The ESCAprobe TPD system with X-ray source consist of vacuum vessels, vacuum pumps, manipulators, fast entry lock chamber, sample transfer system, bake-out system and electronic rack for controllers. The system is specially designed to operate with a dedicated ultra-high vacuum system.
Electron Spectroscopy for Chemical Analysis (ESCA) works on the principle of Photoelectric effect discovered by Heinrich Hertz in 1887. When radiation of appropriate energy incident, electrons are emitted from the surface of the metal. The relation between the energy of the excitation radiation, work function of the metal and the maximum kinetic energy of the emitted electron as proposed by Einstein in 1905 is:
hf = Φ + KEMAX ;
Φ – the work function of the metal, hf – energy of the radiation, KEMax– maximum kinetic energy of the emitted electron. For analyzing the core electronic structure (0 – 1300 eV) of elements, radiation of high energy were used like X-rays , hence the corresponding spectroscopy is termed X-ray Photoelectron Spectroscopy (XPS). Valence electronic structure (0 – 20 eV) is probed using ultraviolet light source and hence the corresponding spectroscopy is termed Ultraviolet Photoelectron Spectroscopy (UPS).
Surface Analysis by XPS is accomplished by irradiating a sample with monoenergetic soft X-rays and analyzing the energy of the detected electrons. Mg Kα (1253.6 eV), Al Kα (1486.6 eV), or monochromatic Al Kα (1486.7 eV) X-rays are usually used. These photons have limited penetrating power in a solid of the order of 1 – 10 µm. They interact with atoms in the surface region, causing electrons to be emitted by the photoelectric effect. The emitted electrons have measured kinetic energies given by:
KE = hf – BE – Φs ;
hf – energy of the photon, BE – binding energy of the atomic orbital from which electron originates, Φs – work function of the spectrometer. The Binding Energy may be regarded as the energy difference between the initial and final states after the photoelectron has left the atom. Because there is a variety of possible final states of the ions from each type of atom, there is a corresponding variety of kinetic energies of the emitted electrons. Moreover there is a different probability or cross section for each final state.
Because each element has unique set of binding energies, XPS can be used to identify and determine the concentration of the elements in the surface. Variation in the elemental binding energies (the chemical shifts) arise from differences in the chemical potential and polarizability of compounds. These chemical shifts can be used to identify the chemical state of the material being analyzed