X-Ray Photoelectron Spectroscopy (XPS) is based on the photoelectric effect, which was first discovered by Albert Einstein in 1905.
The photoemission process from a solid sample takes place when a highly energetic photon interacts with matter, causing an electron to be removed from an atomic orbital or from a band and to reach the vacuum level.The excitation energy must be large enough for the electrons to overcome the work function of the solid. The process has traditionally been described according to the three steps model, which includes : (1) optical excitation; (2) transport to the surface; (3) escape
into the vacuum.
The initial state of the photoelectron can be either a valence band or a core level state. In this respect, soft X-rays, in the range 300-1000 eV, are ideally suited to probe the core levels of a solid, which generally show no dispersion due to their highly localized atomic-like character. The photemission event, which leaves the N-electron system in a core-ionized state, obeys the following energy conservation rule:
hν = EB+Ekinwhere hν is the photon energy, EB is the electron binding energy relative to the vacuum level prior to ionization, and Ekin is the kinetic energy of the photoelectron. In photoeectron spectroscopy, however, reference is generally taken with respect to the Fermi level, so that EB is replaced by EB+Φ (the BE relative to the Fermi level, plus the sample work function), yielding:
hν = EB+Φ+EkinTwo strong points of photoemission spectroscopy are its chemical sensitivity and its suitability for solid surface investigation. The photoelectron BEs, in fact, already convey information on the chemical composition of the sample. In addition, the chemical environment in which the core electron is found prior to the photoemission event (the type of bonding, the oxidation state, the possible presence of adsorbates) results in distinctive BE shift. A particular kind of chemical shifts are Surface Core Level Shifts, which denote the BE shifts between core level electrons originating from the surface and from the bulk. In transition metals, this effect basically ensues from the lower coordination of top layer atoms, which induces a d-band narrowing and a subsequent energy shift to preserve charge neutrality.
The Photoemission Process
The high surface sensitivity of photoelectron spectroscopy is better understood by considering the universal curve of the inelastic photoelectron mean free path in a solid. Electrons in the kinetic energy range 50-100 eV have a mean free path of only few Å. It follows that the photoelectrons typically detected in XPS studies originate from the first few layers of the solid, making this technique a powerful tool for surface analysis.
The excitation sources used in photoelectron spectroscopy can be of different kinds. Conventional sources typically exploit the radiation emitted by an Mg or Al anode to produce photons at a fixed energy of 1256 and 1486 eV, respectively (Mg and Al Kα emission). In the last decades, the development of large synchrotron radiation facilities, as Elettra, has brought about a leap forward in this field.
The main advantages of using synchrotron light with respect to the x-ray we use in the SSL are:
(i) the high brilliance of this kind of radiation, which is orders of magnitude more intense and better collimated than the one produced by anode-based sources;
(ii) the tunability of synchrotron radiation over a wide frequency range;
(iii) the polarized nature of the light;
(iv) the possibility of producing extremely short photon pulses at a frequency as high as a MHz.
|X-ray Photoelectron Spectroscopy|