Principle of X-ray photoelectron spectroscopy detection

(1) Excitation and detection of solid surfaces

X-ray photoelectron spectroscopy (XPS): The excitation source is X-rays, which act on the surface of the sample to produce photoelectrons. Obtain the photoelectron energy spectrum by analyzing the energy distribution of photoelectrons. Used to study the surface composition and structure of samples. Also known as chemical analysis photoelectron spectroscopy (ESCA).

Ultraviolet photoelectron spectroscopy (UPS): The excitation source is ultraviolet light, which can only excite the valence electrons of atoms and is used for quantum chemistry research.

Auger Electron Spectroscopy (AES): Excited by an electron beam, used for rapid analysis of surface composition.

(2) The binding energy of electrons and the photoelectric effect

Fermi level: The highest energy level occupied by electrons in the solid energy band at 0K.

Electron binding energy: The energy consumed by an electron in an atom to transition to its Fermi level after absorbing the energy of a photon.

The work function of the sample φ ： The energy consumed by electrons in the Fermi level to overcome the gravitational pull of the sample lattice and leave the sample surface to enter a vacuum and become stationary electrons.

1. Incident electron beam or X-ray; 2- Optoelectronics; 3-Auger electron

Photoelectric effect: Electrons within a sample atom absorb incident photons. If the energy of the incident photon is greater than the sum of the binding energy of electrons in the atom and the work function of the sample, the electrons that absorb the photon will leave the surface of the sample and enter a vacuum with a certain amount of kinetic energy, which is called the photoelectric effect. As shown in the figure

X-ray photoelectron: The inner electrons of an atom absorb the incident X-rays and detach from the atom to become free electrons, which is called X-ray photoelectron.

When a gas sample absorbs X-rays and produces X-ray electrons:

H ν =  Ek+Eb+Er

H ν- Quantum energy of incident light; Ek - kinetic energy of photoelectrons; Eb - binding energy of electrons; The recoil energy of an Er atom, Er=1/2 (M-m) v2. The recoil energy is very small and can be ignored. Therefore, kinetic energy can be represented as binding energy on the photoelectron spectrum:

Eb=h ν－  Ek

The electron spectrum measures the kinetic energy Ek of photoelectrons, and the horizontal axis of the photoelectron spectrum (energy spectrum curve) is represented by the binding energy Eb.

For solid samples:

H ν =  Ek '+Eb'+ φ kind

φ The work function of the sample. When the solid sample has good electrical contact with the metal sample frame of the instrument and the electron migration reaches equilibrium, the Fermi energy levels of both are at the same level. But the work function is different, and the contact potential difference △ V= φ Sample - φ If the instrument changes the kinetic energy of free electrons from Ek ′ to Ek ″, then:

Ek '+ φ Sample=Ek ″+ φ Instrument=h ν－ Eb ', therefore Eb'=h ν－ Ek ″ - φ instrument

φ The instrument is generally constant (about 4eV), and Ek ″ is measured by electron spectroscopy. Therefore, the electron binding energy Eb ′ of the sample can be calculated.