Why XPS Only Analyze Core Electrons - X-rays Photoelectron Spectroscopy

XPS Analysis: Understanding Core Electrons

Core vs. Valence Electrons in XPS

• In X-ray Photoelectron Spectroscopy (XPS), the focus is on core electrons rather than valence electrons, as the peaks observed in the spectrum originate from these core electrons.

• A comparison between oxygen and titanium dioxide (Titania) illustrates that larger atoms like Titania have higher binding energies for their core electrons, resulting in distinct peak positions in the spectrum.

Elemental Analysis through Binding Energy

• The presence of peaks at lower binding energy indicates smaller atoms such as oxygen, while higher binding energy peaks suggest larger atoms like Titania.

• Core electrons are less influenced by surrounding atoms compared to valence electrons, leading to minimal shifts (0 to 10 eV), termed chemical shifts.

Importance of Binding Energy

• Valence electron binding energies exhibit significant variation due to bonding interactions, making them less reliable for elemental composition analysis compared to core electron binding energies.

• Core electron peaks are crucial for accurate elemental analysis in XPS; they include first S, 2S, and 2P levels.

Cross-section and Ionization Probability

• The term "cross-section" refers to the probability of photoionization; core electrons have a higher cross-section due to stronger nuclear attraction compared to loosely bound valence electrons.

• The use of aluminum and magnesium x-ray sources in XPS targets core electron emissions effectively since their binding energies range significantly higher than those of valence electrons.

Techniques for Electron Emission Analysis

• Ultraviolet Photoelectron Spectroscopy (UPS) is used for analyzing valence electrons due to its lower penetration power compared to x-rays used in XPS.

• When x-rays interact with a sample, they emit both core and valence electrons; however, only core electron peaks appear sharp and distinct while valence electron signals are often weak and difficult to observe.

Comparative Binding Energies

• An example comparing oxygen and Titania shows that Titania's greater number of protons leads to higher attractive forces on its core electrons, resulting in increased binding energy.

• This difference means that when x-rays excite these elements, Titania will show peaks at higher binding energies than oxygen due to its larger atomic structure.

Understanding Core Electrons and Their Binding Energy

The Nature of Core Electrons

• Core electrons are tightly bound to the nucleus, resulting in greater binding energy. This strong binding means they are less influenced by external factors compared to valence electrons.

Influence of Neighboring Atoms

• The binding energy of core electrons can shift due to perturbations from neighboring atoms or orbitals, with a range of 0 to 10 electron volts. This shift is symmetric for insulators but asymmetric for conductors, indicating more variation in conductors.

Binding Energy Range

• The binding energy for elements typically falls around 1100 electron volts, which is sufficient for core electrons but not for valence electrons. This alignment increases the probability and cross-section for photoionization events involving core electrons.

Comparison Between Core and Valence Electrons

• Core level electrons are less affected by their environment than valence electrons, which participate in bonding with neighboring atoms. Valence electrons have lower binding energy and are more susceptible to environmental influences.

Challenges in Extracting Information from Valence Electrons

• Due to their loose binding and constant interaction with surrounding atoms, valence electrons do not provide reliable information about individual atoms. Variations in their environment lead to difficulties in extracting precise elemental data from them.