This lesson is part of XPS Course #500 which provides an XRI Certificate after completion.
The fundamental measurement for photoelectron spectroscopy involves exposing the specimen to be studied to a flux of nearly monoenergetic radiation with mean energy, hv, and then observing the resultant emission of photoelectrons, whose kinetic energies (Ekin) are described most simply by the photoelectric equation:
in which Eb V(k) is the binding energy (BE) or ionization potential (IP) of the kth level as referenced to the energy of the vacuum level and Ekin is the kinetic energy of the photoelectron. In general, both Auger electrons and secondary electrons (usually resulting from inelastic scattering processes) will also be emitted from the specimen, but it is generally possible to distinguish these other types of electrons from true photoelectrons by methods to be discussed later.
In equation form, Einstein found the energy of a photon or photoelectron to be where E is the energy of a photon of frequency f (or v) and h is Planck’s constant. The energy for Aluminum Kalpha X-ray photons can be calculated from Einstein’s equation after we lookup its’ wavelength from a book on X-ray Wavelengths.
E = hf = ℎv
After converting frequency to wavelength using: f = c/λ
E = hf = ℎv = ℎc/λ
The wavelength of X-rays produced by irradiating Aluminum metal with a beam of 12 kV electrons in a sufficient vacuum is 0.83393 nm or 8.3393 Angstroms, which is equivalent to 1486.7 eV of energy. Therefore:
1486.7 eV = hc/8.3393 Angstroms
There are three fundamental properties characterizing each emitted photoelectron:
- its’ kinetic energy,
- its’ directions of emission with respect to the specimen and the exciting radiation, and
- the orientation of its spin
These three properties thus give rise to three basic types of measurements that are possible on the emitted electron flux.
(1) The number distribution of photoelectrons with kinetic energy (KE).
After converting the KE of each electron into its’ corresponding Binding Energy (BE) the distribution is plotted as an Electron Spectrum with Electron Counts (Y axis) versus the Binding Energy (X axis). This measurement produces an electron spectrum which is also known as an Energy Distribution Curve (EDC) tht requires some sort of electron energy analyser or spectrometer, of which several types are currently being utilized. In the dispersive spectrometers most commonly used in XPS, electron spectra are usually measured at fixed angles of electron emission (or over a small range of emission angles) relative to both the photon source and the specimen.
(2) The distribution of photoelectron intensity with angle of emission.
Such angular-resolved measurements can be made relative to the photon propagation direction or to axes fixed with respect to the specimen. Generally, these measurements require kinetic energy distribution determinations at each of several angles of emission.
(3) The spin polarization or spin distribution of the photoelectron intensity.
These measurements require a specimen that has somehow been magnetically polarized, usually by an external field, so that more photoelectrons may be emitted with one of the two possible spin orientations than with the other. Then the relative numbers of spin-up and spin-down photoelectrons are measured. Such spin polarization measurements have been made with ultraviolet radiation and synchrotron light sources for excitation, but they will not be discussed further here.