#Photo
Fundamentals of Electron Spectroscop utilizes photo-ionization and analysis of the kinetic energy distribution of the emittedphotoelectrons to study the composition and electronic state of the surface region of a sample Both PES is based upon a single photon in/electron out process. The energy of a photon of all types of electromagnetic radiation is given by the Planck–Einstein relation: E=hν PES uses monochromatic sources of radiation .UPS is a powerful technique to exam molecular electron structure since we are interested in the molecular orbitals from polyatomic molecules (especially the valence orbitals)In UPS the photon interacts with valence levels of the molecule or solid, leading to ionization by removal of one of these valence electrons. The kinetic energy distribution of the emitted photoelectrons can be measured using any appropriate electron energy analyzer and a photoelectron spectrum can thus be recorded. The difference in energy between the ionized and neutral species and is generally called the vertical ionization energy (IE) of the ejected electron; this then leads to
the following commonly quoted equation, KE=hν−IE or IE=hν−KE The vertical ionization energy is a direct measure of the energy required to just remove the electron concerned from its initial level to the vacuum level (free electron). Photoelectron spectroscopy measures the relative energies of the ground and excited positive ion states that are obtained by removal of single
electrons from the neutral molecule.XPS Background XPS technique is based on Einstein’s idea about the photoelectric effect, developed around 1905– The concept of photons was used to describe the ejection of electrons from a surface when photons were impinged upon it
• During the mid 1960’s Dr. Siegbahn and his research group developed the
XPS technique.– In 1981, Dr. Siegbahn was awarded the Nobel Prize in Physics for the
development of the XPS technique. X-Rays Irradiate the sample surface, hitting the core electrons (e-) of the atoms. • The X-Rays penetrate the sample to a depth on the order of a micrometer. • Useful esignal is obtained only from a depth of around 10 to 100 Å on the
surface. • The X-Ray source produces photons with certain energies:
– MgK photon with an energy of 1253.6 eV
– AlK photon with an energy of 1486.6 eV
• Normally, the sample will be radiated with photons of a single energy (MgK or
AlK). This is known as a monoenergetic X-Ray beam.
Why the Core Electrons?
• An electron near the Fermi level is far from the nucleus, moving in
different directions all over the place, and will not carry information
about any single atom. – Fermi level is the highest energy level occupied by an electron in a neutral solid at absolute 0 temperature. – Electron binding energy (BE) is calculated with respect to the Fermi level. • The core es are local close to the nucleus and have binding energies
characteristic of their particular element. • The core es have a higher probability of matching the energies of AlK and MgK. Core eValence eAtomBinding Energy (BE)These electrons are
attracted to the proton with certain binding energy x This is the point with 0 energy of attraction between the electron and the nucleus. At this point the electron is free from the
atom. The Binding Energy (BE) is characteristic of the core electrons for each element. The BE is determined by the attraction of the electrons to the nucleus. If an electron with energy x
is pulled away from the nucleus, the attraction between the electron and the nucleus
decreases and the BE decreases. Eventually, there wil Ø, which is the work function
At absolute 0 Kelvin the electrons fill from the lowest energy states up.
When the electrons occupy up to this level the neutral solid is in its “ground
state.” XPS Instrument • XPS is also known as ESCA (Electron Spectroscopy for Chemical Analysis). • The technique is widely used because it is very simple to use and
the data is easily analyzed. • XPS works by irradiating atoms of a
surface of any solid material with XRay photons, causing the ejection of
electrons. University of Texas at El Paso, Physics Department
Front view of the Phi 560 XPS/AES/SIMS UHV System
XPS Instrument
The XPS is controlled by using a
computer system.
The computer system will
control the X-Ray type and
prepare the instrument for
analysis.
University of Texas at El Paso, Physics Department
Front view of the Phi 560 XPS/AES/SIMS UHV System and
the computer system that controls the XPS.
XPS Instrument
• The instrument uses different
pump systems to reach the goal
of an Ultra High Vacuum (UHV)
environment.
• The Ultra High Vacuum
environment will prevent
contamination of the surface and
aid an accurate analysis of the
sample.