Photoemission spectroscopy
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Photoemission Spectroscopy refers to two separate techniques/
X-Ray Photoemission Spectroscopy (XPS, formerly known as ESCA - Electron Spectroscopy for Chemical Analysis) was developed at Uppsala University, Sweden in the 1960s by a group headed by Kai Siegbahn, who in 1981 won the Nobel Prize for Physics for his work in developing the technique.
- An article on X-ray photoelectron spectroscopy that complements this article is located elsewhere.
Photoemission Spectroscopy (PES) may also refer to a technique devised by Richard Smalley . With Smalley's technique, X-rays are not used, but instead an UV laser is used to excite the sample. This method is designed primarily to find the binding energy of electrons in molecular clusters.
[edit] Physical principle
The physics behind the XPS technique is really an application of Einstein's photoelectric effect. The material to be analyzed (usually a solid or viscous oil) is exposed to either a focused (10-1000 micrometers) beam of monochromatic X-rays or an unfocused (2-3 cm) beam of non-monochromatic (achromatic) X-rays inducing photoelectric ionization of the sample atoms. The X-rays penetrate several micrometers (1–3 µm) into the sample producing photoelectrons throughout the penetration depth of the X-rays. The energies of the emitted photoelectrons are characteristic of their original electronic states which include bonding state information, configuration interactions and the polarization effects of the adjacent atoms, which is one of the main reasons XPS is very useful. Most of the photoelectrons that were emitted deep inside the sample (0.03 to 3 micrometers) are recaptured and never escape into the ultrahigh vacuum of the instrument. The only photoelectrons that can be measured by typical XPS instruments are those that were emitted from the atoms contained within the top 1-30 atomic layers (0–10 nm]) of the sample.
Typical XPS instruments use magnesium or aluminum K-alpha sources of X-rays, which provide 1253 eV and 1486 eV of kinetic energy. The limited depth of information, which makes XPS a useful surface sensitive technique, occurs because the mean free path of electrons emitted by using the typical X-ray sources is between 1-3 nm. The photoelectrons that actually escaped into the vacuum are collected, energy resolved, slightly retarded and counted, which results in a spectrum of electron intensity as a function of the measured kinetic energy. Because binding energy values are more readily applied and understood, the kinetic energy values, which are source dependent, are converted into binding energy values, which are source independent. This is achieved by applying Einstein's relation <math>E_k=h\nu-E_B</math>. The <math>h\nu</math> term of this equation is due to the energy (frequency) of the X-rays that bombard the sample.
The binding energies of the measured electrons are characteristic of the chemical structure and bonding of the material. By increasing the energy resolution of the electron analyzer (spectrometer) to roughly 0.3-0.5 eV, the user can resolve the presence or absence of various chemical states for most of the metals in the periodic table. Chemical states (species) separated by one oxidation state or polarized by the presence (or absence) of one or more, adjacent, strongly electronegative atom(s) often have binding energy values that are (chemically) shifted by 0.5 to 1.5 eV. By comparing the experimentally measured binding energy values with those available in various reference sources, the chemical nature, state or species of the surface can soon be determined. XPS can also be used to resolve the presence or absence of a very wide variety of chemical states or species localized within the bulk of the sample by freshly exposing bulk regions of the sample.
XPS has the ability to generate very reliable empirical formula for a clean homogenously pure material with a relative uncertainty of less than 3% of the measured atomic percentage values. The smallest analysis area of commercial XPS instruments (in 2005) is roughly 10-20 micrometers diameter while the lateral (spatial) resolution is between 0.5 to 3 micrometers for the imaging type instruments. Synchrotron beam lines, serve as advanced XPS systems, that offer a continuum of soft and hard X-ray energies that allow band mapping and many sophisticated analyses not possible with commercial XPS instruments. By adding an argon ion beam milling (gun) system to an XPS instrument, depth profiling down to 1-2 micrometers is possible, but time consuming. Complementary surface and near-surface chemical analysis instruments include: auger spectroscopy, Time-of-Flight SIMS, Dynamic SIMS, Glow-Discharge Optical Emission Spectrometry (GD-OES), scanning electron microscope with an EDX (EDS) spectrometer, FT-Raman, FT-IR for example.
[edit] Instrument details
Commercial XPS instruments include an x-ray source, e.g. aluminium <math>K_\alpha</math> radiation is common. The beam is monochromatized using Bragg reflection on a crystal and is then directed towards the sample. This whole part of the instrument is kept in an UHV ultra high vacuum environment to avoid sample and instrument contamination due to adsorption of molecules. Most instruments are also equipped with a sputtering gun to remove unwanted molecules from the surface prior to measurement, and for extracting depth profiles.
The photoelectrons are collected most often using an electrostatic lens system, although some magnetic lens designs are available. An electrostatic energy analyzer, most often consisting of concentric hemispheres, is used to determine the kinetic energy of the collected photo-electrons.
