknown elements or for confirming the theoretical electronic structure of a compound. The PES spectrum provides a direct, experimental fingerprint of an atom’s electronic arrangement.